A contribution to The United Nations Task Force on El Niño for implementation of United Nations General Assembly Resolutions 52/200 and 53/185

The Inter-Agency Committee for the Climate Agenda led by the World Meteorological Organization with support from the Intergovernmental Oceanographic Commission of UNESCO the United Nations Environment Programme and the International Council for Science

1999 Front cover: The sea surface warms and rises off the South American coast and out into the Pacific Ocean as the El Niño of 1997–1998 spreads its influence across the planet. Sea level anomaly 10 November 1997 as measured by TOPEX/Poseidon. (NASA)

WMO-No. 905 © 1999, World Meteorological Organization ISBN 92-63-10905-2

NOTE The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. Contents

Page

Foreword ...... 5

Preface ...... 6

Executive Summary ...... 7

Introduction ...... 11

Part I — The Climate System ...... 15 The seasonal cycle of climate ...... 15 The El Niño ...... 15 The Southern Oscillation ...... 19 Ocean-atmosphere coupling — ENSO ...... 21 Global change ...... 22

Part II — The 1997-98 El Niño event ...... 24 Monitoring El Niño ...... 24 The El Niño cycle ...... 26 Overview ...... 26 Prior conditions ...... 27 Commencement ...... 27 Evolution ...... 29 The mature phase ...... 30 The decline ...... 33 An historical comparison ...... 34 Regional climate anomalies and impacts ...... 36 South and Central America ...... 37 North and Central America ...... 42 El Niño effects on coastal fish resources — a case study ...... 46 China ...... 48 Equatorial Asia-Pacific ...... 51 Papua New Guinea — A case study ...... 53 South-West Pacific ...... 56 East and Southern Africa ...... 59 A global assessment ...... 61

Part III — The way ahead ...... 62 Prediction on seasonal timescales ...... 62 Applications of forecasts ...... 64 Bridging the knowledge gulf — a case study ...... 66 Climate Information and Prediction Services ...... 68 Risk and society ...... 69 Economic dimension ...... 70 Environmental dimension ...... 71 3 Page

Developmental dimension ...... 71 Societal dimension ...... 72 An integrating framework for action ...... 72 International scientific infrastructure ...... 73 Regional cooperation ...... 75 National climate programmes ...... 77

Appendix — Climate processes ...... 78 The atmosphere ...... 78 Surface exchange processes ...... 79 The ocean surface layer ...... 80 Sea surface temperatures ...... 81 Monsoon circulations ...... 83 Teleconnections ...... 86 Climate prediction ...... 88 Statistical methods ...... 88 Dynamic models ...... 89 Regional models ...... 91

Selected Bibliography ...... 92

Acronyms ...... 95

Presentations made to the First Global Assessment of the 1997-98 El Niño Event, including rapporteurs for the panel sessions on Risk and Society, held in Guayaquil, Ecuador, 9-13 November 1998...... 96

4 Foreword

This retrospective analysis on the 1997–98 El Niño event has its origins at a major international conference held in Guayaquil, Ecuador from 9 to 13 November 1998. In response to Resolution 52/200 of the United Nations General Assembly, on international cooperation to reduce the effects of the El Niño phenomenon, the United Nations Task Force on El Niño, operating within the framework of the International Decade for Natural Disaster Reduction, convened the First Global Assessment of the 1997–98 El Niño Event. On behalf of the agencies supporting the Climate Agenda, I am pleased to present this important scientific analysis, entitled The 1997–98 El Niño Event: A Scientific and Technical Retrospective, which is in itself a further response by the United Nations system to the General Assembly resolution. There can be no doubt that we have seen enormous strides made in recent years in our understanding of the climate system. Of particular interest here are the complex and interrelated subsystems associated with the El Niño phenomenon and its sibling La Niña. We have also witnessed encouraging progress in making predictions on seasonal timescales that causes us to reflect back to the early, tentative steps of weather forecasting many decades ago. While the current skill in seasonal predictions is limited in terms of its detail and scope, we know that many of our needs for advanced planning do not demand high levels of precision. Information on expected seasonal conditions cast in probabilistic terms, if used wisely, can be of great value to society. This is the challenge of the newly-found predictive skills and the climate science community is keen to ensure that the benefits of its endeavours are put to best use. There are clear lessons for both the climate science community and the wide range of potential users of seasonal predictions in this most recent El Niño episode — some say the most intense in recorded history and certainly comparable to that which occurred in 1982–83. I trust that this publication will serve as a base of information for further analysis to understand El Niño related phenomena, to mitigate their destructive forces and to take advantage of opportunities that they can also provide. Special appreciation is extended to Mr W.R. Kininmonth, Australia, for his efforts and dedication in the preparation of this publication.

(G. O. P. Obasi) Secretary-General

5 Preface

The First Global Assessment of the 1997–98 El Niño Data and analyses have been provided through Event (International Seminar on the 1997–98 El Niño contributors to the international seminar, during the Event: Evaluation and Projections) was carried out in drafting of the Retrospective, or on their Internet sites. Guayaquil, Ecuador, 9–13 November 1998 within the Several scientists with extensive research experience framework of the United Nations General Assembly in climate and the El Niño/Southern Oscillation (ENSO) Resolution 52/200. The meeting was co-sponsored by the phenomenon made time to read the draft manuscript. Government of Ecuador, the United Nations Task Force Their constructive suggestions have added to the final on El Niño, and the Permanent Commission for the South document and are greatly appreciated. Pacific. The 1997–98 El Niño Event: A Scientific and The source of a significant proportion of the Technical Retrospective is an outcome of the First Global material used to illustrate the ENSO phenomenon and Assessment. the evolution of the 1997–98 El Niño event is the climate While every effort has been made to be as monitoring information generated by specialists and comprehensive as possible in this analysis of the 1997–98 organizations and made freely accessible through the El Niño event, scientific research on its onset, Internet. Particular acknowledgement is given to the US development and decay continues. Undoubtedly new National Oceanic and Atmospheric Administration insights will emerge after this Retrospective has been (NOAA) Climate Diagnostics Center, Boulder, Colorado published. Nonetheless it is believed that it will serve as for permission to use the climate analyses from its web a solid reference point for the socio-economic studies site (http://www.cdc.noaa.gov); the US NOAA Pacific under way on the overall social and economic costs of Marine Environmental Laboratory, Seattle, Washington the event and, more importantly, to determine how best for TAO ocean analyses from its web site to deal with the next event that history tells us will (http://www.pmel.noaa.gov); and the US National surely come. For example, the United Nations Aeronautics and Space Administration (NASA) Jet Environment Programme (UNEP), with financial support Propulsion Laboratory for TOPEX/Poseidon (operated from the United Nations Fund for International jointly with France, Centre national d’études spatiale) Partnerships and with the collaboration of the US analyses from the web site (http://topex-www.jpl. National Center for Atmospheric Research, the World nasa.gov/enso97/el_nino_1997.html). Meteorological Organization (WMO) and the United The relatively brief Introduction and description of Nations University, is conducting a series of in-depth the Climate System (Part I) of the Retrospective cannot studies of how 15 countries reacted to warnings of the adequately reflect the extensive international research event and coped with its evolution and aftermath. effort and the historical development of understanding Each of the presenters to the Guayaquil seminar, about the El Niño phenomenon and the coupled ENSO. listed on page 96, has significantly assisted in the drafting The Selected Bibliography reflects the source of material of the Retrospective and their respective involvement is directly used in the Retrospective and is expanded as deeply appreciated. The presenters have recognized partial acknowledgement of the extensive scientific expertise in the subject areas and were nominated by literature. In this context, English has been the working international agencies and national governments. In language of the First Global Assessment and of the addition, the Scientific Steering Group of Climate Retrospective. For this reason, many excellent historical Variability and Predictability (CLIVAR), through its and contemporary studies reported in other languages co-chair, Dr Kevin Trenberth of the US National Center do not get appropriate recognition in the global for Atmospheric Research, submitted a comprehensive assessment, and this is regretted. However, the effort of report as a contribution to the work of the United contributors who have drawn material from non-English Nations Task Force on El Niño. The drafting of the speaking sources has partially offset the deficiency and is Retrospective has only been possible because of scientific appreciated. assessments and material made available by the contributors, and through discussion and additional material provided following the international seminar. National Meteorological and Hydrological Services, in response to a request to Members of WMO by the Secretary-General, Prof. G.O.P. Obasi, have provided reports on the climate anomalies and impacts experienced by their respective countries. (Dr Michael J. Coughlan) Director, WMO World Climate Programme Department 6 Executive Summary

The intensity The strong 1997–98 El Niño event brought to Meteorological and Hydrological Services and global extent international attention the global scale of (NMHSs) and made available to the media of natural risks posed by extremes of climate, and international agencies. disasters particularly for the developing world. Loss WMO, with the United Nations concurrent with of life, destruction of infrastructure, Environment Programme (UNEP), the the 1997–98 El depletion of food and water reserves, Intergovernmental Oceanographic Niño event displacement of communities and outbreaks Commission (IOC) of the United Nations highlights that, of disease all occurred as manifestations of Education, Scientific and Cultural unless a climate-related natural disasters concurrent Organization (UNESCO) and the concerted effort is with the event. The United Nations (UN) International Council for Science (ICSU), made to prevent General Assembly took note of the intensity working with the IDNDR Secretariat within and mitigate the and global extent of natural disasters and the framework of the UN Task Force on El impacts, extremes requested the Secretary General, as Niño, organized the scientific programme of climate reflected in Resolutions 52/200 and 53/185, for the First Global Assessment of the variability will to develop a strategy within the framework 1997–98 El Niño Event carried out at continue as a of the International Decade for Natural Guayaquil, Ecuador, the First Global yoke of natural Disaster Reduction (IDNDR) to prevent, Assessment (International Seminar on the disasters mitigate and rehabilitate the damage caused 1997-98 El Niño Event: Evaluation and burdening by the El Niño phenomenon. An Projections) was co-sponsored by the especially the intergovernmental UN Task Force on Government of Ecuador, the UN Task developing world. El Niño was established. Force on El Niño, and the Permanent This Retrospective reviews existing Commission for the South Pacific. knowledge and capabilities for monitoring The term El Niño, although not yet and forecasting El Niño/Southern rigorously defined, is associated with a Oscillation (ENSO) in order to establish a major warming of the surface layers of the sound basis for new strategies to mitigate central and eastern equatorial Pacific the negative impacts and capitalize on Ocean. An El Niño event occurs when potential positive benefits. Although the warm water flows eastward from the warm focus of the Retrospective is on the pool of the western tropical Pacific Ocean scientific and technical aspects of and there is a reduction in upwelling of monitoring and prediction of ENSO, it also cold water in the eastern equatorial Pacific identifies productive linkages to Ocean and along the Pacific coast of the multidisciplinary impact assessment studies Americas. Once initiated, an El Niño event necessary to support preparedness and typically lasts about a year, although management of the economic, climate anomalies in some parts of the environmental, developmental and societal globe may persist longer. dimensions of natural disaster reduction. During mid-1997, sea surface Within the UN Task Force on El Niño, temperatures across the central and eastern the World Meteorological Organization equatorial Pacific Ocean became (WMO) took a lead role to coordinate the significantly warmer than normal and a gathering of scientific and technical major El Niño event developed. Deep information about the 1997–98 El Niño event tropical atmospheric convection shifted and its primary impacts. In particular, during eastward from the region of Asia and the the course of the event WMO prepared a western Pacific Ocean and, as a series of publications, called El Niño consequence, unusually heavy rainfall Updates, providing information on the occurred over many parts of the normally current status; these were issued to national dry Pacific coastal regions of South 7 America. As the deep tropical atmospheric • Waterlogging of fields as a result of The inseparability convection shifted eastward subsiding dry recurring periods of rain reduced of El Niño from air and reduced rainfall became the agricultural production in many parts. broader issues of prevailing conditions over the western • In other regions, the absence of the climate Pacific Ocean and parts of South East Asia. usual seasonal storms and rains led to variability and The El Niño event ended in mid-May 1998 prolonged dry spells, loss of crops and change when sea surface temperatures rapidly reduction in water supplies. underscores the returned to normal (and then somewhat • Outbreaks and spread of wildfires were importance of a cooler than normal). also more frequent during extended dry concerted effort The dramatic changes in atmospheric periods. on the part of circulation across the Pacific Ocean • Increased incidence of disease was an governments associated with El Niño are one extreme of outcome of prolonged disruption to and non- what is referred to as the Southern weather and rainfall patterns over many governmental Oscillation, and the overall coupled ocean- months that resulted in contamination organizations to atmosphere processes are referred to as of water supplies or a more favourable continue research El Niño/Southern Oscillation, or ENSO. The environment for disease-carrying insect into climate other extreme of the Southern Oscillation is vectors. variability, to associated with colder than normal waters As an outcome of several decades of improve forecast over the eastern equatorial Pacific Ocean climate research and observing system skill, and to and a piling up of warm waters in the west, development there is now an extensive body develop referred to as a La Niña event. These of knowledge about the climate system and appropriate extremes are often referred to as the warm a capacity to monitor in real-time aspects of policies for phase and cold phase of ENSO, indicating its variability. The ability to watch the mitigating the that they appear to be part of a single 1997–98 El Niño develop was made possible impacts of phenomenon. by the systems that had been established climate extremes. The eastward movement of deep over the previous decade. There is also a atmospheric tropical convection during developing capability to predict climate 1997–98 also triggered a shift in seasonal anomalies up to several seasons in advance patterns of weather systems over many over some parts of the globe, for some subtropical and mid-latitude parts of the seasons of the year. globe. The abnormal location of Pacific Science and technology provide tools Ocean convection changed the source of that are essential to build better community tropical heating of the atmosphere and, preparedness against the hazards of climate through dynamic processes in the extremes and provide early warning of atmosphere (called “teleconnections”), events. For example, historical climate data, affected the locations and mobility of and understanding of the processes of the subtropical and mid-latitude cyclones and climate system, provide the basis for anticyclones. Some regions received more assessment of climate risk and vulnerability seasonal storms and rainfall than normal to natural disasters. Monitoring and while other regions received less than prediction of the climate system provide normal seasonal rainfall, with attendant early warning for implementation of rapid increased potential for drought. response in the event of climate extremes, The shifting of seasonal weather including those associated with El Niño. patterns that was triggered by the 1997–98 El Niño event produced climate extremes over The 1997–98 El Niño event clearly many parts of the globe, often with major demonstrated useful and developing socio-economic impacts. capabilities in the areas of climate • More than 24 000 lives were lost monitoring and prediction, but the gaps because of high winds, floods or storm in knowledge of the climate system, the tides that occurred during intense gaps in monitoring coverage, and the storms. early stage of development of climate • More than 110 million people were forecasting models indicate the affected and more than six million potential for significant improvement. people were displaced as community infrastructures, including housing, food The global climate changes through a storage, transport and communications, combination of natural and anthropogenic were lost during storms. influences. Some of these changes appear • The direct value of losses exceeded through shifts in patterns of weather and US $34 billion. regional climate, and especially in changing 8 Executive Summary

The patterns of risk associated with extreme climate information services. Accessible mitigation of the events, such as El Niño. climate information allows community negative impacts involvement at all levels in disaster of the El Niño preparedness and promotes good design phenomenon and The Climate Agenda and an appropriate pattern of development. other extremes of climate The Climate Agenda is the existing An essential prerequisite to building variability will organizational framework for coordinating preparedness for climate risk is a require ongoing international climate activities and for national commitment to the international developing regional and global climate development of publicly accessible support for the infrastructures. The Climate Agenda national climate archives. Climate Agenda, provides the scientific and technical with special capability necessary to support a global, Despite recent advances in global emphasis on multidisciplinary approach to mitigating the climate monitoring there are still serious technology negative impacts of climate extremes and data gaps. WMO, the IOC of UNESCO, transfer, capacity for the promotion of sustainable UNEP and ICSU are cooperating in building and development. planning for a Global Climate Observing meeting the needs Co-sponsors of the Climate Agenda are System (GCOS) to provide comprehensive of developing relevant agencies of the United Nations led meteorological, oceanographic and related countries. by WMO, and non-governmental bodies led environmental data necessary for detecting by the ICSU. The four pillars of the Climate climate change, for climate research, climate Agenda for addressing global climate issues forecasting and operational services. GCOS are: includes new observing instruments and • Dedicated observations of the climate systems that have been proven through system; research, such as the Tropical Atmosphere • New frontiers for climate science and Ocean (TAO) array of moored buoys across prediction; the equatorial Pacific Ocean and the • Studies of climate impact assessments altimeter of the TOPEX/Poseidon satellite. and response strategies to reduce GCOS will build upon the long established vulnerability; and World Weather Watch system of WMO. • Climate services for sustainable development. Research

Observations Many benefits from developments in climate monitoring and prediction were International Information about the climate of a locality demonstrated during the 1997–98 El Niño commitment to is fundamental to understanding and event but there are still many unknowns the development developing preparedness against the about the phenomenon and the associated and operation of hazards associated with climate extremes. teleconnections that affect global weather Global Climate Local climate records, in computer- patterns. The Climate Variability and Observing System compatible format, are the means for Predictability (CLIVAR) project has been is critical. identifying and assessing the potential established within the framework of the dangers of local climate extremes. It is for World Climate Research Programme. this reason that WMO, supported by the Amongst its aim is the extension of the Human donations of Member countries to the capability for climate prediction to larger impact on the Voluntary Cooperation Programme, is geographic regions and longer timescales for climate system, assisting developing countries to preserve the ultimate benefit of the world’s particularly on early manuscript climate records through the communities. the pattern of Data Rescue (DARE) project. WMO is also weather and assisting the national Meteorological and climate extremes, Hydrological Services of developing Vulnerability and disaster is a fundamental countries to establish computer-based issue to be climate archives using a standardized data preparedness resolved through management package. The CLICOM the CLIVAR (Climate Computing) project has been the Data on the full extent of climate anomalies research project. means through which many developing during the 1997–98 El Niño event are not countries have made the necessary available in many parts of the globe. For transition towards modern computer-based many countries it is only possible to 9 provide general estimates of the type and Services extent of impacts from weather and climate extremes, including loss of life, destruction WMO has initiated the Climate Information Effective climate and damage to housing and infrastructure, and Prediction Services (CLIPS) project to information and and the extent of disease during the assist national Meteorological and prediction aftermath. Communities and economies Hydrological Services in delivering an services require were affected differently and UNEP is enhanced range of operational climate an appropriate taking the lead, within the framework of services, including prediction on seasonal to framework where the Climate Agenda, for coordinating and interannual timescales. CLIPS will strengthen users recognize arranging support for impact assessment the interface between national providers and what is possible to studies. An initial study of the impacts of sectoral users so that climate services are predict, where the the 1997–98 El Niño event, supported delivered in a framework that assists the providers through the United Nations Fund for various decision-making processes. recognize what is International Partnerships and with a The development of regional climate essential to be duration of 19 months, will cover impacts centres, within the CLIPS framework, will predicted, and in 15 countries. establish a focus for cooperation in data where the management, monitoring and prediction, scientific The UN Task Force on El Niño was and will assist technology transfer and information flow created within the framework of the capacity building. is in a form that International Decade for Natural can be readily Disaster Reduction (IDNDR) and assimilated in demonstrated the immense value of National climate programmes decision-making. multidisciplinary approaches in the global disaster reduction effort. The Climate-related natural disasters, such as successor arrangements to IDNDR will those linked to El Niño, are a major require continuation of an inter- consideration by governments in the overall agency mechanism for concerted action scope of community protection and well- on El Niño. being. The management of climate risk is multidimensional and involves agencies with To be fully effective, impact economic, environmental, social and assessment studies at the national and developmental objectives. Many regional levels should be multidisciplinary governments have established a formal and policies for the mitigation of the national climate programme as a framework impacts of climate extremes should be for coordination and to ensure that, through integrated into sustainable development the national Meteorological and strategies. Multidisciplinary risk assessment Hydrological Service, firstly, the appropriate provides the basis for an effective scientific and technical infrastructures are preparedness and early warning system for adequately supported, and secondly, to the mitigation of natural disasters associated ensure that information and prediction with climate extremes, such as El Niño services are accessible to policy and events. decision makers for planning, early warning and better management across a range of sectors, including natural disaster reduction.

10 Introduction

Over the closing decades of the twentieth causing scarcity of food in Egypt and century the term El Niño has become indicating drought in Ethiopia. Famine synonymous with social, economic and accompanied the drought in China and environmental crises in many parts of the India: nine to 13 million people are globe. El Niño signals a major departure estimated to have died in northern China; from the normal climate patterns, over eight million deaths in India could be particularly those affecting tropical regions. attributed to famine and outbreaks of For some countries an El Niño event is disease. There were also very heavy rains typically associated with abnormal heat and and flooding along the Pacific Coast of drought, whereas for others it is persisting South America and a rare hurricane struck rain and devastating flooding. Loss of crops, Tahiti in February 1878. stock and food reserves, destruction of The failure of the 1877 monsoon rains shelter and community infrastructure, and over India prompted Henry Blanford (first outbreaks of disease are some of the impacts Imperial Meteorological Reporter to the that impose hardship and set back social and Government of India) to seek additional economic development. meteorological observations in Australia and However, drought, flood and other across the Indian Ocean. It was this manifestations of climate extremes are not scientific initiative that laid the groundwork recent phenomena. What is new is the for gathering essential meteorological data knowledge to link a pattern of for study of the Indian summer monsoon, simultaneously occurring climate extremes and later understanding of the seasonal within a global framework. This knowledge, atmospheric circulation of the Indian and with a developing capability to predict a Pacific Ocean basins. These meteorological season or more in advance in some regions, data provide the basis for linking social and is providing new tools for preparedness and economic hardship with a recurring climate early warning to reduce the risks and better pattern. Figure 1 manage the impacts of climate extremes, Drought again affected many parts of Regions with significant and to underpin strategies for sustainable the globe in 1888. Particularly severe rainfall deficiency during 1877. development. impacts were reported in India, Ethiopia, (Allen et al., 1996) The suggestion of a possible linkage northern Brazil and Australia. Famine, between climate extremes in different parts accompanied by disease, was experienced + of the globe has its origins in reviews of the in India and Ethiopia and led to many > 45 31-45 calamitous events of 1877 and 1878. During deaths. More than one and a half million 16-30 this period drought was widespread across people perished in India and it was 0-15 northern China, India, southern Africa, estimated that more than one third of the 0-15 16-30 northeastern Brazil, Australia, and the population of Ethiopia was lost. 31-45 islands of the South Pacific. Also, the Nile The pattern of drought has recurred <45 River was very low during the period, many times in the latter part of the - 60°N nineteenth and early twentieth centuries. Systematic studies by scientists using data <10 from the expanding network of <30 30°N <10 meteorological observing stations have provided a coherent picture of the seasonal EQ >35 patterns of climate, and of the major <10 changes that occurred during periods of 30°S >10 <20 major drought. During the early decades of the 60°S twentieth century scientists began to 90°E 180° 90°W 11 understand the basis of the global scale of El Niño was originally a phenomenon meteorological processes. Studies led by Sir of the coastal waters of northwestern South Gilbert Walker, the second Director-General America. The cold northward flowing of British Observatories in India, Peruvian current, also known as the demonstrated that a correlation existed Humbolt current, has a marked seasonal between surface air pressures over the cycle in tropical latitudes. The prevailing Indo-Australian region and those over the offshore south-easterly winds cause eastern South Pacific Ocean and South upwelling in the waters of the coastal zone America. In particular, failures of the Indian for most of the year and bring high nutrient summer monsoon were associated with levels in the surface layers. The coastal higher than normal surface air pressure waters support abundant fish stocks that in extending from India to Australia; at the early times provided an important food same time surface air pressure over the source for the coastal communities that have eastern South Pacific Ocean was generally traditionally benefited from the rich offshore lower than normal. Strong monsoons were marine resources. Since the 1950s significant associated with lower than normal surface export industries have become established air pressure over the tropical region from to commercially exploit the seemingly India to Australia and higher than normal abundant fish stocks. surface air pressure over the eastern South El Niño was originally the name given Pacific Ocean. Walker, in 1909, referred to by local fishermen to the annual appearance the relationship between monsoon intensity of a warm southward flowing current in the and the cross-Pacific Ocean surface pressure surface waters off coastal Ecuador and Peru patterns as the Southern Oscillation. in the Southern Hemisphere summer. The Walker also noted that the monsoons first appearance of the warm waters was at of India and South East Asia have their about the time of Christmas, the time origins in warm moist air generated over associated by Christians with the infant Jesus the northern Indian and Pacific Oceans. A (Spanish: El Niño — the boy child). The major overturning atmospheric circulation recurring annual warming of the coastal along the equator, with easterly Trade waters has been observed for centuries by Winds feeding rising air in the rain- people along this coastline and is associated producing clouds over Asia and returning with a seasonal dispersal of fish stocks and westward at high levels to sink over the low catches. eastern Pacific Ocean, is known as the The coastal communities also Walker circulation in recognition of this recognized that in some years the cold work. Periods of drought over India and nutrient-rich waters failed to return during South East Asia are associated with the following year, giving a poor fish harvest weakening of the Walker circulation. and with disastrous consequences on local It was not until the 1960s that a linkage food stocks and community welfare. It is between the Southern Oscillation and the El now recognized that these periods also Niño began to be formed. The International probably involved abnormal and prolonged Geophysical Year (IGY) of 1957–58 was a warming of the eastern equatorial Pacific stimulus for international cooperation in Ocean. These prolonged periods of Figure 2 observations and multidisciplinary studies. In abnormal warming are now generally Major coastal upwelling particular, it was a stimulus for studying the referred to as El Niño events. However, it regions of the world and interactions between the atmosphere and the was not until the mid-1960s that the El Niño the sea level atmospheric pressure systems that oceans that led to knowledge of the phenomenon was recognized as more than influence them. important role of the oceans in modifying local significance. Several scientists drew (Glantz et al., 1987) seasonal climate. attention to potential linkages between the El Niño of the eastern equatorial Pacific Atlantic Ocean Ocean and the Southern Oscillation that California current North Asia America affected weather patterns across the tropical Europe Pacific Ocean. By the late 1960s Peru was a major Canary current Africa Pacific Ocean fishing nation supplying international

Peru current markets with fish meal, an animal feed South Benguela America current Somali current supplement. The 1972–73 El Niño was Australia Indian Ocean devastating for the Peruvian fishing industry and the Peruvian economy. The failure of the anchovy fishery to return to the high 12 Introduction pre-El Niño levels of productivity became a TOGA observing system, which had been focus of international concern during the completed by 1995. mid-1970s. Studies of the 1972–73 El Niño One legacy of TOGA is the new event drew attention to the major changes to monitoring systems that include satellite- the surface waters of the central and eastern borne instruments for remote sensing over Pacific Ocean and their linkage to data-sparse regions of the globe, moored contemporaneous climate anomalies, ocean buoys and other instruments for direct particularly droughts or floods, in different observations of the climate system. For the parts of the globe. first time ocean data are rapidly available. Linkages between the El Niño High-speed computers now analyse the phenomenon and global climate anomalies continuous data stream from around the were recognized by some scientists at the globe and have provided enhanced time of the 1982–83 event. By early 1983 awareness of the state of the climate system, major drought periods were in progress in including a capability for early warning. The many regions around the globe, and other TAO array of moored ocean buoys across regions had experienced devastating the equatorial Pacific Ocean was crucial for floods. The Indian summer monsoon of providing timely information during the 1982 was late, erratic and withdrew early evolution of the 1997–98 event, particularly causing reduced summer crop yields; the changing subsurface temperatures of the drought in the northern China plains led to ocean. decreased grain yield; a severe drought The 1982–83 event and studies of affected the dry season crop of Indonesia; subsequent events have established that El Australia experienced one of the worst Niño events are associated with negative droughts of the century and out-of-control values of the Southern Oscillation (higher wildfires claimed 75 lives; drought in than normal surface pressure over the Indo- southern Africa threatened famine and Australian region and lower than normal disease; and drought in Northeast Brazil surface pressure over the eastern South affected more than 14 million people. Pacific Ocean). The studies have also There was severe flooding on the Pacific identified that positive values of the Southern Coast of South America and in Ecuador 600 Oscillation, when there is a stronger than lives were lost in landslides. Six tropical normal summer monsoon over India and cyclones affected French Polynesia leaving Asia, are associated with colder than normal 250 000 homeless in Tahiti. The overall sea surface temperatures over the eastern losses associated with the 1982–83 El Niño Pacific Ocean, called La Niña events. event were estimated to exceed 1 500 The El Niño/Southern Oscillation, or deaths and US $13 billion. ENSO, is now a well-developed model to The World Climate Programme (WCP) link the fluctuations in sea surface was established following the first World temperature over the eastern equatorial Climate Conference in 1979. The World Pacific Ocean with regional climate extremes Climate Research Programme (WCRP), jointly in many parts of the globe. The recurring sponsored by the World Meteorological pattern of droughts and floods observed Organization (WMO) and the International during 1877–78, 1888–89, 1972–73 and Council for Science (ICSU), was established 1982–83 broadly describe the climate to coordinate and promote international extremes associated with a warm ENSO (or climate research. A major initiative of the El Niño) event. A cold ENSO event, or La WCRP has been the Tropical Ocean Global Niña (i.e. strong upwelling and colder than Atmosphere (TOGA) project to describe, normal sea surface temperatures over the model and predict variability of the coupled eastern Pacific Ocean) is associated with a ocean-atmosphere system on timescales of strong Asian summer monsoon. However, months to years. The decade-long project, in the impacts of the cold ENSO, while in planning from 1982, commenced in 1985 and general terms reflecting the reverse efforts were directed at describing the characteristics to the warm ENSO, are not tropical oceans and global atmosphere as a strictly the opposite. time-dependent system, identifying the Recent studies of El Niño events that mechanisms and processes underlying the have occurred over the past century have variability, and initiating modelling and also identified a characteristic pattern of observing systems for prediction over the development, evolution and decay linked to relevant timescales. The 1997–98 El Niño the seasonal cycle. The evolution of an El event was the first to be observed by the full Niño event is generally first detected in the 13 second quarter of the year (May–June) as an Permanent Commission for the South unusual warming of the surface waters of Pacific, held in Guayaquil, Ecuador, the eastern equatorial Pacific Ocean. Within 9–13 November 1998 (International a few months an El Niño event is clearly Seminar on the 1997–98 El Niño Event: observed as unusually warm water (a warm Evaluation and Projections), other national anomaly) that extends eastward from the studies and from the scientific research central equatorial Pacific Ocean and spreads community. The Retrospective has been along the South American coast. The warm divided as follows: anomaly tends to decline later in the year Part I — The climate system and generally dissipates early in the following year. This section reviews the seasonal cycle of It is now clear that of the climate- climate and describes El Niño/Southern related disasters that strike communities Oscillation (ENSO) as an outcome of the many, particularly those in tropical and coupled ocean-atmosphere system. subtropical regions, are associated with Part II — The 1997–98 El Niño event ENSO. This knowledge leads to a degree of predictability because once an event (either This section reviews the development and warm or cold) is observed to have evolution of the 1997–98 El Niño event and commenced some typical impacts are likely the global pattern of climate extremes. This to occur. However, no two events are section also examines many of the regional identical and prediction skill of the onset of climate anomalies and provides information an event well in advance is still developing. on the extent of human impacts and losses Furthermore, ENSO is only one factor, albeit on communities in the regions covered. an important one, contributing to regional Part III — The way ahead climate anomalies. The development of computer models of the coupled ocean- This section examines how science and atmosphere system is heralding a powerful technology, supporting climate information new tool for prediction of climate a season and prediction services, can be used in the or more in advance. Experimental seasonal service of community preparedness, early model-based forecasts of temperature and warning, and management of climate risk in precipitation are being routinely produced society, particularly through the integration by the European Centre for Medium-range of science and technology with natural Weather Prediction (ECMWF) and the disaster reduction and sustainable International Research Institute for Climate development planning and policies. Prediction (IRI). Appendix The various national and international responses to climate disasters around the This section reviews relevant knowledge and globe during the 1997–98 El Niño event processes of the climate system, including point to a need for better application of the Asian Monsoon and El Niño. existing climate knowledge and prediction capability, as well as further development of The current levels of knowledge about the reliability of these capabilities. National ENSO and prediction skill provide a and regional plans within a framework of foundation to better cope with climate risk. international cooperation are essential to Furthermore, integration of climate prepare for and cope with climate risk. knowledge and prediction skills into multi- Months Northern Southern Clearly, knowledge is being applied to disciplinary preparedness and emergency Hemisphere Hemisphere beneficial effect because the magnitude of response strategies will lead to saving of Dec deaths from famine and disease reported lives and development of communities that Jan Winter Summer during the nineteenth century has are more resilient to the impacts of climate Feb

significantly been reduced in the twentieth extremes, whether or not linked to ENSO. Mar century. But the level of loss and suffering Since ENSO produces global-scale Apr Spring Autumn from ENSO (warm and cold) events and effects throughout a calendar year, it is May their impacts are still unacceptably high and necessary to explain events in both Jun can be further reduced. hemispheres in all seasons. The box Jul Summer Winter The Retrospective draws on a series of opposite can be used as a guide when Aug presentations at the First Global Assessment determining the months of a year applicable of the 1997–98 El Niño Event, co-sponsored to the season on a given hemisphere. Sep Oct Autumn Spring by the Government of Ecuador, the United Nov Nations Task Force on El Niño and the 14 Part I

The Climate System

The seasonal cycle of climate important factor in establishing the large- scale ocean circulations and it is the oceans The annual north-south seasonal movement that are the major contributor of latent in the latitude band of maximum solar energy (water vapour) and heat to the heating imposes an annual cycle on local atmosphere. The overall motions of both climates and is the dominant cause of systems transport heat from the tropics to variability of the global climate system. the polar regions. However, the distribution of land and ocean The El Niño/Southern Oscillation, or between the hemispheres makes an ENSO, is an outcome of the dynamic important contribution to the seasonally coupling between the oceans and the changing characteristics of regional tropical atmosphere. The El Niño has its focus in the climates. Land surfaces heat up and cool surface waters of the Pacific Ocean and the down more than the neighbouring oceans life cycle of each event is different, but during the summer and winter. As a generally about a year in duration. The consequence, seasonal wind circulations, or relatively long life cycle of the El Niño and monsoons, are established. The Asian the persistent anomalous forcing of the monsoon, the impacts of monsoon winds on circulation of the atmosphere mean that, in ocean surface temperatures, and the many parts of the globe, El Niño is the most interaction between seasonal circulations in important contributor to atmospheric the tropics and the mid-latitudes are each variability after the annual cycle of solar important in the context of the year-to-year heating. variability of climate. The following sections describe the In addition to external forcing by solar El Niño phenomenon of the equatorial radiation and seasonally changing land Pacific Ocean, the Southern Oscillation that surface characteristics, the climate system is a major factor in the interannual variability has variability in response to internal of the cross-Pacific atmospheric circulation, processes, particularly those relating to and the coupled ocean-atmosphere El energy feedback within the oceans and Niño/Southern Oscillation system (ENSO). atmosphere. The dynamics of these fluids The variability of the climate system and their boundary interactions are associated with ENSO is influenced by and complex. As a consequence, in the acts upon the broader range of climate atmosphere scales of motion vary from the processes. As background, the Appendix seasonal planetary waves of the upper contains brief descriptions of some relevant atmosphere to turbulent eddies related to climate processes and these add to the wind gusts at the surface. The cyclones and description of ENSO and its components that anticyclones that make up the weather follows. systems and the buoyant convection of local storms also have their place in the variability of the climate system. For their part, the The El Niño oceans have scales of motion that range downward from ongoing inter-basin A pattern of abnormal warming of the exchanges and basin-wide mid-latitude surface coastal waters off Ecuador, Peru and gyres. Chile has become known as El Niño. Each The exchange of heat, moisture and event has typical characteristics, but each is momentum between the ocean and the unique in its actual time of onset, rate of atmosphere provides the linkage between development and intensity. The local the two fluid systems. Wind stress is an warming is a manifestation of changes in the 15 upper ocean layers and is linked to autumn and early winter months. During the Figure I.1 Monthly mean sea processes extending across the equatorial Southern Hemisphere winter and early surface temperatures (left Pacific Ocean. El Niño affects more than the spring months (see Figure I.2b), the Trade panel) and anomalies Pacific Coast of South America. This section Winds are relatively strong and induce (right panel) across the will focus on the ocean surface layers of the upwelling and advection of deep water that Pacific Ocean just along the equator (2°N to 2°S) equatorial Pacific Ocean and describe how is transported westward as a tongue of cold for the period 1986 to prolonged abnormal warming observed in surface water along the equator into the 1998. There is a marked the near-shore waters of South America is central equatorial Pacific Ocean. annual cycle of sea surface temperature over but part of a more geographically extensive In some years the onset of cooling in the eastern equatorial set of ocean processes. the eastern Pacific Ocean appears to be late Pacific Ocean (left), There is a pronounced annual cycle of and relatively weak. Instead of coastal and particularly near the South American coast. warming and cooling over the eastern equatorial upwelling there is a transport of When the annual cycle is equatorial Pacific Ocean. The temporal warm water from the west. During these removed (right) the characteristics of the warming and cooling periods, as uniformly warm water spreads periods of anomalous warming and cooling are along the equator can be seen in longitude- eastward across the equatorial Pacific identified. Warm and time section (or Hovmöller diagram) of Ocean, the usual (though seasonally cold periods are Figure I.1. The left panel represents the varying) gradient of sea surface temperature identified as El Niño and change with time (vertical axis) of actual sea across the equatorial Pacific Ocean is La Niña events respectively if they are surface temperature across the longitudes of reduced and may even disappear. A sufficiently intense and the equatorial Pacific Ocean (horizontal characteristic of El Niño events is the long lived to meet axis). The annual appearance of warming off abnormally warm sea surface temperatures definitional criteria. (NOAA/PMEL (TAO the coast of Ecuador (the right margin of the that appear and persist for many months Project), USA) panel) occurs late in the year (about the across the central and eastern equatorial time of Christmas, or the Christ Child — El Pacific Ocean. Niño). The magnitude of the departure of sea The maximum of sea surface surface temperatures from the normal temperature usually is reached in late seasonal cycle during warming and cooling summer of the Southern Hemisphere and is events can be seen in the anomalies of followed by strong cooling during the temperature in the right panel of Figure I.1.

A Definition of El Niño The term El Niño has evolved in its meaning over the In this Retrospective, 1997–98 is recognized as an years, leading to some confusion in its use and El Niño event because of the strong warm anomaly of application. Because the El Niño phenomenon has sea surface temperature across the central and eastern achieved strong public recognition through its equatorial Pacific Ocean that first made its appearance association with interannual climate variability around in April/May of 1997 and became more extensive and the world, there is a need to provide a more definitive persisted well into 1998. The region of sea surface meaning to its use. There have been a number of temperature anomaly associated with the event attempts to define El Niño, both qualitatively and reached maximum extent and strongest intensity in quantitatively, but none has achieved universal late 1997 before collapsing dramatically, especially in recognition. A quantitative definition of El Niño the region of the central equatorial Pacific Ocean proposed by the Climate Variability and Predictability during mid-1998. Reference to onset, maturity and (CLIVAR) project (Trenberth, 1997) is based on cessation generally apply to characteristics of the accepted concepts and designed to be consistent with event. previously recognized events. The criteria require that The extent of the abnormally warm sea surface the five-month running mean of temperature anomaly temperature and the maximum intensity of the in the region of the central equatorial Pacific Ocean, anomaly are what characterize the 1997–98 El Niño referred to as NIÑO 3.4 (5°N to 5°S and 170°W to event as possibly the strongest warming event of the 120°W), exceeds a threshold value of 0.4°C for a Pacific Ocean of the twentieth century, and at least of minimum of six months. This definition takes account comparable magnitude to the 1982–83 event. of what was considered to be the key region for Until a universally agreed definition of an El Niño coupled ocean-atmosphere interactions. However, event emerges there will continue to be debate as to the Trenberth acknowledged that the definition is not status of a number of the weaker warming episodes that completely satisfactory and is still evolving with have been observed, including some of those for the developing understanding of the characteristics of the period 1986–96. There will also be debate as to whether phenomenon. The recommendation is that any use of the period 1991–95 comprised a sequence of discrete the term should state which definition is being used. events, or was one long event.

16 I. The Climate System

the sequence and will be discussed separately in Part II. There are also periods of abnormal cooling across the eastern and central

12 equatorial Pacific Ocean, indicating abnormally strong upwelling and westward 10 advection of cold water. A strong cold event, or La Niña, occurred in 1988 and there was the onset of another cooling event in 1998. A weak cold event occurred in 1995. The timing of the anomalous warming is important in the ocean forcing on the dynamics of the atmosphere and the impact

1998 1996 1994 1992 1990 1988 1986 on climate. For example, although the 1987

140°E 160°E 180° 160°W 140°W 120°W 100°W 140°E 160°E 180° 160°W 140°W 120°W 100°W event achieved a warm anomaly of about (°C) (°C) 2°C, the 28°C isotherm did not extend to the 20 22 24 26 28 30 -4 -2 0 2 4 eastern equatorial Pacific Ocean (see the left This panel shows departure of sea surface panel of Figure I.1). During late 1991 and temperature from the long-term seasonal early 1992 the 28°C isotherm extended much average with time at the equator (vertical further eastward but the magnitude of axis) across the longitudes of the Pacific abnormal warming achieved was only about Ocean (horizontal axis). Several periods of the same magnitude as 1987. The 1987 warm warm anomaly (shaded yellow to red) can event was collapsing during the Southern be identified, but each with different Hemisphere summer of 1987–88 and the magnitude and characteristics. During 1987, event of 1993 is observed as a late onset of 1991 and 1994 the sea surface temperature the seasonal cycle of cooling. The warm over the central equatorial Pacific Ocean is events of 1991–92 and 1994–95 had abnormally warm and, although the developed early in the first year and magnitude of anomaly is small, warmer than persisted into the second year; they were at normal sea surface temperatures also extend their peak during the Southern Hemisphere to the eastern equatorial Pacific Ocean. summer. During 1993 the weak warm anomaly is The variations in timing and geographic confined mainly to the eastern equatorial patterns of anomalous warming identify the Pacific Ocean. The major event of 1997–98 is difficulty in establishing quantitative and very strong compared to the other events in universal definitions for El Niño and La Niña

TOGA – The Tropical Ocean Global Atmosphere Project The TOGA project was a decade-long (1985–94) (XBT — instruments dropped from ships that report activity of the World Climate Research Programme. temperature and salinity with depth during descent). TOGA was established through international The XBT are released by merchant vessels of the cooperation to encompass observations, empirical and WMO-coordinated Volunteer Observing Ships (VOS) theoretical studies, and modelling of ocean and project. atmosphere processes. Its objective was to better Data from the TAO array and the XBT observations understand processes linking the ocean and have provided new insights into the climatology and atmosphere circulations, particularly ENSO. An variability of the subsurface layers of the oceans, important process to be studied was forcing of large- particularly those of the equatorial Pacific Ocean. These scale atmospheric circulations by tropical ocean data have also been crucial for modelling and setting temperature anomalies such as El Niño, with a view to initial conditions for climate predictions. improved predictions on seasonal to interannual The CLIVAR project is the successor to TOGA and timescales. will adapt and enhance existing atmosphere and ocean The focus of new TOGA observations was the observing systems. A network of moored buoys similar Tropical Atmosphere Ocean (TAO) array of moored to the TAO array is being established across the buoys across the equatorial Pacific Ocean, completed equatorial Atlantic Ocean. The CLIVAR scientific plan in 1994. The moored buoys provide surface includes a network of moored buoys across the meteorological and subsurface ocean observations. equatorial Indian Ocean but implementation in this Subsurface ocean observations were also expanded by latter region requires a commitment of funds by increased numbers of expendable bathythermograph governments.

17 (a) equatorial Pacific Ocean behaves as a 30°N delayed oscillator and its evolution on 20°N interannual timescales is governed by the

10°N interplay between large-scale equatorial ocean wave processes and ocean- EQ atmosphere feedbacks. An important 10°S precursor to an El Niño event is the build-up 20°S of heat content in the surface layers of the

30°S western Pacific Ocean. Persisting Trade Winds are important for generating 100°E 120°E 140°E 160°E 180° 160°W 140°W 120°W 100°W80°W60°W m/s downwelling Rossby waves that travel 23456 7 8 9 10 11 westwards and produce a deepening of the (b) thermocline in the western Pacific Ocean. 30°N An expansion in the overall volume of warm 20°N surface water in the western Pacific Ocean is

10°N associated with the deepening thermocline. At the western side of the Pacific Ocean, EQ reflection off the equator of ocean Rossby 10°S waves can generate a Kelvin wave that

20°S propagates eastwards along the equator and generates downwelling and deepening of 30°S the thermocline in the east. 100°E 120°E 140°E 160°E 180 160°W 140°W 120°W 100°W80°W60°W m/s More recently attention has been 2345796 8 10 11 directed toward the role of tropical surface winds and their intraseasonal variations in Figure I.2 events. The anomalous warming and cooling the development of El Niño events. Known Average surface vector are part of the natural variability of the as Madden-Julian Oscillations, waves in the winds over the Pacific Ocean during a) surface layers of the equatorial Pacific atmosphere with a 30- to 60-day period Northern Hemisphere Ocean. The amount, the duration and the originate over the Indian Ocean and winter (December, geographical focus of warming and cooling propagate eastwards. Deep atmospheric January and February); are important in forcing anomalous convection and low-level westerly winds that and b) Southern Hemisphere winter (June, circulations in the atmosphere. occur with the Madden-Julian Oscillation are July and August). The The annual cycle of upper layer cooling usually only associated with relatively warm Peruvian current and of the central and eastern equatorial Pacific ocean surface waters (warmer than coastal upwelling off South America are Ocean waters is strongly influenced by the approximately 28°C to 29°C). The Madden- strongest during the seasonal waxing and waning of surface Julian Oscillation goes through a normal Southern Hemisphere winds. The Trade Winds north of the seasonal cycle with largest amplitude in the winter when the Trade Winds over the region equator are strongest during the Northern summer and autumn of the Southern are strongest. During the Hemisphere winter (see Figure I.2a) and Hemisphere (December through May). Northern Hemisphere strongest south of the equator during the The potential role of the Madden-Julian winter, when the Southern Hemisphere winter (see Figure Oscillation in the development of an El Niño intertropical convergence zone moves southward, I.2b). The easing of the southeast winds event can be identified through westerly there is an easing of the over the eastern equatorial Pacific Ocean wind bursts lasting several days to weeks Trade Winds and during the Northern Hemisphere winter is that are associated with these waves. A reversal of the current offshore from Ecuador very evident in Figure I.2a. Therefore, it is westerly wind burst over the western Pacific and northern Peru. not unexpected that the Peruvian current Ocean causes lateral drift of local eastward (NOAA/CDC, USA from bringing cold water towards the equator and directed wind-driven currents towards the NCEP/NCAR reanalysis, USA) the offshore upwelling both have an annual equator. The resulting convergence at the cycle and are at a peak during the Southern ocean surface induces downwelling and Hemisphere winter. downward pressure on the thermocline. The Despite the influence of seasonal winds overall effect of a westerly wind burst of the on the annual cycle of warming and cooling, Madden-Julian Oscillation is to generate a the periods of anomalous warming of the Kelvin wave that propagates eastward and surface layers of the central and eastern both deepens the thermocline and transports Pacific Ocean appear to have their origins in warm surface water. A Kelvin wave has a the ocean dynamics operating across the travel time of approximately two months for equatorial Pacific Ocean. its crossing of the equatorial Pacific Ocean. A prevailing theory of ENSO is that the All El Niño events since the 1950s have ocean and atmosphere coupling across the been associated with elevated levels of 18 I. The Climate System

mm 200 surface westerly wind forcing. However, Figure I.3 150 episodic wind forcing is not a sufficient Monthly sea level anomaly 100 for Santa Cruz, Ecuador condition for El Niño events to occur, since 50 (blue line) and Pohnpei such forcing is evident during non-El Niño 0 (red line), Federated States Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. 91 91 92 92 of Micronesia showing rise years as well. -50 in sea level (mm) in the -100 During an El Niño event the cross- east and fall of sea level in Pacific sea level setup relaxes as the Trade -150 the west during the El Niño Pohnpei -200 event of 1991–92. Winds weaken and warm water moves Santa Cruz -250 (University of Hawaii Sea eastward. The change in equatorial cross- Months Level Center) Pacific setup can be seen from the records 60°N of sea level, both in the western Pacific Ocean and the eastern Pacific Ocean. Figure 40°N I.3 shows the anomaly of sea level, 20°N corrected for atmospheric pressure, at Santa Cruz, Ecuador (lat 0.45°S, long 90.19°W) and EQ

Pohnpei, Federated States of Micronesia (lat 20°S 6.59°N, long 158.15°W) during the 1991–92 El Niño event. In the west (Pohnpei) the sea 40°S level initially fell about 20 cm and then 60°S 0° 60°E120°E 160 120°W60°W0° recovered while in the east (Santa Cruz) the °C sea level initially rose about 20 cm and then -3 -2 -101 2 3 returned to normal. These patterns are consistent with eastward flowing water identified the tropical cross-Pacific Figure I.4 across the Pacific Ocean as the El Niño atmospheric circulation that now is generally Pattern of sea surface temperature anomaly for event develops. referred to by his name. The Walker near the mature phase of The spatial extent of warmer than Circulation is a simple picture of the zonal an El Niño event (January normal sea surface temperature over the cross-Pacific airflow that takes account of 1992). The abnormal central and eastern Pacific Ocean during the neither seasonal variability nor year-to-year warming of this event is mainly in the equatorial mature phase of an El Niño event can be differences. However, the description central Pacific Ocean gauged from the anomaly pattern for captures the essential zonal atmospheric and the warming over the January 1992 (Figure I.4). For this particular overturning processes across the Pacific eastern equatorial Pacific Ocean is relatively weak. event the focus of warming is along an Ocean: (NOAA/NCEP, USA) equatorial band in the central Pacific Ocean, • Surface atmospheric pressure is higher but there are also warm anomalies in the over the east than the west; eastern Pacific Ocean and extending along • Surface Trade Winds blow from east to the coastal margins of tropical South and west and accumulate heat and moisture Central America. from the underlying tropical oceans; The pattern of warm sea surface • Deep tropical atmospheric convection temperature anomalies of Figure I.4 outlines clouds, with rising air and heavy the area of warm water that accumulates in rainfall, are experienced in the the surface layers of the central and eastern converging Trade Winds and monsoon equatorial Pacific Ocean and illustrates the over the western tropical Pacific Ocean extensive rearrangement of energy storage and Asia respectively; that occurs during an El Niño event. The • Westerly winds provide a return flow in relatively warm water across the central and the high atmosphere; and eastern equatorial Pacific Ocean becomes • Clear skies, subsiding air and relative available to act as a source of energy to the aridity are experienced over the eastern atmosphere. The unusual warming of the Pacific Ocean. eastern Pacific lasts for up to a year and Significant year-to-year variability in the represents a long period of anomalous tropical cross-Pacific surface atmospheric forcing of the atmospheric circulation by the pressure gradient and the Trade Winds are El Niño event. features of the Walker Circulation that have been known for nearly a century. Lack of data prevented detailed elaboration of the The Southern Oscillation variability but an early linkage was made between the cross-Pacific surface During the early decades of the twentieth atmospheric pressure gradient and the century, Sir Gilbert Walker, then Director- strength of the Asian summer monsoon. The General of British Observatories in India, year-to-year variability of the Walker 19 Figure I.5 80°N Walker Circulation). Similarly, during years Correlation between when surface pressure is higher than normal surface atmospheric 60°N over Asia and northern Australia it is lower pressure anomaly at 40°N L Darwin, Australia and L than normal over the central and eastern other available data 20°N Pacific Ocean (a weakening or reversal of points around the globe. EQ 20°S L the Walker Circulation). The analysis of (Trenberth, 1976) H

40°S Figure I.5 is based on annual mean data and

60°S confirms that the Southern Oscillation is

0° 60°E 120°E 180° 120°W60°W associated with interannual variability of the Figure I.6 climate system. Monthly and filtered (a) 4 For each phase of the Southern values of a) Southern 3 SOI Oscillation there is spatial variability in the Oscillation Index (SOI); 2 b) Darwin and Tahiti 1 anomaly pattern of climate indicators, 0 surface atmospheric particularly of rainfall, because of the -1 pressure anomaly (hPa); -2 day-to-day weather systems. The summer and c) standard deviation -3 monsoon of Asia tends to be weaker than of outgoing longwave -4 radiation (OLR) over the 80 82 84 86 88 90 92 94 96 98 normal when the Walker Circulation is weak (b)4 central equatorial Pacific 3 Tahiti and, on average, there will be fewer rain Ocean (160°E–160°W). Darwin 2 events and longer dry spells. Historical (NOAA/NCEP, USA) 1 climate records demonstrate, however, that 0 -1 even during a weak monsoon some areas

Anomaly (hPa)-2 Standard deviation will receive average or above average -3 -4 rainfall as a result of a few very active 80 82 84 86 88 90 92 94 96 98 (c) 4 weather disturbances and their generally 3 OLR limited spatial impact. 2 The Southern Oscillation represents a 1 0 departure from long-term average of the -1 seasonal circulations of the tropics. By -2 Standard deviation -3 averaging over periods from a month to a -4 80 82 84 86 88 90 92 94 96 98 season the fluctuations of the day-to-day weather systems and the intraseasonal Circulation and the associated “see-saw” of Madden-Julian Oscillation are smoothed out the cross-Pacific surface atmospheric and the signal represents the climate pressure gradient are known as the Southern anomaly. Oscillation. By convention, the strength of the More recent studies based on long Southern Oscillation is now measured by the historical records have quantified the large- normalized difference in surface scale spatial coherence of seasonal and atmospheric pressure anomaly between longer-term surface pressure anomalies. An Tahiti and Darwin. This is the Southern example is the correlation between the Oscillation Index, or SOI*. Negative values annual mean surface atmospheric pressure of the SOI represent a weak cross-Pacific anomaly at Darwin, Australia with the surface pressure gradient, or even a reversal, annual mean surface atmospheric pressure while a positive SOI is associated with a anomaly at all other available data locations strong cross-Pacific pressure gradient and (Figure I.5). The extensive area of positive Walker Circulation. correlation confirms that surface The series of graphs in Figure I.6 atmospheric pressure values over the region underscores the overall coherence of the from south Asia to northern Australia tend to year-to-year variability of climate parameters vary in unison. The analysis also identifies a across the equatorial Pacific Ocean. The out- large area over the central and eastern of-phase behaviour of surface atmospheric Pacific Ocean where surface atmospheric pressure anomaly at Tahiti and Darwin *SOI: There are slight differences in the value of pressure values rise and fall in unison but reflects the “see-saw” behaviour and SOI calculated at some out of phase with those of south Asia and contributes to the variation of the Southern climate centres because northern Australia. Oscillation Index. The standard deviation of of the period of years used to calculate the In those years when surface outgoing longwave radiation (OLR) reflects long-term mean and atmospheric pressure is lower than normal the waxing and waning of tropical standard deviation. Also over Asia and northern Australia it tends to convection over the equatorial central the Troup SOI used in some countries is be higher than normal over the central and Pacific Ocean averaged between 160°E and 10*SOI. eastern Pacific Ocean (a strengthening of the 160°W. OLR values are below normal 20 I. The Climate System

(negative values of standard deviation in (a) Normal conditions Figure I.7 Figure I.6) indicating more deep convection Stylized graphics of the Convective loop ocean surface layer and in the central Pacific Ocean when the SOI is atmosphere across the negative and the Walker Circulation is equatorial Pacific Ocean weaker. during a) normal conditions; and b) an Equator El Niño event. During normal conditions there is Ocean-atmosphere a well-developed Walker Circulation (the coupling — ENSO ”Convective Loop”) in the Thermocline atmosphere and the There is a synergy in the behaviour of the 120°E 80°W thermocline in the ocean ocean and atmosphere across the equatorial surface layer slopes (b) El Niño conditions upward toward the east. Pacific Ocean, particularly the strong During an El Niño event coherence between El Niño events and the focus of deep Increased atmospheric convection negative phases of the SOI. This has led to convenction has shifted eastwards, the usage of the term El Niño/Southern Walker Circulation has lost its coherent structure, Oscillation, or ENSO. El Niño is the ocean Equator component and the Southern Oscillation is and the slope of the thermocline in the ocean the atmospheric component. The warm has flattened. phase of ENSO coincides with El Niño (or (NOAA/PMEL (TAO ocean warming) and negative SOI; the cold Thermocline Project), USA) phase of ENSO coincides with La Niña (or 120°E 80°W ocean cooling) and positive SOI. The prevailing characteristics of the of warm water and the reduction in ocean and atmospheric circulations across upwelling is to produce warmer than normal the equatorial Pacific Ocean arise because of water in the surface layer of the central and the positive feedbacks from wind stress of eastern equatorial Pacific Ocean. The the Trade Winds acting on the ocean surface reduced equatorial cross-Pacific sea surface and from the transfer of heat and moisture temperature gradient weakens the overlying (latent energy) to the atmosphere. The surface atmospheric pressure gradient and coupling of the ocean and atmosphere the strength of the Trade Winds. The through positive feedbacks assist in anomalous warm water over the central and maintaining the Walker Circulation of the eastern equatorial Pacific Ocean is also a atmosphere and surface layer characteristics source of heat and moisture to the across the equatorial Pacific Ocean as atmosphere and, in the absence of the usual follows: Trade Winds, supports local deep • The wind stress of the Trade Winds acts atmospheric convection further east than to maintain the equatorial cross-Pacific normal. sea surface temperature gradient, The ocean and atmospheric particularly through upwelling of cold characteristics that are generally prevailing water in the east. and those during an El Niño event are • In their passage across the Pacific shown schematically in Figure I.7. The Ocean the Trade Winds accumulate eastward spread of warm water and the heat and moisture to provide the energy deepening of the thermocline in the east are source of convection over the western the significant features of the ocean surface equatorial Pacific Ocean, Asia and layers during an El Niño event. In the Australasia. atmosphere the feature of note is the • High surface atmospheric pressure is occurrence of deep atmospheric convection favoured over the colder water of the further to the east than normal. The eastern Pacific Ocean. contraction of the Walker Circulation is • The cross-Pacific surface atmospheric linked to the weakening of the Trade Winds. pressure gradient maintains the strength West of the deep atmospheric convection a of the Trade Winds. reverse circulation is established with During an El Niño event, however, surface westerly winds assisting in the warm surface water spreads eastward across convergence of heat and moisture over the the equatorial Pacific Ocean towards South warmer than normal water of the central America. Also, the thermocline deepens in equatorial Pacific Ocean. the east and upwelling of cold water is El Niño events tend to be linked to the reduced. The combined effect of the influx annual cycle. Typically, the onset of 21 anomalous warming of sea surface (a) temperatures is first detected around the middle of the year (May) and the maximum anomaly of temperature is reached towards 60°N the end of the year. Generally by May of the 50°N following year the areas of significant sea 40°N 30°N surface temperature anomaly over the 20°N equatorial Pacific Ocean have vanished. 10°N EQ During a strong El Niño event the 10°S 20°S reversal of the Walker Circulation over the Wet 30°S Dry western and central Pacific Ocean can cause Warm 40°S subsiding air and clear skies to dominate 50°S over the western Pacific Ocean and parts of (b) 0° 20°E60°E 100°E 140°E 180° 140°W 100°W 60°W 20°W Asia and Australia. The subsiding air inhibits convection and results in below average

rainfall and even drought in parts of those 60°N

regions. Over the eastern equatorial Pacific 50°N Ocean the anomalous convection often 40°N brings flood rains to coastal parts of Ecuador 30°N 20°N and Peru. 10°N In addition to the direct impact on the EQ 10°S ° Walker Circulation an El Niño event has Wet 20 S ° other impacts on the global circulation of the Dry 30 S Warm 40°S

atmosphere. Over the tropical Pacific Ocean 50°S region the changed pattern of atmospheric 0° 20°E60°E 100°E 140°E 180° 140°W 100°W 60°W 20°W overturning and the changed equator to pole atmospheric temperature gradients impact El Niño events are consistent from one event on the subtropical westerly winds and the to the next. However, in some parts of the trough-ridge patterns of the mid-latitudes. globe and for some seasons of the year there Through teleconnections (see Appendix) are characteristic patterns of rainfall and there are downstream impacts on the temperature anomaly that recur with each El seasonal weather patterns over both North Niño event. Anomaly patterns of rainfall and America and South America and other parts temperature that have been identified are of the globe. shown in Figure I.8. These repeating Studies of historical records from island patterns are the basis for providing alerts and land stations of the globe show that about dangers from potential climate there is a degree of consistency in the extremes. Computer models of the global precipitation and temperature anomaly climate system that include forcing of the patterns during El Niño events. The most atmosphere by regions of anomalous sea dramatic and consistent change occurs across surface temperature show skill at predicting the tropical Pacific Ocean as convection and some characteristics of the observed rainfall shift eastward and the intertropical response of the atmosphere, including convergence zone is drawn further south seasonal rainfall and temperature anomalies. than usual, closer to the equator. At the same Computer models that couple the dynamics time there is suppressed seasonal rainfall of the oceans and atmosphere are being over the western Pacific Ocean and often a developed as the basis for more robust late onset to a weak Asian summer monsoon. prediction schemes. An El Niño event generally reaches its mature phase during the Northern Hemisphere winter when the atmospheric Global change westerly flow of the Northern Hemisphere is also at its peak. In addition to the The development of knowledge about ENSO precipitation anomalies of the Pacific Ocean, has been based on observations of the there is evidence of impacts reaching into climate system covering more than a the Indian Ocean basin and Africa, and into century, but especially with improved the middle and higher latitudes affecting observations and data management methods East Asia and the Americas. of the past 50 years. Many characteristic Not all patterns of anomalous attributes associated with the warm and cold precipitation and temperature attributed to phases of ENSO have been identified by 22 I. The Climate System

Figure I.8 comparing respective events over the period enhancement or reduction in the frequency Patterns of precipitation and noting the recurring common features. of cyclones and of the seasonal rainfall and temperature anomaly that are relatively More data are available from the more distribution. If the recurring frequency consistent between recent events, which tend to bias findings, and/or intensity of El Niño and La Niña El Niño events a) during but it is possible to review earlier events in a events were to change then there would be the Northern Hemisphere search for indicators that those same profound changes to many local and winter and b) during the Southern Hemisphere characteristics may have been present. Thus regional climate extremes, with consequent winter. we know from contemporary descriptions of societal impacts. (NOAA/PMEL (TAO the intensity and global patterns of societal The available observations indicate that Project), USA after Ropelewski and Halpert, impacts during 1877 and 1878 that the the characteristics of ENSO variability over 1987) climate anomalies are almost certainly linked the second half of the century may have to a major El Niño event, despite having changed. There have been more El Niño only limited scientific data from the period. events, particularly since the early 1970s, A serious concern about the application with an almost continuous sequence during of knowledge of ENSO constructed from the early 1990s and two major events in analysis of earlier events relates to potential 1982–83 and 1997–98. Some scientists have long-term change of the climate system, suggested that the changed frequency and whether through natural variability or intensity of recent El Niño events is outside anthropogenic forcing. Over the past 150 the expected variability of the climate years there has been a small (less than 1°C) system (based on observations of the past increase in the average global temperature century), and that the changes may be at the Earth’s surface, both over land and linked to, or evidence of, anthropogenic over the oceans. The Intergovernmental influence on the climate system. There are Panel on Climate Change (IPCC) has other scientists who suggest that a century is concluded “the balance of evidence suggests too short a period on which to make such that there is a discernible human influence judgements, particularly if there is multi- on global climate”. decadal variability in the recurrence of El As yet, there is no consensus on how Niño events. The IPCC is addressing these global warming may impose shifts in the important issues. seasonal pattern and intensity of weather Notwithstanding the continuing systems. There is, however, consensus that a scientific discussion on the changed changing climate will be reflected in characteristics of El Niño events over the changing patterns of weather and climate past three decades, and the attribution of extremes. Even a change in the average state cause, there is consensus that issues related of the local climate will be reflected in a to El Niño are inseparable from the broader profile of higher extremes at one end of the issues of climate change. ENSO is part of the range and lower extremes at the other. Also, recurring patterns of extreme weather events during the global warming of the recent past and persisting climate anomalies that have there are parts of the globe that have enormous societal impacts, especially in experienced a cooling trend and this developing countries. A concerted effort is underscores the potential for increased required on the part of governments and climate variability and extremes. non-governmental organizations to develop Warm and cold ENSO events contribute appropriate policies to mitigate climate to the pattern of variability of weather change, where feasible, and to prepare their systems and to extremes of local and communities for periodic impacts of climatic regional climate. As a result of complex non- extremes as a basic for sustainable linear interactions between components of development. the climate system during ENSO events there are seasonal shifts in the frequency of occurrence and intensity of regional weather systems in many parts of the globe. For example, in some regions the seasonal incidence of tropical cyclones is suppressed during an El Niño event but enhanced during a La Niña event and the seasonal risk of wind damage and flooding changes; in other regions it is the opposite. In some mid-latitude regions, through teleconnections, there is either an 23 Part II

The 1997–98 El Niño Event

Monitoring El Niño • Drifting buoys for measuring sea surface conditions. A range of new instruments to monitor the The initiatives, strongly supported by climate system has been implemented since the World Climate Research Programme, the major El Niño event of 1982–83. These reflect recognition by scientists that the developments must be tempered by understanding and prediction of climate realization that there has also been an variability and climate change will not overall decline in the number of operational advance satisfactorily without essential national surface and upper-air observations, particularly from across and meteorological observing stations in many beneath the ocean surface. In addition, countries around the globe over the period. because of limits to the observing capability New or expanded observing systems that existing at the time, many significant have been introduced and from which data processes associated with the 1982–83 El are freely available include: Niño were not recognized or could only be • Satellite instruments for remote inferred. Many features were only identified sensing; through research well after the event was • An array of fixed buoys across the over. The previous deficiencies significantly equatorial Pacific Ocean (the Tropical reduced the capacity for effectively warning Atmosphere Ocean (TAO) array); of the existence of an El Niño event and the • A fleet of cooperating volunteer associated potential for climate extremes. A observing ships for deploying priority of the World Climate Research expendable bathythermographs (XBT) Programme, put to the Second World that measure ocean temperature and Climate Conference in 1990, has been for salinity at depth; and more international cooperation and

The Global Climate Observing System — GCOS The Global Climate Observing System is an initiative that terrestrial, cryospheric, hydrospheric and ecosystem has its origins at the 1990 Second World Climate processes, as well as space-based observations and data Conference, held in Geneva, Switzerland. Scientists systems and management. called for a greater effort to better observe the climate Several initiatives are under way to increase critical system in support of research, monitoring variability and climate observations from the oceans. In the tropical trends, prediction of future events, and services in support Pacific Ocean the TAO array is operational. The TAO of safety of life and sustainable development. WMO, the array has proven so successful that it is being extended Intergovernmental Oceanographic Commission (IOC) of to the tropical Atlantic Ocean and there is a plan for a the United Nations Educational, Scientific and Cultural network in the tropical Indian Ocean. New projects to Organization (UNESCO), International Council for Science deploy drifting buoys in the Southern Ocean have been (ICSU) and United Nations Environment Programme initiated and an additional project has been proposed to (UNEP) have joined as co-sponsors of the GCOS programme. provide global data assimilation of both space-based A priority of GCOS is to ensure that the data needs and in situ observations. are met for seasonal-to-interannual climate prediction The implementation and operational management and for detection and attribution of long-term climate of GCOS components are through the dedicated contri- trends. The detailed scientific plans for GCOS were butions by national governments. The ultimate success developed during the 1992–95 period and in 1996 the of GCOS will require ongoing commitment by all implementation phase began. The GCOS plans include governments to contribute funds and the services of observational elements relating to atmospheric, oceanic, scientific and technical staff.

24 II. The 1997–98 El Niño event improved instrumentation for monitoring of the global climate system. The Tropical Ocean Global Atmosphere (TOGA) project was a decade-long research initiative (1985–94) of the World Climate Research Programme, managed by an international consortium, to improve monitoring of the Pacific Ocean as the basis for understanding and predicting the El Niño phenomenon. At the heart of the TOGA project is the TAO array of approximately 70 moored ocean buoys extending across the equatorial Pacific Ocean to systematically measure ocean and meteorological data (Figure II.1). The Atlas buoys of the array are moored and measure surface winds, air Figure II.1 temperature and relative humidity, and The main components of temperatures in the upper 500 metres of the the Pacific Ocean climate Wind sensor observing system installed ocean. The Current Meter buoys measure 3.8 m above water surface for the decade-long ocean currents and additional parameters ARGOS antenna (1985–94) Tropical Temperature sensors such as shortwave radiation and rainfall. Humidity sensor Ocean Global Torroidal buoy Data logger/ Atmosphere (TOGA) The TAO array is managed by the Data 2.3 m diameter transmitter project to research the Buoy Cooperation Panel and coordinated El Niño phenomenon. Sea surface Components include the with WMO and the IOC. Observations from temperature Sensor cable sensor Temperature sensors TAO array of moored the TAO array are automatically collected buoys (red dots), fixed using the Global Telecommunication System tide gauges (yellow dots), (GTS) of WMO and are available to national drifting buoys (orange 3/8" wire rope Meteorological Services and for Temperature arrows) and the tracks of sensor cable volunteer observing ships meteorological and oceanographic research. 500 m deploying XBT (dark A typical Atlas buoy of the TAO array is in blue). These systems Figure II.2. Data are also collected at the US continued to be available 700 m during the 1997–98 National Oceanic and Atmospheric 3/4" nylon line El Niño event. Administration (NOAA) Pacific Marine (NOAA/PMEL (TAO Environment Laboratories (PMEL) for Project), USA) archival and analysis. PMEL maintains an

Internet site and provides free access to Acoustic release Figure II.2 data, and to a range of products obtained by A schematic diagram of analysis of the data. an Atlas buoy of the TAO (Anchor 4200 lbs.) array of the equatorial Additional information on the vertical Pacific Ocean showing temperature structure of the ocean is given instrumentation and by XBT dropped from merchant ships of a measurements. Satellite data and analysed mooring. (NOAA/PMEL (TAO fleet of Volunteer Observing Ships products that are relevant to climate Project), USA) coordinated by WMO. The XBT instruments monitoring are also available over the provide better vertical resolution and wider Internet. geographical coverage than do sensors on Over the past two decades satellite moored buoys but significantly less frequent instruments to measure sea surface temporal sampling. The data are collected conditions have improved. The Advanced via the GTS of WMO for use in research and Very High Resolution Radiometers (AVHRR) meteorological operations. on the NOAA weather satellites provide Satellites, providing near-global routine near-global coverage of sea surface coverage, give spatial representation for temperature with good accuracy when many oceanographic and meteorological blended with ship and buoy observations. parameters not available from in situ Since 1992 the TOPEX/Poseidon measurements. However, the frequency of satellite, a joint US and French mission, has sampling from satellite instruments is provided near-global sea level anomaly generally less than can be achieved from in data. These new sea level data have spatial situ instruments. It is necessary to maintain coverage not previously possible from the calibration of the satellite sensors through existing network of coastal and island tide regular in situ and other specialized gauges, but depend on these gauges for 25 (a)S (b) (c) throughout 1996. This led to the formation O N of a warm pool in the western Pacific Ocean 1996 D J with raised sea level and deeper than F M normal thermocline. The period of westerly A M wind bursts over the western equatorial J J

1997 Pacific Ocean associated with Madden-Julian A S Oscillations, which is suggested as being an O N important factor in the initiation of the D J 1997–98 El Niño event, can be seen in the F M A wind field from December 1996 through M 1998 J April 1997 (shaded red). Periods of westerly J A winds recur over the western Pacific Ocean 140°E 180° 140°W 100°W 140°E 180° 140°W 100°W 140°E 180° 140°W 100°W through most of 1997 but do not extend the (m/s)-8 -4 0 4 8 (°C) 19 21 23 25 27 29 31 (W/m2)160 200 240 280 full width of the Pacific Ocean at any time. Kelvin waves initiated by the westerly Figure II.3 calibration. The TOPEX/Poseidon satellite is wind bursts propagated eastwards along the Mean monthly surface expected to continue operation until May equator and acted to raise sea level and lower zonal wind (a), sea surface temperature (b) 2000 and then to be replaced by its the depth of the thermocline. Sea surface and outgoing longwave successor, JASON-1. Surface wind speeds temperatures exceeding 27°C (shaded red in radiation (c) from over the oceans were measured for about Figure II.3b) appeared at most longitudes September 1996 to nine months using the NSCAT system across the central and eastern Pacific Ocean August 1998 along the equator across the Pacific mounted on the ADEOS satellite before it between April and May 1997. The warm sea Ocean. failed in June 1997. surface temperatures were maintained over (McPhaden, 1999a) It was fortuitous, therefore, that prior to the central and eastern equatorial Pacific the onset of the 1997–98 El Niño event the Ocean until rapid cooling set in about May Global Observing System for weather 1998. forecasting had been augmented by the new The changing outgoing longwave instruments for climate research. Not only radiation fields are consistent with eastward were the systems for observing the progress of the focus of deep atmospheric phenomenon unprecedented but the data convection across the equatorial Pacific were rapidly made available to the scientific Ocean (see Figure II.3c). There is community and national meteorological consistency between the eastward progress services. The development and of the region of reduced outgoing longwave intensification of the El Niño phenomenon radiation and the eastern edge of the region and the spatial extent of many climate of sea surface temperature in excess of 28°C anomalies were well monitored and accurate to 29°C. The anomalous warming under the advice was available, through national influence of the developing El Niño event meteorological services, as a basis for action meant that sea surface temperatures over the by governments and individuals. central equatorial Pacific Ocean provided a heat and moisture source necessary to support deep atmospheric convection. The El Niño cycle Sea surface temperatures over the eastern equatorial Pacific Ocean did not Overview exceed the 28°C to 29°C threshold until about October 1997, and in coastal regions The evolution and decay of the equatorial of South America these temperatures were characteristics of the 1997–98 El Niño event not exceeded until December 1997. During can be visualised from concurrent equatorial the Southern Hemisphere summer of longitude-time sections (or Hovmöller 1997–98, when the intertropical convergence diagrams) for zonal wind, sea surface zone was normally at its most southern temperature and outgoing longwave extent, the warm waters were a strong radiation shown in Figure II.3. Each of the source of heat and moisture. The panels represents values along the equator significantly reduced values of outgoing (horizontal axis — Date Line 180) with time longwave radiation over the eastern along the vertical axis commencing equatorial Pacific Ocean at this time September 1996 and ending in August 1998. identified deep atmospheric convection A period of easterly winds persisted adjacent to the South American coast. over the central and eastern equatorial As the focus of deep atmospheric Pacific Ocean (shaded blue in Figure II.3a) convection moved eastward across the 26 II. The 1997–98 El Niño event

equatorial Pacific Ocean during the 60°N developing El Niño event it can be inferred that it had an impact on the atmospheric 30°N circulation. The deep atmospheric convection and ascending air of the Walker Circulation was no longer over the EQ longitudes of the warm pool of the western Pacific Ocean but over the central and 30°S eastern equatorial Pacific Ocean. Surface winds over the western Pacific Ocean reversed in response to the eastward shift in 60°S ° ° ° ° ° ° ° convection and in a positive feedback 0 80 E 120 E 180 120 W60W0 process, further heat and moisture were fed (°C) into the convection zone. Thus, as the -4-3 -2-101234 convection moved eastward the Walker

Circulation contracted and a reverse (a) 140°E 160°E 180° 160°W 140°W 120°W 100°W (°C) Figure II.4 circulation was established over the western 0 32 Sea surface temperature 28 anomaly for January equatorial Pacific Ocean with subsiding air 100 24 1997 during the in the west. commencement of the 20 200 El Niño event. 16 12 Temperatures are warmer 300 Prior conditions Depth (m) 12 than normal in the 8 western Pacific Ocean 400 10 and cooler than normal Early in 1997 some computer models 4 predicted the development of an El Niño in the east. 500 0 (NOAA/CDC, USA) event. However, overall the different computer models provided conflicting (b) 140°E 160°E 180° 160°W 140°W 120°W 100°W (°C) Figure II.5 0 12 (a) Monthly mean advice and none indicated the development 0 8 temperature and (b) 100 of a very intense event. There remains 3 temperature anomaly fundamental scientific debate about what are 2 1 4 during January 1997 at 200 the commencement of the the necessary conditions to initiate an El 0 El Niño event along the 300 Niño event. A major issue is the relative Depth (m) -4 equator. The thermocline importance of internal processes within the is deeper than normal 400 ocean and of westerly wind bursts over the -8 under the warm surface layer in the western and western equatorial Pacific Ocean to initiate 500 -12 central Pacific Ocean, and intensify eastward propagating but slightly elevated with equatorial Kelvin waves. The early onset and deeper than normal over the western Pacific colder than normal water at the surface over the rapid intensification of the 1997–98 event Ocean. To the north of Papua New Guinea eastern Pacific Ocean. were not anticipated but once identified the temperature was nearly 2°C warmer than (NOAA/PMEL (TAO several computer models described the normal at a depth of near 100 metres, and Project), USA) evolution well. near the Date Line the temperature was about A precondition for an El Niño event had 4°C above normal at a depth of 150 metres. developed during the latter half of 1996 as an extensive warm pool became established Commencement in the surface layers of the western Pacific Ocean. By January 1997 sea level over the During the Southern Hemisphere summer of region was up to 20 cm above normal and 1996–97 there were periods of sustained sea surface temperatures were slightly westerly wind bursts in the western warmer than normal west of the Date Line. equatorial Pacific Ocean that may have The global map of sea surface temperature been significant in the initiation and anomaly for January 1997 (see Figure II.4) subsequent evolution of the 1997–98 El shows an area of slightly warmer than Niño event. The first period of westerly normal temperatures over the western winds was in December 1996, when there equatorial Pacific Ocean and slightly cooler was a pair of tropical cyclones in the temperatures over the eastern equatorial western Pacific Ocean, one north and the Pacific Ocean. other south of the equator. The longitudinal The equatorial depth-section of extent and duration of this westerly wind temperature and temperature anomaly is burst were limited. shown in Figure II.5. The thermocline A major westerly wind burst over the associated with the pool of warm water was western equatorial Pacific Ocean occurred 27 during March 1997 and was associated with 20°N a long-lived intense tropical cyclone, Justin, in the Coral Sea south of Papua New 10°N

Guinea. Not only did the westerly winds EQ persist for most of the month but also the longitudinal extent was approximately 40 10°S degrees. The mean surface wind field over 20°S the tropical Indian and western Pacific 60°E° 80°E 100°E 120°E 140°E 160°E 180 160°W° (m/s) Oceans for March 1997 is in Figure II.6 and 26810124 shows the persisting westerly winds

northeast of Papua New Guinea. (a) 140°E 160°E 180° 160°W 140°W 120°W 100°W (°C) Figure II.6 12 The spatial extent of higher than normal 0 Monthly vector mean 0 2 8 winds at the surface over sea level over the warm pool in the western 100 1 1 0 the Indo-Pacific region for 2 Pacific Ocean and its subsequent movement 4 0 4 3 March 1997. The eastward can be tracked by analysis of sea 200 0 northwesterly winds over level anomalies measured by the 300 the western equatorial Depth (m) TOPEX/Poseidon altimeter in March, April -4 Pacific Ocean northeast 400 of Papua New Guinea and May 1997 (Figure II.7). The -8 associated with tropical TOPEX/Poseidon image of March is 500 -12 cyclone Justin may have coincident with the December 1996 westerly Means been an important factor (b) 140°E 160°E 180° 160°W 140°W 120°W 100°W (°C) to initiate the 1997–98 wind burst that is believed to have generated 0 12 El Niño event. 6 (NOAA/CDC, USA) an eastward propagating Kelvin wave on the 8 100 thermocline. By the end of April the 4 5 2 3 4 Figure II.8 eastward travelling equatorial Kelvin wave, 200 0 1 1 Anomalies of subsurface identified by the equatorial band of above 0 temperature across the 300 normal sea level, had arrived at the coast of Depth (m) -4 equatorial Pacific Ocean during a) March 1997; 400 South America. Deflection of the Kelvin -8 and b) May 1997. The waves north and south along the coast deepening thermocline 500 -12 occurred during May 1997 as the (as indicated by the thermocline deepened, forced by the shows that the Kelvin wave generated by the warm anomalies of temperature at depth) propagating equatorial Kelvin wave. major westerly wind burst of March 1997 shows the progress of the The impact of the Kelvin wave on the arrived at the coast of South America during Kelvin wave that thermocline and subsurface temperatures May 1997. There was significant deepening propagated to the east. (NOAA/PMEL (TAO can be seen in the pair of equatorial-depth of the thermocline and warming of the Project), USA) cross-sections of temperature anomaly in subsurface temperatures across the eastern Figure II.8. Figure II.8a (for March 1997) Pacific Ocean in May 1997. shows the Kelvin wave generated by the The monthly mean anomaly of sea limited west-wind burst of December 1996 surface temperature for March 1997 (Figure arriving at the coast of South America, II.9a) does not show the progress of the causing deepening of the thermocline and a Kelvin wave that was generated by the warm anomaly of temperature at depth. limited burst in December 1996. The first There is also further deepening of the significant warming in the sea surface is thermocline in the central Pacific Ocean seen at the end of April 1997, when the (more than 4°C at 200 metres) resulting from Kelvin wave reaches the coast of South the sustained westerly winds during March America approximately two months after and the eastward propagation of another being initiated by the westerly wind in early Figure II.7 Kelvin wave. Figure II.8b (for May 1997) March 1997. The anomalous warming in the Sea level anomaly as measured by the TOPEX/ Poseidon altimeter during 25 March 1997 25 April 1997 25 May 1997 the onset of the 1997–98 El Niño event. The eastward progress of the Kelvin wave across the equatorial Pacific Ocean is identified by the moving pattern of elevated sea level. White is more than 12 cm above normal, and mauve areas are more than 12 cm below normal. (NASA/JPL, USA) 28 II. The 1997–98 El Niño event

(a) 60°N characteristic indicators in Figure II.10 outlines the extent of development of the El 30°N Niño event by July 1997. Over the eastern equatorial Pacific Ocean the area of above EQ normal sea level expanded (Figure II.10a), the thermocline depth deepened (Figure 30°S II.10b) and the area of warm sea surface

60°S temperature anomaly also expanded 0° 80°E 120°E 180° 120°W60°W0° significantly (Figure II.10d). (°C) -4-3 -2-101234 By July 1997, despite the marked (b) changes that had taken place in the 60°N distribution of heat content of the equatorial Pacific Ocean (Figure II.10d), there was no 30°N apparent coherent pattern of reorganization of the tropical surface wind field at that time EQ (Figure II.10c). Nevertheless, the development of clear patterns of anomaly of 30°S outgoing longwave radiation points to rapid

60°S coupling between the developing El Niño ° ° ° ° ° ° ° 0 80 E 120 E 180 120 W60W0event of the Pacific Ocean, through deep (°C) -4-3 -2-101234 atmospheric tropical convection, and the atmospheric circulation. Figure II.9 eastern equatorial Pacific Ocean, and the The outgoing longwave radiation Monthly mean anomalies southward extension along the coast of pattern for June 1997 is in Figure II.11. The of sea surface temperature for a) March 1997; and South America following the arrival of the extensive area of increased outgoing b) May 1997. Warmer Kelvin waves, can be seen in the map of sea longwave radiation over the tropics, than normal temperatures surface temperature anomaly for May 1997 stretching from India through Indo-China, developed off the Pacific coast of South America in Figure II.9b. the Philippines and Indonesia to east of during May associated Papua New Guinea, indicated there was less with the deepening ther- Evolution deep atmospheric convective cloud and mocline that followed the rainfall over the region. However, reduced arrival of the Kelvin waves. (NOAA/CDC, USA) The El Niño continued to develop during outgoing longwave radiation over the June. By July, sea surface temperature central and eastern equatorial Pacific Ocean Figure II.10 anomalies had reached from 3°C to 5°C indicated increased tropical deep A montage of above the seasonal normal in the eastern atmospheric convection and precipitation characteristic El Niño indicators showing equatorial Pacific Ocean, the largest had shifted eastwards. There was also development during July recorded during Southern Hemisphere reduced outgoing longwave radiation, 1997. Sea level has winter months. The montage of indicating increased cloudiness and risen across the eastern equatorial Pacific Ocean ° ° ° ° ° ° ° ° (white shading more than (a) 25 June 1997 (b) 140 E 160 E 180 160 W 140 W 120 W 100 W ( C) 12 12 cm above normal (a). 0 The thermocline of the 0 8 eastern equatorial Pacific 100 Ocean has deepened 3 2 1 4 (b). Westerly wind 200 anomalies have 0 developed over the 300 Depth (m) western and central -4 equatorial Pacific Ocean 400 (c). The area of warm -8 sea surface temperature anomaly over the eastern 500 -12 equatorial Pacific Ocean has expanded (d). (c) (d) Easterly wind anomalies 60°N 60°N and cold sea surface temperature anomalies 30°N 30°N have developed over the eastern equatorial Indian EQ EQ Ocean. ° (a) NASA/JPL, USA; b) 30°S 30 S NOAA/PMEL (TAO Project), 60°S 60°S USA; c) and d) 0° 80°E 120°E 180° 120°W60°W0° 0° 80°E 120°E 180° 120°W60°W0° NOAA/CDC, USA from (m/s) (°C) NCEP/NCAR reanalysis) 134562 -4-3 -2-101234 29 ° precipitation, over the western equatorial 60 N Indian Ocean adjacent to East Africa. ° Surface ocean currents over the Pacific 30 N Ocean have been estimated using the EQ NSCAT winds (the NSCAT scatterometer was mounted on the ADEOS satellite, 30°S which was functioning during the early development of the El Niño event but 60°S failed in June 1997) and the sea level 0° 80°E 120°E 180° 120°W60°W0° topography of the TOPEX/Poseidon (W/m2) satellite. The calculated surface currents -50-40 -30 -20 -10 0 10 20 30 40 50 and the anomaly of sea surface (a) temperature for December 1996, April 1997 20°N and June 1997 are shown in Figure II.12. 10°N The maps provide a view of the impact of EQ (°C) the evolving El Niño event on surface 10°S 2 ocean currents, particularly of the reversal 20°S of the wind-driven westward directed 50 cm/s 1.5 80 120 160 200 240 280 surface current, and the transport of 1 surface water across the Pacific Ocean. (b) During December 1996, (Figure II.12a) 20°N 0.5 there was a strong wind-driven westward 10°N flowing current over the central and eastern EQ 0 equatorial Pacific Ocean. This was consistent 10°S -0.5 with the continuing Trade Winds 20°S 50 cm/s maintaining an equatorial wind-driven -1 80 120 160 200 240 280 current and the development of a warm pool in the surface layers of the western (c) -1.5 ° Pacific Ocean. Northward of the equatorial 20 N ° -2 wind-driven current is the North Equatorial 10 N Counter Current that flows eastward. EQ ° By April 1997 (Figure II.12b) the 10 S 20°S equatorial wind-driven surface current had 50 cm/s reversed and there was eastward flow across 80 120 160 200 240 280 the full width of the equatorial Pacific Basin. Water was being transported from the warm pool in the west towards the east. The The mature phase Figure II.12 eastward flowing equatorial surface current Surface ocean currents (arrows) calculated using strengthened between April and June 1997 The El Niño event continued to evolve TOPEX/Poseidon (Figure II.12c) and the advection of warmer during the second half of 1997 and reached topography and NSCAT water from the west and deepening of the its mature phase December 1997 through winds from the ADEOS satellite, and with mean thermocline contributed to strong warming of May 1998. At the time of the mature phase anomaly of sea surface the eastern equatorial Pacific Ocean. The sea levels over the western equatorial Pacific temperature (colour) for a) establishment of a westward flowing wind- Ocean were at a minimum. For example, the December 1996; b) April driven surface current in the equatorial Indian sea level at Pohnpei, Federated States of 1997; and c) June 1997. Over equatorial Ocean west of Indonesia (Figures II.12b and Micronesia was 24.6 cm below normal latitudes the westward II.12c) was also important in the context of during December 1997 while, at the same directed wind-driven upwelling and subsequent cooling of the sea time, over the eastern equatorial Pacific current prevailing in December 1996 was surface that appeared in the region. Ocean sea level was at a maximum and replaced by an eastward The new satellite instruments and TAO reached 39.3 cm above normal at Santa directed current by April array of buoys that were available during the Cruz, Ecuador. 1997 that gained in strength to June 1997. evolution of the 1997–98 El Niño event have During the later part of the evolution of (Lagerloeff et al., 1999) given insights into ocean processes that the El Niño event many impacts on the were previously not possible. Analyses tropical atmospheric circulation could be based on these data also allowed testing of readily identified. By June 1997 the Southern other hypotheses that had been developed Oscillation Index (SOI) was strongly from careful examination of previous events negative, indicating a reduced cross-Pacific but for which there were limited data to Ocean pressure gradient consistent with the draw solid conclusions. changed sea surface temperature gradient. 30 II. The 1997–98 El Niño event

Figure II.11 The anomaly of vector wind and anomaly of although thick smoke from fires may have Monthly mean anomaly of outgoing longwave sea surface temperature during October biased the satellite instrument measurements radiation for June 1997. 1997 are shown in the maps in Figure II.13. towards colder values. The positive anomaly Enhanced outgoing By that time the area of warmer than normal of sea surface temperature in the coastal longwave radiation (blue to mauve shading) water over the eastern equatorial Pacific waters off equatorial East Africa was indicating reduced Ocean had expanded further and anomalous possibly linked to the changed circulation tropical deep westerly winds directed towards the warmer and a failure of seasonal upwelling of cold atmospheric convection water had appeared over the western water off the Somali coast. extends from southern India through the islands equatorial Pacific Ocean. The outgoing The development of a reverse zonal of Indonesia and the longwave radiation pattern of June 1997 (see atmospheric circulation across the Indian Philippines to east of Figure II.11) persisted through the second Ocean, apparently linked to the El Niño and Papua New Guinea. Reduced outgoing half of 1997, indicating reduced convection the reversed zonal atmospheric circulation longwave radiation over the region of Indonesia and Papua New over the equatorial Western Pacific Ocean, (orange to red shading) Guinea, and enhanced convection over the has significance for the weather and climate identifies increased tropical deep warmer waters of the central and eastern of the basin and surrounding countries. atmospheric convection equatorial Pacific Ocean. However, only few upper-air, surface and over the central and The breakdown of the Walker ocean subsurface data are available from eastern equatorial Pacific Ocean. Increased deep Circulation of the Pacific Ocean can be across the Indian Ocean and the nature of atmospheric convection inferred by the enhanced convection over linkages between El Niño and regional can also be seen over the central and eastern equatorial Pacific climate anomalies can only be inferred. the western equatorial Ocean, the reduced convection (subsidence) Better observations are essential to support Indian Ocean. (NOAA/CDC, USA) over Indonesia and westerly wind anomalies research if understanding of the processes over the western equatorial Pacific Ocean. that will lead to better climate predictions Similarly, the establishment of a reverse for surrounding countries is to be achieved. circulation over the Indian Ocean can be The eastward shift of deep atmospheric inferred from the subsidence over convection across the tropical Pacific Ocean Indonesia, convection over the western can be related to eastward movement of Indian Ocean and strengthened easterly warm water and the raising of the sea winds along the equator. surface temperature during the evolution of As the atmospheric circulation changed, the El Niño event. Under usually observed the easterly winds that became established tropical conditions, deep atmospheric across the Indian Ocean induced a wind- convection does not occur except in regions driven current in the surface layers of the of surface wind convergence and where sea Indian Ocean. Offshore upwelling of cold surface temperatures exceed a threshold of water was indicated west of Sumatra, about 28°C to 29°C.* This threshold of sea surface temperature is not usually exceeded (a) over the central and eastern equatorial 60°N Pacific Oceans.

30°N *Over the tropical oceans, where there is an abundant source of latent energy, converging air that has a EQ temperature above about 28°C to 29°C and is lifted to condensation will generally rise buoyantly to the high 30°S atmosphere in deep convective clouds because of the

60°S release of latent energy. Below about 28°C to 29°C the ° ° ° ° ° ° ° 0 80 E 120 E 180 120 W60W0surface air will not achieve sufficient buoyancy (except in (m/s) regions of lowered surface pressure) and deep 23 4 56789 convective clouds will not develop. Over the tropical (b) Pacific Ocean deep atmospheric convection is usually 60°N observed over the warm waters of the west and along the intertropical convergence zone. 30°N

EQ Figure II.13 Anomalies of a) surface wind; and b) sea surface ° temperature for October 1997 during the developing El 30 S Niño event. The westerly wind anomalies of the western equatorial Pacific are associated with a breakdown of 60°S ° ° ° ° ° ° ° the Walker Circulation. The anomalous easterly winds 0 80 E 120 E 180 120 W60W0across the equatorial Indian Ocean are apparently ° ( C) associated with a reverse zonal atmospheric circulation. -5-4 -3 -2-1012345 (NOAA/CDC from NCEP/NCAR reanalysis, USA) 31 For each of three regions of the equatorial (°C) 30 Pacific Ocean the changing average sea 28 (a) surface temperature during the El Niño 26 24 event is shown in Figure II.14. The upper 22 panel is for the central equatorial Pacific 20 Dec. 96 Feb. 97 Apr. Jun. Aug. Oct. Dec. 97 Feb. 98 Apr. Jun. Aug. Oct. 98 Ocean (NIÑO 3.4) the middle panel is for 30 the eastern equatorial Pacific Ocean (NIÑO 28 (b) 26 3) and the lower panel is for coastal South 24 America (NIÑO 1+2). The actual 22 temperatures for 1997–98 are compared with 20 Dec. 96 Feb. 97 Apr. Jun. Aug. Oct. Dec. 97 Feb. 98 Apr. Jun. Aug. Oct. 98 the seasonally varying long-term average for 30 28 (c) each region, and the magnitude of the warm 26 anomaly is shaded orange. Each of the 24 regions has a seasonal cycle of sea surface 22 20 temperature and the amplitude of the cycle Dec. 96 Feb. 97 Apr. Jun. Aug. Oct. Dec. 97 Feb. 98 Apr. Jun. Aug. Oct. 98 10°N increases to the east. Under normal Niño 3,4 Niño 1+2 conditions the sea surface temperature does EQ 10°S Niño 4 Niño 3 not exceed 28°C at any time of the year for 120°E 150°E 180° 150°W 120°W90°W60°W any of the three regions, although the threshold temperature is approached during temperature values exceeded the threshold March–April in all regions. 28°C because of the combined impact of The duration of sea surface temperature seasonal and El Niño warming. For as long as values favourable for deep convection over coastal sea surface temperatures continued to the three regions can also be seen in Figure be above the threshold value, deep II.14. Warmer than normal sea surface convection was possible. The sea surface temperature was first evident off the coast of temperatures over coastal South America Figure II.15 South America during February 1997, but continued to be favourable to promote deep A montage of within two months had appeared across all convection until May 1998, when the characteristic El Niño regions. Sea surface temperature approached seasonal cooling and the collapsing El Niño indicators during January 1998 at about the time the threshold value of 28°C in the central caused sea surface temperatures to again fall of the mature phase. Sea region about mid-1997 and remained above below the threshold value. level continues to be high that value for approximately 12 months The El Niño event remained strong over the eastern (NIÑO 3.4). Throughout the period ocean through the Southern Hemisphere summer equatorial Pacific Ocean but has fallen over the temperatures in the central equatorial Pacific of 1997–98. The continuing warm sea west (a — TOPEX/ Ocean were favourable for deep atmospheric surface temperatures across the central and Poseidon; white shading convection and rainfall. Across the eastern eastern equatorial Pacific Ocean have been is more than 12 cm above normal, mauve equatorial Pacific Ocean (NIÑO 3) the sea noted. A montage of characteristic indicators shading is more than 10 surface temperatures were near the threshold of El Niño for January 1998 during the cm below normal). The value of 28°C from May 1997, but only mature phase is in Figure II.15. thermocline of the eastern equatorial Pacific Ocean consistently exceed the value from The mature phase of the El Niño event continues to be deeper November 1997 to May 1998. The waters off was reached about January 1998. Figure than normal in the eastern the coast of South America (NIÑO 1+2) only II.15a shows that sea level elevations were equatorial Pacific Ocean but has risen significantly exceeded 28°C from January to early May significantly lower than normal over a wide in the west; (b — 1998. Also, it was the first time since the area of the western equatorial Pacific Ocean. equatorial longitude-depth preceding strong event of 1982–83 that The more than 5°C negative anomaly of section of temperature anomaly across the equatorial sea surface temperatures in the temperature at a depth of 120 metres in the western equatorial Pacific Pacific coastal region of South America had equatorial longitude-depth section of Figure Ocean). Westerly wind exceeded 28°C. II.15b identifies the elevated thermocline of and easterly wind anomalies persist over the The time histories of Figure II.14 the western and central Pacific Ocean. The central equatorial Pacific demonstrate why the local maximum impact elevated thermocline and colder than Ocean and the equatorial of El Niño may not be at the time of normal waters at depth contribute to the Indian Ocean maximum development of the event. The lowering of the sea level over the region. In respectively (c). Warm sea surface temperature maximum anomaly of sea surface the eastern equatorial Pacific Ocean the anomalies persist over the temperature over the coastal waters of South deepening of the thermocline during eastern equatorial Pacific America is reached in July 1997 but, because January 1998 was not as great as earlier in Ocean (d). (a) NASA/JPL, USA; b) of the normal seasonal cycle, temperatures the development but was still significantly NOAA/PMEL (TAO Project), continue to cool. The anomaly remained deeper than normal. The positive anomaly USA; c) and d) relatively strong and during the Southern of temperature continued to be in excess of NOAA/CDC from NCEP/NCAR reanalysis, Hemisphere summer of 1997–98 sea surface 7°C at 50 metres. The situation of raised USA) 32 II. The 1997–98 El Niño event

Figure II.14 thermocline in the west and deeper Africa and the sea surface temperature Average sea surface thermocline in the east is typical of the gradient across the Indian Ocean was temperature from Jan 1996–Oct 1998 over mature phase of an El Niño event. maintained, consistent with the continuing a) the central equatorial The easterly wind anomaly of the easterly wind anomaly. Pacific Ocean (NIÑO tropical Indian Ocean and the westerly wind The pattern of positive outgoing 3.4 — lat 5°N–5°S, long 170°W–120°W); anomaly of the tropical Pacific Ocean longwave radiation anomaly across the b) the eastern equatorial persisted as features of the wind circulation equatorial Indo-Pacific region that had been Pacific Ocean (NIÑO 3 (Figure II.15c). The southward shift in the established during the development of the El — lat 5°N–5°S, long latitude of maximum anomalies is consistent Niño persisted during the mature phase. The 150°W–90°W); and c) the coastal waters off with the normal southward shift of the January 1998 pattern of outgoing longwave South America (NIÑO intertropical convergence zone during the radiation anomaly is shown in Figure II.16 1+2 — Eq–10°S, long Southern Hemisphere summer. The surface and suppressed convection (positive 90°W–80°W). The average seasonal cycles wind field anomalies indicate that the anomalies) is noted to extend from the (blue lines) and the Walker Circulation is not as strong as eastern Indian Ocean through Indonesia to approximate threshold normal, and the likely presence of a reverse the western Pacific Ocean. Deep convection temperature for tropical deep atmospheric circulation across the Indian Ocean. The is indicated over the western equatorial convection (28°C — red anomaly of sea surface temperature Indian Ocean off the coast of East Africa and line) are shown for continued to be strong over the eastern over the central equatorial Pacific Ocean, comparison. equatorial Pacific Ocean during the mature mainly south of the equator. (Du Penhoat (personal communication) based on phase but over the western equatorial data from NOAA/CPC, Pacific Ocean the pattern of sea surface The decline USA) temperature anomalies had reverted to near normal (Figure II.15d). The equatorial longitude-depth section of Over the equatorial Indian Ocean the temperature anomaly across the Pacific changes in the pattern of sea surface Ocean for January 1998 (Figure II.15b) temperature anomaly that had taken place indicates that the decline of the El Niño during the development of the El Niño event event may have already commenced at that in October 1997 (see Figure II.13) were still time. The thermocline depth was rising evolving in January 1998. Although progressively from the west to the east anomalously cold waters west of Indonesia across the equatorial Pacific Ocean and the were no longer evident there was further normal Trade Winds returned in the east warming off the coast of equatorial East during May 1998. The winds caused mixing

(b) 140°E 160°E 180° 160°W 140°W 120°W 100°W (°C) (a) 8 Jan. 1998 0 12

0 8 100 3 2 1 4 200 (m) 0 300 Depth -4

400 -8

500 -12

(c) (d) 60°N 60°N

30°N 30°N

EQ EQ

30°S 30°S

60°S 60°S 0° 80°E 120°E 180° 120°W60°W0° 0° 80°E 120°E 180° 120°W60°W0° (m/s) (°C) 12 34567 -4-3 -2-101234 33 and upwelling and sea surface temperatures 60°N rapidly returned to normal. The rapid decline from May 1998 can be seen in the 30°N time sections of sea surface temperature (Figure II.14). Also, temperatures recorded EQ by some TAO buoys cooled by about 8°C over a 30-day period in the cold tongue of 30°S the central equatorial Pacific Ocean. 60°S By July 1998 colder than normal water 0° 80°E 120°E 180° 120°W60°W0° along the equator covered the central (W/m2) equatorial Pacific Ocean and further -50-40 -30 -20 -10 0 10 20 30 40 50 eastward (Figure II.17). Sea surface

temperature anomalies off the tropical coast 60°N of South America continued to be above

normal but the actual sea surface 30°N temperatures were again below the threshold of 28°C necessary to support deep EQ atmospheric convection (as seen previously in Figure II.14). Over the Indian Ocean, very 30°S warm waters had developed around 60°S Indonesia and South East Asia but cooler 0° 80°E 120°E 180° 120°W60°W0° than normal waters had appeared off the (°C) equatorial East African coast, the latter -4-3 -2-101234 indicating a return of the seasonal south winds and upwelling. The sea surface is important to know how the characteristics Figure II.17 temperature gradients across the western of the event fit into an historical context. Anomalies of sea surface temperature for July equatorial Pacific and the Indian Oceans had The SOI, sea surface temperatures of 1998. Colder than returned to their normal direction and were the central and eastern equatorial Pacific normal water has stronger than normal, indicating the Ocean, and departures in magnitude of appeared across the potential for strengthening of the Walker outgoing longwave radiation all respond to central equatorial Pacific Ocean but warm Circulation and the circulation over the an El Niño event. However, each of these anomalies persist near the Indian Ocean that supports the Asian indicators is a measure of different coast of South America. summer monsoon. characteristics of the climate system of the (NOAA/CDC, USA) By June or July 1998 the ongoing region. The SOI has proven to be a useful influence of the 1997–98 El Niño on the objective characteristic to make quantitative atmosphere was effectively exhausted. In comparisons between events of different particular, colder than normal waters had years. The value of the SOI as an indicator appeared in the central equatorial Pacific for comparison is enhanced because of the Ocean and were expanding eastward. Across availability of surface pressure records at the central and eastern equatorial Pacific Darwin and Tahiti and an historical record Ocean sea surface temperatures were below with few breaks can be constructed back to the threshold necessary to support deep the late nineteenth century. atmospheric convection. Thus, the linkage A disadvantage of using SOI for that allowed the anomalous energy source of historical comparison is that it is the ocean to force a response in the representative of only one characteristic of atmospheric circulation was effectively the El Niño/Southern Oscillation (ENSO), severed. namely the tropical cross-Pacific surface atmospheric temperature gradient. Other characteristics of ENSO are generally not An historical comparison completely in phase with the SOI and it is still not clear what are the most important During the 1997–98 El Niño event there factors for characterizing socio-economic Figure II.19 were significant changes from normal to the impacts. Although coupled in a general A comparison of the ocean and atmospheric circulations across sense, sea surface temperature fields, wind Multivariate ENSO Index for the seven strongest El the tropical Pacific Ocean. In using fields and the location of tropical deep Niño events since 1950 information about the 1997–98 event for atmospheric convection are not locked to showing the magnitude, future planning, particularly to mitigate the the surface pressure fields. The absence duration and life cycle negative impacts of future events and to of a universally agreed quantitative characteristics of each event. adopt sustainable development strategies, it definition for El Niño also increases the (Wolter and Timlin, 1998) 34 II. The 1997–98 El Niño event

Figure II.16 complexity of making objective historical The MEI values spanning a two-year Anomaly of outgoing longwave radiation for comparisons. duration for seven of the strongest El Niño January 1998 during the The Multivariate ENSO Index (MEI) has events since 1950 are shown graphically in mature phase of El Niño. been proposed as one such objective Figure II.19. Each event starts from near zero There is reduced deep method to combine a set of characteristics in at the beginning of the calendar year and atmospheric convection (positive anomaly — blue a way to generate a series suitable for builds strongly during the middle of the year to mauve shading) over comparison of different events. The MEI before reaching a plateau. However, there are Indonesia, the Philippines uses six variables from the tropical Pacific clear differences in the patterns of and Mexico and enhanced deep Ocean: sea level atmospheric pressure, zonal development between events. The 1982–83 atmospheric convection and meridional components of the surface event had the distinction of achieving the (negative anomaly — wind, sea surface temperature, surface air highest MEI departure, and that was early in yellow to red shading) over the central temperature and cloudiness fraction. These the second calendar year. The 1997–98 event equatorial Pacific Ocean observations have been collected and almost reached the same MEI departure value and off equatorial East published for many years. The MEI is but in the third quarter of the first calendar Africa. computed on a sliding bimonthly time-step year and six months in advance of the (NOAA/CDC, USA) that provides a degree of intraseasonal 1982–83 event. However, the 1997–98 event smoothing. In order to keep the MEI remained strong and achieved a second comparable, all seasonal values are departure maximum early in the second year. standardized to the respective seasonal time- In these comparisons using the MEI as a step and to 1950–93 as a reference period. measure, the 1982–83 and 1997–98 El Niño The MEI is displayed as a standardized events are clearly the strongest of the past 50 Figure II.18 Time series of the departure. years. Unfortunately, data are not adequate standardized departure of The historical MEI series from 1950 is to make comparisons with earlier events. the Multivariate ENSO graphically displayed in Figure II.18. The Even the relatively simple SOI series cannot Index (MEI) from 1950 to 1998. The MEI is an positive and negative departures represent be reconstructed before the late nineteenth objective combination of the warm and cold ENSO periods century and there are gaps in the series until six atmospheric and respectively. The series shows different the early twentieth century. Many aspects of ocean variables characteristics of each event, particularly in the 1877–78 event and others during the associated with ENSO. (Wolter and Timlin, 1998) terms of magnitude and duration. early period, particularly the socio-economic impacts, have been documented. However, it is a matter for further research as to 3 whether the 1877–78 event was comparable 2 to those of 1982–83 and 1997–98. 1 Nevertheless, El Niño events since 1950, 0 particularly the two major events of 1982–83 and 1997–98, provide valuable data about -1 Standardized departure weather and climate extremes that can be -2 expected to recur and for which mitigation 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 steps should be taken.

3

2

86/87 1 91/92 57/58 0 82/83 65/66 97/98

Standardized departure -1

-2 72/73 Jun./Jul. Jun./Jul. Dec./Jan. Dec./Jan. Dec./Jan. Mar./Apr. Mar./Apr. Sep./Oct. Sep./Oct. 35 Climate Anomalies and Impacts

The disruption to firstly artisan fishing and, acted as an energy source and established later, commercial fishing during the suitable conditions for deep atmospheric appearance of abnormally warm water off convection to occur further east than usual. the equatorial Pacific coast of South America The deep atmospheric convection was a has been the historical focus of socio- linkage through which the sea surface economic impacts associated with El Niño. It temperature anomaly was able to force an is now recognized that the abnormally warm atmospheric response. surface water that is the characteristic feature In the tropics and subtropics of the of El Niño events can trigger episodes of Pacific Ocean the altered pattern of deep extreme weather and seasonal climate atmospheric convection resulted in changes extremes in many parts of the globe. These to the large-scale zonal and meridional episodes have a relatively consistent pattern circulations of the atmosphere, such as the from one event to the next and produce Walker Circulation and Hadley Cell severe socio-economic impacts in many respectively. The changes in weather and regions. climate patterns across the tropical Pacific The 1997–98 El Niño was a major event Ocean can therefore be directly linked to the that provided strong forcing to the warmer sea surface patterns. atmospheric circulation. The global pattern The changes to the meridional of weather and climate during 1997 and overturning of the Hadley Cell also 1998 included near extremes over many strengthened the high atmosphere parts of the globe. Some of the more westerly flow of the subtropics, causing significant extremes are presented in the changes in the location and strength annual summaries of 1997 and 1998 of the jetstreams. In middle to high reproduced in Figure II.20, although not all latitudes the linkage between the El Niño of these anomalies can be attributed directly and climate anomalies was through or indirectly to the El Niño event. Direct or teleconnections involving atmospheric indirect linkages can, however, be made Rossby waves. The new locations of tropical between many of the climate anomalies and deep atmospheric convection triggered the El Niño event especially over tropical downstream waves in the upper atmosphere and subtropical latitudes. westerly flow. The eastward spread of abnormally A change in seasonal strength and/or warm sea surface temperatures associated location of the jetstreams is one with the El Niño event enhanced the local manifestation of teleconnections between exchange of heat and moisture between the the tropics and middle latitude weather ocean and the atmosphere over the eastern systems. From December 1997 through equatorial Pacific Ocean. The warm waters February 1998 (during El Niño’s mature

MAJOR GLOBAL CLIMATE ANOMALIES AND EPISODIC EVENTS IN 1997

Wet spring & Oct.-Dec. Periodic warmth much of year Cold/snow early (2-5 times normal) record spring snowmelt floods Record July flooding Periodic regional Warm/dry Wet Nov.-Dec. wetness Mar.-Sep. wildfires (July) Apr.-Oct. drought: Cold 55%-70% of normal Very dry August Cool in some precipitation Jan.-Mar. floods areas Feb.; Rain & snowmelt Apr.-May; Sep.-Oct. Numerous floods (Jan.-Feb.) Heavy tropical systems Jan. Heavy Oct.-Dec. snow Near-record mid-Aug. heat Summer summer & autumn; precipitation (Oct.) drought unseasonably wet Wet spring: floods, Oct.-Dec. Spotty floods/hail (July-Aug.) severe weather Remnants of Nora (Sept.) Excessive Danny (July) Oct.-Nov. rains Strong hurricane: Aug.: no named (10-60 times normal) Paka hits Guam (Dec.) area’s strongest on record, tropical storms possible world-record Dry end to Wet May.-Sept. Linda (Sept.) stays over water in Atlantic basin wind gust (380 km/h) wet season & Nov.-Dec.; Pauline (1st time since 1961) Rare snow/sleet (Dec.): devastates cool May-June Dry Oct.-Dec. first snow in 100 years Acapulco, Wet Oct.-Dec. rain in Guadalajara, Mexico surpluses 200-600 mm Mexico (Oct.) Frequent Intense rains Very strong El Niño develops, Severe flooding & several tropical early Oct.-Dec.; near or exceeding 1982-83 record- warmth Wet Jan.-Feb. systems water up to 5°C above normal Excessive 725-1240 mm Jan.-Mar.; Dec. above-normal rainfall Nov.-Dec. Dry Mar.-Apr. rains & all flooding Warm & dry Severe drought & last half of year year wildfires May-Oct.; regional May-Dec. Figure II.20 Storms & Heavy rain/tropical systems/ deficits 400-1600 mm; flooding (June) spotty flooding Jan.-Apr.; increased rainfall Nov.-Dec. Major climate anomalies slow start to 1997-98 wet season Dry Jan.; Apr.-June; Nov.-Dec.; and episodic events of Wet early; wet Feb.-Mar.; dry Mar.-May; warm Oct. & Dec. 1997 and 1998. very wet Oct.-Dec. (200-800 mm surplus) (NOAA/CPC, USA, WMO-No. 877 and WMO-No. 896) Source: Climate Prediction Center, NOAA, USA 36 II. The 1997–98 El Niño event

60°N phase) the deep atmospheric convection (m/s) over the central and eastern equatorial

Pacific Ocean caused increased upper 30°N 70 atmosphere divergence over the region. 60 Upper atmosphere anticyclones 50 EQ 40 strengthened poleward of the convection 30 region in both hemispheres and increased 20 ° the speed of the subtropical jetstreams. 30 S 10 Across the eastern North Pacific the subtropical jetstream extended further east 60°S ° ° ° ° ° ° ° than normal and influenced weather systems 0 80 E 120 E 180 120 W60W0 over the Pacific Ocean–North America regions. 60°N The mean upper atmosphere (200 (m/s) hPa — approximately 12 km altitude) 30°N wind flow and anomaly pattern for 25 January to March 1998 are shown in Figure 20 II.21. The strengthened anticyclonic EQ 15 circulations straddling the equator in the 10 5 eastern Pacific Ocean and the abnormally 30°S strong subtropical jetstreams of each hemisphere are clearly seen in the 60°S anomalous flow. 0° 80°E 120°E 180° 120°W60°W0° In the following sections a more detailed description will be given of South and Central Figure II.21 weather extremes and significant Mean and anomaly maps America of high atmosphere wind seasonal climate anomalies attributed to the flow (200 hPa is El Niño event that occurred in some regions approximately 12 km of the globe during the 1997–98 El Niño When there is anomalous sea surface altitude) during event, and the associated socio-economic temperatures and deep atmospheric January–March 1998. There was a well- impacts that were experienced. Neither the convection over the central and eastern developed anticyclone regional climate anomalies, nor the impacts, Pacific Ocean the continent of South pair straddling the should be taken as a complete America can be expected to be affected in equator over the eastern Pacific Ocean. documentation. The examples do, however, two ways. Firstly, the tropical parts will be Strengthened subtropical provide a picture of the geographical extent affected by the changes to the zonal westerly jetstreams and intensity of weather and climate circulations, including a contraction of the poleward of these systems extend across the extremes that occurred and had direct or Walker Circulation. Secondly, the active southern United States in indirect linkage with the 1997–98 El Niño convection and atmospheric heating will the Northern Hemisphere event. lead to a more active subtropical jetstream and across South America in the Southern Hemisphere. MAJOR GLOBAL CLIMATE ANOMALIES AND EPISODIC EVENTS IN 1998 (NOAA/NCEP, USA)

May-Aug. floods: Mild Jan.-Mar. May-Jun. up to 2 168mm rain heat wave Wetness/flooding surpluses to 772mm temp. to 48°C Hot & dry Frequent Jun.-Aug. Sep.-Nov. warmth Cold Nov.-Dec. Jun.-Aug. Wet/severe weather throughout heat waves year Severe Jan. Warm & dry/ Apr.-Jun. wildfires Stormy ice storm Jul.-Oct. Wet/cool Nov.-Dec. Very dry Dry Jul.-Oct. up to Jan.-May & Dec. Jun.-Dec. Oct.-Dec. Flooding 2 870mm rain; Periodic warmth Crop losses Dry Bonnie (Aug.) up Apr.-May surpluses to 915mm throughout year Very Jul.-Aug. Jan.-Jun. Jul.-Sep. to 250mm rain Severe flooding Brief but severe drought hot Unseasonably wet Jan.-Mar. Jun. 50% of Aug. flooding Death Valley, CA. Dry July May fires Abundant tropical normal rain Warm & dry Extreme Wet Jan.-Jun. approaches 54°C Warm Oct.-Nov. fires rains Jul.-Nov. Jan.-Mar. Oct.-Dec. Jul.-Aug. warmest in North America Zeb (Oct.) Georges (late Sep.) much flooding for 36 years (Jul.) Mitch Wetness/flooding O3B (Jun.) Babs (Oct.) severe damage to of the Charley (Aug.) (late Oct.) northern Caribbean; Jul.-Sep. Dry Jun.-Jul. wind damage, Wet up to 450mm rain heavy rain, year Wet/numerous ends drought up to 685mm rain central USA Gulf Coast Sep.-Oct. & flooding tropical systems Hot & dry Mar.-Jul. Dry Wet Sep.-Dec. Powerful El Niño Oct.-Dec. (Up to US$ 8 billion gives way to Jan.-Apr. drought damage moderate La Niña in southern USA) Wet Nov.-Dec. Sep. 97-May 98 11 to 49 times normal rainfall Dry Feb.-May Very warm & wet Warm & dry/ Wet Jan.-May wildfires Jun.-Dec. Stormy Severely Dry Jan.-May; Jan.-Apr. Oct.-Dec. Wetness/flooding Indonesian fires Jan.-May Largest Sep.97-May 98 Dry Sep.-Dec. Rainfall deficits: Philippines: 2 472mm Highest global annual average surface Indonesia: 1 613mm temperature on record Malaysia: 1 430mm

Source: Climate Prediction Center, NOAA, USA 37 and the downstream propagation of Rossby (a) (°C) (b) (hPa) waves that will affect subtropical and mid- 5 5 latitude weather systems. Both of these 4 4 EQ EQ responses were observed at times during 3 3 2 2 1997 and 1998. 10°S 10°S 1 1 Significant climate anomalies of 20°S 20°S 0 0 1997–98 over the region of South America ° ° 30 S -1 30 S -1 that can be attributed to the influence of the 40°S -2 40°S -2 El Niño event were: -3 -3 50°S 50°S • An abnormally warm surface layer in -4 -4 ° ° the waters of the eastern equatorial 60 S -5 60 S -5 120°W6080°W°W30°W 120°W6080°W °W30°W Pacific Ocean along the coast and (c) (m/s) (d) (W/m2) offshore (the El Niño itself); 14 50 • Unusual storm activity with above 12 normal rainfall along the central coast EQ EQ 30 ° ° of Chile, particularly during the 10 S 10 10 S 10

Southern Hemisphere winter and spring 20°S 20°S periods; 8 0 ° ° • Frequent and often heavy rains over the 30 S 30 S 6 -10 subtropics east of the Andes Mountains, 40°S 40°S 4 leading to severe flooding during May 50°S 50°S -30 1998; ° 60°S 2 60 S -50 • Intense summer rains with flooding and 120°W6080°W °W30°W 120°W6080°W °W30°W mudslides over the usually dry coastal regions of southern Ecuador and heavy rainfalls east of the Andes Mountains. Figure II.22 northern Peru; and The “blocking” high pressure systems Climate characteristics during August through • Suppressed rainfall over much of the contributed to a reduction of rainfall along October 1997 during the tropical region east of the Andes the far south coast of Chile and southern developing stage of the Mountains with drought over parts of Argentina. The montage in Figure II.22 El Niño event: anomalies the northeast of the continent until April shows three-month composite anomaly of a) sea surface temperature; b) surface 1998. maps for selected climate indicators covering atmospheric pressure; c) the period August through October 1997. 250 hPa (approximately Warmer than normal sea surface 10.5 km altitude) vector June–November 1997 mean wind; and d) temperatures were well established over outgoing longwave The El Niño event was developing during coastal waters during the Southern radiation. August through October 1997. The broad Hemisphere winter of 1997 and had (NOAA/CDC, USA) influences of the evolving El Niño on the extended westward into the eastern weather patterns of South America during equatorial Pacific Ocean. Actual sea surface these seasons were essentially twofold: temperatures rose during spring, reaching • Mid-latitude storms tended to be further values approaching the threshold for deep northward and more persistent under tropical atmospheric convection by October the influence of the enhanced 1997. Over the southeast Pacific Ocean and subtropical jetstream; and over the Atlantic Ocean sea surface • Convection and rainfall over the tropical temperatures were near normal from parts of the continent east of the Andes August–October 1997 (Figure II.22a). Mountains, including the Amazon River Surface air pressure that was below basin to the northeast coast, were normal extended over much of the tropical suppressed as a consequence of the and subtropical southeastern Pacific Ocean changed zonal circulations and subsiding and South America (Figure II.22b). The low air over the region. surface pressures, reflecting the eastern arm The direct influence of the subtropical of the cross-Pacific Southern Oscillation, jetstream was manifest as a tendency for were consistent with the negative values of “blocking” high pressure systems southwest the SOI at that time. The middle to high of the continent and for winter cyclones to latitude surface pressure anomaly field also follow a more northerly track from the indicated that the seasonal high pressure Pacific Ocean to the central regions of Chile. systems of the southeast Pacific Ocean were Much of central and northern Chile further to the south than usual. The region experienced above average winter rainfall. of lower than normal pressure offshore from There were also more storms with some Chile reflected the more frequent winter 38 II. The 1997–98 El Niño event

(a) (°C)(b) (hPa) Mountains, and the increased rainfall, is 5 5 identified by the reduced outgoing 4 4 longwave radiation over these parts of the EQ 3 EQ 3 continent. 10°S 2 10°S 2 1 1 20°S 20°S 0 0 December 1997–April 1998 30°S 30°S -1 -1 A positive anomaly of sea surface -2 -2 40°S 40°S temperature was maintained over the -3 -3 50°S 50°S eastern equatorial Pacific Ocean during -4 -4 ° January–March 1998 (Figure II.23a). Warmer 60 S -5 60°S -5 120°W6080°W °W30°W 120°W6080°W °W30°W than normal surface waters were also (c) (m/s)(d) (W/m2) 14 50 experienced southward along the coast to Chile. Surface atmospheric pressure also EQ 12 EQ 30 continued to be below normal over the

° 10°S 10 S 10 10 eastern equatorial Pacific Ocean, and this was consistent with the continuing negative 20°S 20°S 8 0 values of the SOI. The frequent strong 30°S 30°S anticyclones that tended to block to the 6 -10 40°S 40°S southwest of the continent maintained a

50°S 4 50°S -30 strong positive anomaly of surface pressure in that region (Figure II.23b). ° ° 60 S 2 60 S -50 120°W6080°W120°W30°W °W6080°W °W30°W During December 1997 through February 1998, sea surface temperatures Figure II.23 cyclones that tracked into the region of the over the Pacific Ocean offshore from Climate characteristics central coast of Chile, and that brought Ecuador and Peru exceeded the threshold during the Southern heavier than normal winter rains. value (about 28°C) necessary to support Hemisphere summer (January through March The upper atmosphere westerly winds deep convection. This was an outcome of 1998) during the mature of the subtropics were stronger than normal the combined influences of the warm phase of the El Niño (Figure II.22c — 250 hPa is at approximately anomaly of El Niño and seasonal warming. event: anomalies of a) sea surface temperature; 10.5 km altitude). The strengthened westerly The anomalous equatorial easterly winds of b) surface atmospheric jetstream was driven by the enhanced release the upper atmosphere (Figure II.23c) and pressure; c) 250 hPa of latent energy in the deep atmospheric the reduced outgoing longwave radiation (approximately 10.5 km altitude) vector mean convection occurring over the warmer waters (Figure II.23d) are consistent with a region wind; and d) outgoing of the equatorial central and eastern Pacific of deep atmospheric convection over the longwave radiation. Ocean. The eastern extension of the eastern equatorial Pacific Ocean and over (NOAA/CDC, USA) anomalous deep atmospheric tropical the coastal margins of Ecuador and Peru. convection over the eastern equatorial The intertropical convergence zone Pacific Ocean is identified by the region of moved further south than usual during the reduced outgoing longwave radiation (Figure Southern Hemisphere summer of 1997–98 II.22d). The increased outgoing longwave and was active over the coastal margins of radiation over equatorial regions of southern Ecuador and northern Peru. Deep continental South America was consistent convection developed over the normally dry with suppressed convection and reduced coastal regions of Ecuador and Peru. cloudiness associated with subsiding airflow. Torrential rains with floods, mudslides and The region covering Paraguay, northern destruction of infrastructure (including Argentina, southeastern Brazil and Uruguay highways and buildings) occurred over often receives more rainfall than normal in Ecuador from November 1997 to May 1998. winter and spring during El Niño events and Over northern Peru the rains started a month 1997 was consistent with the pattern. later but also continued until May 1998. Monthly precipitation records were Monthly values of departure from exceeded at different locations in central normal of outgoing longwave radiation over and northern Argentina during March, June the central and eastern equatorial Pacific and October 1997. Many rivers of northern Ocean (long 120°W to 180°W and long 60°W Argentina rose to levels significantly higher to 120°W respectively) for the period May than normal during the latter half of 1997. 1997 to August 1998 are plotted in Figure The increased frequency of winter and II.24. Also plotted is an index of precipitation spring storms tracking from the Pacific over the coastal margin of Ecuador. The Ocean and to the east of the Andes consistently negative departures of outgoing 39 20 10 000 Argentina, Paraguay and Uruguay exceeded 10 9 000 critical levels and were comparable to the 8 000 0 floods of 1983 and 1992, also during El Niño 7 000 2 -10 6 000 events. The below average outgoing Precipitation -20 5 000 longwave radiation was an indicator of the Eastern 4 000 teleconnection influence on the subtropical Central WATT/m -30

3 000 Precipitation -40 2 000 regions east of the Andes Mountains. -50 1 000 0 Impacts Jul. 98 Jul. 97 Jan. 98 Jun. 98 Jun. 97 Apr. 98 Apr. Feb. 98 Oct. 97 Sep. 97 May 98 May 97 Mar. 98 Mar. Dec. 97 Nov. 97 Nov. Aug. 98 Aug. 97 The El Niño phenomenon originally came to notice because of its significant negative Figure II.24 longwave radiation confirm the presence of impact on the biological productivity and Values of median monthly anomalous deep atmospheric convection marine resources along the Pacific Coast of outgoing longwave radiation over the central through the period to May 1998. However, it South America. The cessation of upwelling (long 120°W–180°W) was after the coastal sea surface temperatures cold water and the cut-off of the supply of and eastern (long exceeded the threshold for tropical deep nutrients alter the marine environment and 60°W–120°W) Pacific Ocean and an index of atmospheric convection and the intertropical the capacity of ecosystems to maintain the monthly precipitation over convergence zone moved south of the marine food chain and fish populations. the coastal region of equator during the Southern Hemisphere During 1997 and 1998 the abnormal Ecuador during summer that torrential rainfalls were warming off the coast of Ecuador exceeded 1997–98. (Cornejo (personal experienced over southern Ecuador and 5°C in the water column from the surface to communication) based on northern Peru. These normally dry regions 100 metres for a period exceeding data from INAMHI, received from 10 to nearly 50 times normal 10 months. An abundance of wild shrimp Ecuador) rainfall between September 1997 and May larvae to seed fish ponds meant that 1998. commercial hatchery products were in low Because the intertropical convergence demand. This led to a collapse of the shrimp zone was further south than normal, rainfall hatchery industry (made up of approximately over northern Colombia and Panama was 300 hatcheries employing 6 000 people). very much below average during December However, shrimp export revenues from 1997 and January 1998. However, as the Ecuador increased by 40 per cent in 1997. very active intertropical convergence zone There was, however, a migration of pelagic began to retreat northward following the species away from the warmer waters, annual cycle, very much above average resulting in a virtual collapse of the pelagic rainfall was received over Colombia and fisheries of Ecuador, Peru and Chile. Panama in February 1998. Despite several decades of research and The summertime pattern of positive the implementation of improved monitoring anomalies of outgoing longwave radiation systems there are still many unknown factors over tropical South America (Figure II.23d) relating to the various fish populations and was not as strong nor coherent as during the their sensitivity and vulnerability to the El earlier spring period (Figure II.22d). The Niño phenomenon. These need to be seasonal southward travel of the resolved if the marine resources are to be intertropical convergence zone was, to some managed as a sustainable industry. Systematic extent, countering the subsidence from the monitoring from a network of offshore anomalous convection over the eastern observing systems is required to generate equatorial Pacific Ocean. However, very dry necessary data to better understand sensitivity conditions persisted along the northeast of species to temperature and salinity Atlantic Coast from Brazil to Venezuela. changes, species migration and changing The anomalous subtropical westerly fertility during El Niño events. Collection, winds in the upper atmosphere weakened ongoing archival and analysis of the data in between September and December 1997. standard formats will assist multidisciplinary However, atmospheric teleconnection studies that are needed to reduce the processes associated with the still stronger economic and environmental risks and than normal upper atmosphere jetstream underpin sustainable development of the continued to be favourable for active marine resources. weather systems across the mid-latitudes of The El Niño event had a direct the continent. Very heavy rains during April influence on weather systems along the 1998 compounded the effects of earlier Pacific coast from Panama to southern Chile, storms and many river levels of northern with periods of flood or drought 40 II. The 1997–98 El Niño event

experienced. Over Panama and into mm mm 3 500 1 500 Normal northern Colombia rainfall was suppressed 3 000 Tumbes Cañaveral 1982/83 and accumulated totals from June 1997 to 2 500 1997/98 1 000 January 1998 were much below average. 2 000 1 500 500 Crops were lost and food supplies were 1 000 seriously affected. Shortages of water 500 0 0 Sep. Oct. Nov. Dec. Jan. Feb.Mar. Apr. May Sep. Oct. Nov. Dec. Jan. Feb.Mar. Apr. May disrupted operation of the Panama Canal mm mm and restricted ship traffic. Later, as the 1 600 4 000 1 400 Talara 3 500 Chulucanas intertropical convergence zone shifted 1 200 3 000 1 000 2 500 northwards and the El Niño influence was 800 2 000 600 1 500 rapidly declining during 1998, drought 400 1 000 turned to flood and more than 300 000 200 500 0 0 people required government assistance in Sep. Oct. Nov. Dec. Jan. Feb.Mar. Apr. May Sep. Oct. Nov. Dec. Jan. Feb.Mar. Apr. May mm mm Panama. 350 2 500 300 Reque Olmos 2 000 The accumulated rainfall from 250 September 1997 to May 1998 at several 200 1 500 150 1 000 locations on the north coast of Peru, and the 100 500 comparison with normal rainfall and the 50 0 0 1982–83 El Niño event, are shown in Figure Sep. Oct. Nov. Dec. Jan. Feb.Mar. Apr. May Sep. Oct. Nov. Dec. Jan. Feb.Mar. Apr. May II.25. In many locations the accumulated totals of summer rainfall were more than ten Corrientes Province of northern Argentina Figure II.25 times normal values. exceeded previous monthly rainfall records Accumulated rainfall for selected stations of Peru The heavy rainfall running off steep in December 1997 as well as in January and during 1997–98 and terrain along coastal Ecuador and northern April of 1998 (Figure II.27). comparison with normal Peru during the Southern Hemisphere The frequent heavy spring and summer summer rainfall and the summer caused extensive erosion and rains over northern Argentina, Paraguay, 1982–83 El Niño event. (Cornejo (personal mudslides, and resulted in loss of houses, Uruguay and southern Brazil caused rivers communications) based on roads and agricultural infrastructure, to rise and there was extensive flooding, data from SENAMHI, Peru) including irrigation systems. In Ecuador, especially after the heavy rains of April 1998. 90 000 families were evacuated and 70 per The data at Table II.1, showing the cent of the population suffered direct and maximum levels reached for locations in indirect losses. More than 30 000 people Argentina along the Paraná River and were left homeless as 7 500 houses and 440 Paraguay River, are typical for the region schools were destroyed or damaged. The and show that peak levels reached during death toll exceeded 200 persons as a result May 1998 were comparable to levels reached of drowning, landslides and related causes. during the 1982–83 and 1991–92 El Niño Seventy per cent of the coastal highway, events. including 15 bridges, was destroyed with the Below average rainfall was recorded overall loss estimated to exceed over a region extending from northeastern US $2 billion.

The very heavy rains over the region of 200 Figure II.26 southern Brazil, Paraguay, Uruguay and Monthly rainfall for 1997 Normal Argentina during the second half of 1997 150 and the 1961–90 Precipitation 1997 normal for Comodoro and early 1998 produced flooding and posed 100 Rivadavia, Argentina. problems for agriculture and water (National Meteorological 50 Service, Argentina) management. During the Southern Precipitation (mm)

Hemisphere winter of 1997 the heaviest 0 rains were in higher latitudes and a number 1 23456789101112 Months of locations in and near Chubut Province of Argentina received record rainfall during June 1997 (see Comodoro Rivadavia, Figure II.26). 600 Maximum Nov. 97 to Aug. 98 Over late spring and summer the 500 Normal 61/90 abnormally heavy rain was further north 400 Figure II.27 over the headwaters of major rivers. Monthly 300 Monthly, normal and rainfall records were exceeded at a number 200 previous maximum

Precipitation (mm) monthly rainfalls for Paso of locations during October and December 100 de los Libres, Argentina. 1997 and January, February, March and April 0 Nov. 97Dec. 97 Jan. 98 Feb. 98 Mar. 98 Apr. 98. May 98 Jun. 98. Jul. 98 Aug. 98 (National Meteorological of 1998. For example, Paso de los Libres in Months Service, Argentina) 41 Brazil to Panama. Many of the communities Rio Paraná of the affected area suffered drought. The Puerto PH(m) AC(m) AE (m) Alt 1983 (m) Alt.1992 (m) Alt. 97/98 (m) period of below average rains commenced Corrientes 3.23 5.50 6.70 9.04 – 18/Jl 8.64 – 08/Jn 8.39 – 04/My during June 1997 and continued until April La Paz 3.24 5.40 6.15 9.04 – 29/Jn 7.37 – 19/Jn 7.11 – 11/My 1998. A number of locations in Guyana had Paraná 2.53 4.20 5.50 6.83 – 05/Jl 6.89 – 21/Jn 6.72 – 13/My accumulated rainfall deficits exceeding 1 000 Rosario 2.54 4.00 5.30 6.15 – 23/Mr 6.27 – 25/Jn 6.43 – 15/My mm over the period and by March 1998 the San Nicolás 3.00 5.00 6.00 – 26/Mr 6.08 – 29/Jn 6.23 – 19/My discharge from many rivers was reduced to San Pedro 1.49 3.00 3.60 5.72 – 13/Jl 5.03 – 01/Jn 5.52 – 19/My about 20 per cent of normal flow. Ibicuy 2.50 2.02 514–12/Jl 4.20 – 02/Jl 4.73 – 20/My Crops over the region were severely affected by lack of rainfall and reduced river Rio Paraguay flow affected irrigation, especially in the Pilcomayo 3.18 5.50 6.00 9.22 – 30/My 8.79 – 02/Jn 7.60 – 06/My lower reaches where saline encroachment Formosa 3.81 7.00 7.75 10.78 – 05/Jn 10.13 – 11/Jn 9.46 – 07/My was an added problem. The land became parched under the hot tropical sun and wildfires were frequent during early 1998, Some of the climate anomalies over the Table II.1 many burning out of control until relief rains region of North and Central America during River levels (metres) reached at various arrived. Also, pastures were diminished the 1997–98 El Niño event were: locations on the Rio through drought or were burned by wildfire • An expanded area of tropical cyclone Paraná and Rio Paraguay and cattle were severely affected. activity over the eastern Pacific; following the abnormal Severe health problems compounded • Greatly reduced hurricane activity over Southern Hemisphere summer rains of the impacts of drought and crop failure over the tropical Atlantic Ocean during the 1997–98. Reference the northeast. Water supplies became 1997 season; levels and levels reached contaminated as creeks and streams stopped • Abnormally warm waters spreading during the 1982–83 and 1991–92 El Niño events flowing. As a consequence of contaminated along the Pacific Coast of the United are included for water many people were affected by States and Mexico; comparison. diarrhoea and water purification tablets were • Extreme drought over Mexico and most (National Meteorological Service, Argentina) distributed throughout communities. Crop of Central America from June 1997 Legend failures meant that local food supplies were through June 1998; PH: Historical inadequate and malnutrition particularly • Wetter than normal conditions over the average height affected children. Smoke caused respiratory Pacific West Coast and southeastern AC: Critical height AE: Height problems for many people in areas affected United States during November 1997 requiring by wildfires. through February 1998; evacuation • Much warmer than normal conditions Alt. Peak height in over the northern United States and (year): the year indicated North and Central sections of Canada during December America 1997 through February 1998; and • Extreme drought and heat over the During the last half of 1997 sea surface southern United States during April temperatures throughout the central and through June 1998. eastern equatorial Pacific Ocean remained Outside of the tropics in the Northern between 28°C and 29°C, which were Hemisphere the largest climate anomalies record values for that time of year. Strong associated with El Niño occur during the warm (ENSO) episode conditions continued winter and spring seasons. This seasonality until May 1998, when ocean surface in the El Niño response is also the time of temperatures cooled rapidly, signifying the maximum pole-to-equator temperature end of oceanic warm episode conditions. gradient. It is driven by the anomalous The persisting deep atmospheric convection patterns of heating by deep atmospheric over the eastern equatorial Pacific Ocean convection in the tropics and the direct and the meridional circulation of the Hadley interaction with the wintertime East Asian Cell had a major influence on the upper jetstream. As the seasons progress from fall atmosphere westerly flow and the to winter the East Asian jetstream normally subtropical jetstream. The influence of strengthens and shifts equatorward in subsiding air associated with the descending response to the strengthening Hadley Cell branch extended across Mexico. The United circulation. During strong El Niño episodes, States was affected by increased wintertime when deep tropical convection shifts to the Pacific storms and, through teleconnections, central and eastern equatorial Pacific abnormally heavy rainfalls over the Ocean, the East Asian jetstream extends southeast. further eastward than normal over the 42 II. The 1997–98 El Niño event

Figure II.28 (a) (°C)(b) (°C) 60°N 60°N Composite maps of a) 30 4 28 sea surface temperature; 50°N 26 50°N 3 and b) anomaly of sea 24 22 2 surface temperature for 40°N 40°N the three months August 20 18 1 30°N 30°N to October 1997. Note 16 0 that the warmest water is 14 20°N 12 20°N north of the equator but -1 the maximum anomalies 10 8 10°N 10°N -2 are south of the equator 6 and along the coast of 4 -3 EQ EQ Baja California. 2 0 (NOAA/CDC, USA) 130°W90110°W °W70°W50°W 130°W90110°W °W70°W50°W -4

eastern North Pacific Ocean to near cyclones were near normal but those that southern California, with only a slight formed occurred over an expanded area decrease in intensity. The jetstream then from normal. continues on a more southerly track than Easterly winds in the upper atmosphere normal across northern Mexico and the over Africa and the tropical Atlantic Ocean northern Gulf of Mexico. are crucial for creating an environment Figure II.29 favourable for tropical cyclone formation Tropical storm and hurricane tracks over the June–October 1997 over the Atlantic Ocean. In the absence of eastern Pacific Ocean these easterly winds, the vertical wind shear during the 1997 The abnormally warm sea surface throughout the Atlantic basin remained too hurricane season. (NOAA/NHC, USA) temperatures over the tropical eastern strong during 1997 to allow tropical storm equatorial Pacific Ocean during 1997 (see formation and hurricane activity was greatly 1 T Andres Jun 1–7 Figure II.28) created conditions favourable suppressed over the region (Figure II.30). 2 T Bianca Jun 8–12 3 T Carlos Jun 25–28 for an expanded area of tropical cyclone During what is normally the most active 4 H Dolores Jul 5–12 activity. The 1997 season featured 17 named period of the Atlantic Ocean hurricane 5 H Enrique Jul 12–16 storms, of which eight became hurricanes season (August through October) only one 6 H Felicia Jul 14–22 and six were classified as major hurricanes; hurricane was observed, making 1997 the 7 H Guillermo Jul 30–Aug 15 8 T Hilda Aug 10–15 this compares to an average of 17 named least active August–October period on 9 T Ignacio Aug 17–19 storms of which about 10 become record. 10 H Jimena Aug 25–30 hurricanes and about five become intense 11 T Kevin Sep 3–7 12 H Linda Sep 9–17 hurricanes. Hurricane Linda, during November 1997–March 1998 13 T Marty Sep 12–16 September 1997, was the area’s strongest on 14 H Nora Sep 16–26 record but stayed over water. Four of the Climate characteristics during January–March 15 T Olaf Sep 26–Oct 12 16 H Pauline Oct 5–10 1997 cyclones moved west of 140°W 1998 in North America can be seen in Figure 17 H Rick Nov 7–10 compared to a normal incidence of one. II.31. The abnormal deep convection across Two cyclones moved north of usual latitudes the central and eastern Pacific Ocean Hurricane (H) Tropical storm (T) with Nora moving into the southwestern enhanced the Northern Hemisphere winter- Tropical dep. United States and Pauline striking Acapulco time Hadley Cell over the Pacific Ocean and Position at 000 UTC 21 Position/date at 1200 UTC and causing extensive damage (Figure II.29). contributed to a stronger than normal 3 Tropical cyclone number Overall, the number and intensity of tropical subtropical jetstream in the upper atmosphere over the north Pacific Ocean. The Pacific-North America teleconnection pattern contributed to the Aleutian Low being more intense than normal, and to storms and stronger than normal westerly winds being directed toward the Pacific Northwest of the United States (Figure II.31b). Active storms affected the Pacific West Coast of the United States during the late fall and winter but it was particularly during February 1998 that heavy rains adversely affected California. Accumulated rainfall graphs for three widespread sites in California are in Figure II.32. As is characteristic for strong El Niño episodes, the high atmosphere subtropical jetstream continued eastward across 43 northern Mexico and the northern Gulf of Mexico (Figure II.31c). This strong jetstream was also part of the Pacific-North America teleconnection pattern and was a primary factor in the development of a large number of storm systems that affected the southeastern and eastern United States, producing wetter than normal conditions throughout the region. Several places from Louisiana to Florida received record or near record amounts of precipitation between December 1997 and March 1998. Graphs of accumulated rainfall at three locations are in Figure II.33. One consequence of the changed winter circulation was the strong flow of maritime air from the Pacific Ocean into the western United States. The generally more westerly flow and reduced incursions of very cold arctic air resulted in persistently warmer than normal conditions extending from the Hurricane (H) Pacific North West of the United States Tropical storm (T) eastward to New England. The anomalies The Pacific North West coast of the Tropical dep. Subtropical storm (ST) and percentile value of average daily United States was regularly battered by mid- Subtropical dep. temperature for the December 1997 to latitude storms with strong winds and rain + + + Extratropical Position at 000 UTC February 1998 period over North America during fall and winter. During February 21 Position/date at 1200 UTC are shown in Figure II.34. Temperature 1998, exceptionally heavy rainfall affected 3 Tropical cyclone number anomalies exceeded 3°C over a large area the whole State of California and some ppp minimum pressure (mb)

with maximum anomalies greater than 7°C; communities experienced flooding and Figure II.31 many temperatures exceeded the 80 mudslides. Record and near record Climate characteristics percentile value with those over a significant December through March accumulated over North and Central area exceeding the 90 percentile value. rainfall totals were received from many America during January to March 1998: a) locations over the region. anomaly of sea surface April–July 1998 The extensive area of significantly temperature; b) anomaly below average rainfall over Central America of surface vector mean wind; c) 250 hPa As the upper atmosphere jetstream retreated and Mexico during the fall and winter, (approximately 10.5 km poleward during the Northern Hemisphere shifted northward across the southern altitude) vector mean spring, the pattern of subsiding air on its United States during April through June wind and d) anomaly of outgoing longwave equatorward side also shifted northward. 1998. This was the driest such period in 104 radiation. The drier than normal conditions south of years of records in New Mexico, Texas, (NOAA/CDC, USA) the jetstream that had been confined to Mexico expanded northward to the southern (a) (°C) (b) (m/s) 60°N 60°N USA. Extremely dry conditions developed 4 6 5.5 50°N 3 50°N over many parts, but were most severe in 5 2 ° ° Florida and Texas. Accumulated rainfall 40 N 40 N 4.5 1 4 deficits at a number of locations In Texas are 30°N 30°N 0 3.5

shown in Figure II.35. 20°N 20°N -1 3

2.5 10°N -2 10°N 2 -3 Impacts EQ EQ 1.5 -4 130°W90110°W °W70°W50°W 130°W90110°W °W70°W50°W 1 (c) (m/s) (d) (W/m2) 60°N 60°N Tropical cyclones extended over a larger 50

area of the southeastern North Pacific Ocean 50°N 60 50°N 40 and increased the risk of landfall and loss of 30 40°N 50 40°N life and infrastructure damage to exposed 20 10 30°N 40 30°N coastal communities. Indeed, Acapulco, 0

20°N 20°N Mexico suffered severe damage as a result of 30 -10 -20 10°N 10°N a direct hit from tropical cyclone Pauline 20 -30

and tropical cyclone Nora crossed into the EQ EQ 10 -40 southwestern United States. 130°W90110°W °W70°W50°W 130°W90110°W °W70°W50°W -50 44 II. The 1997–98 El Niño event

Figure II.30 inches mm Louisiana and Florida. During the period, 110 2 750 Tropical storm and Blue Canyon, California 2 475 many locations across Florida and Louisiana hurricane tracks during 88 Accumulated observed 2 200 1 925 Accumulated normal received less than half of normal rainfall and the 1997 Atlantic 66 1 650 Hurricane Season. 1 375 broad sections of Texas and New Mexico 44 1 100 (NOAA/NHC, USA) 825 received less than 25 per cent of normal 22 550 275 precipitation. Extreme heat accompanied the 1 ST — Jun 1–2 0 0 Jun. Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. 2 T Ana Jun 30–Jul 4 1997 1998 dry conditions, which lasted into early inches mm 3 H Bill Jul 11–13 50 1 250 summer, even though the El Niño conditions 4 T Claudette Jul 13–16 San Francisco, California 1 125 40 1 000 were then rapidly dissipating. The heat and 5 H Danny Jul 16–28 875 6 H Erika Sep 3–15 30 750 drought in Florida contributed to extensive 625 7 T Fabian Oct 4–8 20 500 fires that burned nearly one-half million 8 T Grace Oct 16–17 375 10 250 acres. The dryness across the southern 125 0 0 United States was a dramatic change from Jun. Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. 1997 1998 inches mm the surplus precipitation observed in most of Figure II.32 40 1 000 Los Angeles, California 900 this region from late 1997 through early Accumulated rainfall and 32 800 700 March 1998. excess over normal at 24 600 three locations across 500 The abnormally warm coastal waters 16 400 California, USA: Blue 300 from Mexico to northern California had a Canyon; San Francisco; 8 200 100 major impact on marine life and fishing and Los Angeles. 0 0 Jun. Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. industries. The region includes the transition (NOAA/CPC, USA) 1997 1998 between the tropical and temperate eastern inches mm North Pacific Ocean — referred to as the 50 1 250 Tampa International Airport, Florida 1 125 Eastern Transitional North Pacific (ETNP). At 40 Accumulated observed 1 000 875 the transition zone the highly variable 30 Accumulated normal 750 625 conditions amplify the general problems of 20 500 375 lack of knowledge about physical and 10 250 125 biological mechanisms operating, and of 0 0 16 Nov . 1 Dec. 16 Dec. 1 Jan. 16 Jan. 1 Feb 16 Feb. 1 Mar. 16 Mar. 1 Apr. 1997 1998 lack of historical data and operational inches mm 50 1 250 monitoring systems. Mobile, Alabama 1 125 40 1 000 Even from the available data, however, it 875 30 750 is clear that the environmental temperatures 625 20 500 and the nutrient supply necessary for bio- 375 10 250 logical enrichment were altered during the El 125 Figure II.33 0 0 Niño event. Figure II.36 shows time series of 16 Nov . 1 Dec. 16 Dec. 1 Jan. 16 Jan. 1 Feb 16 Feb. 1 Mar. 16 Mar. 1 Apr. Accumulated rainfall and 1997 1998 inches mm sea surface temperatures at various locations excess over normal at 50 1 250 New Orleans/Moisant, Louisiana 1 125 along the ETNP coast. During the 1997–1998 three locations across 40 1 000 southeastern United 875 El Niño event temperatures reached record 30 750 States: Tampa, Florida; 625 high values at many locations. At least part of Mobile, Alabama; and 20 500 375 the change in temperature at the ETNP arises New Orleans, Louisiana. 10 250 (NOAA/CPC, USA) 125 from the relaxation of wind that is the 0 0 16 Nov . 1 Dec. 16 Dec. 1 Jan. 16 Jan. 1 Feb 16 Feb. 1 Mar. 16 Mar. 1 Apr. primary forcing agent for coastal upwelling. 1997 1998

(a) (b)

Figure II.34 a) Surface temperature anomalies (°C); and b) surface temperature expressed as percentiles of the normal (Gaussian) for December 1997–February 1998. Anomalies are departures from the 1961-–90 base period means. % (NOAA/CPC, USA) 10 20 30 70 80 90 45 El Niño effects on coastal fish resources — a case study*

* From: Anomalous environmental conditions in California peninsula. Preliminary data on Lluch-Cota D.B., coastal waters have several immediate effects landings indicate that the catch of brown Lluch-Belda D., on fish resources. The distribution of shrimp (Penaeus californiensis) peaked at Lluch-Cota S.E., López-Martínez J., migratory species changes, mortality 836 tons during the first half of 1998, while Nevárez-Martínez M.O., increases, there is decreased food availability for the same period of the previous year Ponce-Díaz G., for some species, and there can be increased (without El Niño influence) it only accounted Salinas-Zavala C.A. and Vega-Velazquez. predators; all these contribute to generally for some 100 tons. Only a minor fraction of Impactos en el sector poor fishing. The 1997–98 El Niño event the total catch of the species normally comes pesquero. In: Magaña provides examples of the impacts of from this area because of the nearby R.V. and A. Trasviña (eds), El Niño en anomalous environmental conditions on fish temperate limit of the normal species, México. CCA-UNAM, populations in the vicinity of the Gulf of distribution, and the contribution during CONACyT-CICESE, California. 1998 was much larger than expected. Mexico (in preparation). It is usual for fishing efforts to be Low mobility species are unable to alter directed to specific fishing grounds and often their normal distribution during an El Niño any changes in the population distribution event and are subject to severe through environmental change and migration environmental stress. The rocky community will be a problem. For example, the sardine associated with giant kelp (Macrocystis fishery of the Gulf of California is the largest pirifera) off the central Pacific Coast of the Mexican fishery in volume but during El Baja California peninsula clearly showed Niño years the population remains at the Big such impacts. This community is important, Islands and does not occupy the normal not only from the ecological standpoint, but spawning grounds of the central gulf. To also because of the fishing value of its highly reach the fish populations during El Niño prized components. El Niño has been years requires longer than normal trips and directly blamed for the decline in abalone higher operating costs. As a result, sardine (Haliotis spp.) catches, apparently through production in early 1998 decreased 40 per lack of food and direct physiological effects Table II.2 (below left) cent in Sonora and 24 per cent in Sinaloa, of high temperatures. Another fishery National Mexican marine with a loss of approximately US $4.5 million resource associated to these communities is fisheries exports, first semester (January to June) in value. Other important pelagic fisheries, the spiny lobster (Panulirus interruptus). 1997 and 1998. (Note: including tuna and squid, were also severely Catches during the first semesters plummeted Catch in metric tons, affected by distribution changes, reflected as from 781 tons in 1997 to 291 tons in 1998, value in US$ thousands.) a dramatic catch decrease in the former and with an associated loss of US $4 million. almost disappearance of the latter. Other In addition to the direct effects there are fish-related industries, such as those based other mid-term effects operating, including Table II.3 (below right) on small coastal fisheries and tourist sport through feeding, survival and reproductive Production (catch and fishing, were also affected. These activities success. Unfortunately, those effects have value) from the main are less important in terms of catch volumes, been poorly studied for most fisheries and fisheries of the Mexican Pacific, first semester but not in terms of societal impacts. are hard to estimate. Some observations are (January to June) 1997 Distribution changes can also result in available to identify deleterious and 1998. (Note: temporary benefits at the regional scale. One consequences of south-to-north advection catch in metric tons, value in US$ example from the 1997–98 El Niño event associated with coastal-trapped waves. The thousands.) occurred on the west coast of the Baja clearest example is from the advection of planktonic larvae of many marine species Difference 1997 1998 north of their normal geographic range of (1997–98) development. Environmental conditions, Catch Value Catch Value Catch Value such as temperature, in these new areas are Seaweeds 1 954 695 61 54 –1 893 –641 temporarily different and after returning to Tuna 29 817 39 357 33 166 46 712 3 349 7 355 normal are not suited for advected larvae. Squid 18 706 19 220 7 127 9 954 –11 579 –9 266 Shrimp 9 436 112 819 15 093 190 285 6 657 77 466 Another strong effect that has been observed Lobster 781 9 572 291 5 367 –490 –4 205 is the alteration of the reproductive cycle of Others many species resulting in off-season Canned 13 339 50 130 2 853 27 482 –10 486 –22 648 spawning. Since reproductive success is Other presentations 33 436 74 494 32 113 59 897 –1 323 –14 597 closely dependent on synchrony between Non-edibles 12 981 7 124 13 990 10 477 1 009 3 353 breeding and environmental conditions, off- TOTAL 120 450 313 411 104 694 350 288 –15 756 36 817 season spawning commonly results in poor recruitment. 46 II. The 1997–98 El Niño event

inches mm 20 500 Houston, Texas 16 400 Accumulated observed Accumulated normal An interim attempt to evaluate the 12 300 impact of the 1997–98 El Niño on regional 8 200 fisheries is presented in Tables II.2 and 4 100 0 0 II.3. During the first semester of 1998, 16 Apr. 1 May 16 May 1 Jun. 16 Jun. 1 Jul. 16 Jul. 1 Aug. three out of four massive fisheries in the 20 500 Dallas–Fort Worth, Texas region (sardine, squid and tuna) 16 400 underwent significant declines, above 12 300

200 000 tons, with a loss of some US $37 8 200 million. Fortunately, losses were partially 4 100 compensated by the increase in high 0 0 priced shrimp landings (some 6 000 tons, 16 Apr. 1 May 16 May 1 Jun. 16 Jun. 1 Jul. 16 Jul. 1 Aug. 20 500 Figure II.35 a marginal gain as volume) with an Austin, Texas 16 400 Accumulated rainfall and additional value of nearly US $30 million; deficits for three locations 12 300 thus, the net loss was reduced to about in Texas, United States: US $7 million. Nationwide, although the 8 200 Houston; Dallas-Fort 4 100 Worth; and Austin for export volume of fish products decreased April–August 1998. 0 0 about 15 per cent, their value increased by 16 Apr. 1 May 16 May 1 Jun. 16 Jun. 1 Jul. 16 Jul. 1 Aug. (NOAA/CPC, USA) 12 per cent, mostly because of increased shrimp landings. In societal terms, added benefits from one fishery do not compensate the industry and individuals of those fisheries USA suffering massive loss. While a few people accessing shrimp benefited during the anomalous conditions, many from other Mexico fishing industries suffered severe consequences through lost jobs and underperforming investments in fishing vessels and industrial processing plants and equipment. Clearly, ignorance about the sensitivity and vulnerability of different species is limiting the potential productivity of the industries and jeopardizing sustainable development. Improved knowledge of the behaviour of the species and an ability to integrate predictions of environmental conditions a season or more in advance can be achieved through multidisciplinary research and an investment in monitoring infrastructure. Without such an effort the industry remains in danger of catastrophic failure through a combination of financial crisis and accidental species destruction Figure II.36 during a future El Niño event. Sea surface temperature anomalies (°C) time series (1970–98) for some 2°×2° boxes along the 1997 1998 Catch Value Eastern Transitional North catch catch variation variation Pacific and indication of (t) (t) (per cent) ($000) ENSO events. (Luch-Cota, personal Squid 101 291 23 095 –77 –5 326 communication) Tuna 204 603 151 277 –26 –5 301 Sardine 718 577 508 133 –29 –9 680 Shrimp 39 187 50 393 +29 55 959 Total 1 063 658 732 898 –31 –4 348

47 China (a) (hPa)(b) (m/s) 60°N 60°N 5 16 4 One of the historical objectives that 14 50°N 50°N stimulated climate research was the search 3 2 12 for an explanation of recurring drought over ° ° 40 N 1 40 N 10 northern China. Many of the early studies 0 ° ° 8 were critical in assisting scientists develop 30 N -1 30 N an understanding of the Southern Oscillation -2 6 ° ° 20 N -3 20 N early in the twentieth century. 4 China extends from the subtropics in -4 10°N 10°N 2 the south to the middle latitudes in the 70°E80°E 90°E 100°E 110°E 120°E 130°E 140°E -5 70°E80°E 90°E 100°E 110°E 120°E 130°E 140°E north. During the Northern Hemisphere summer the Asian monsoon draws warm June–November 1997 Figure II.37 a) Sea level pressure moist tropical air over eastern China and anomaly; and b) surface generally produces high seasonal rainfall. The El Niño event was associated with a vector mean wind at 925 hPa (approximately The winter monsoon generates an outflow weak summer monsoon circulation over 750 m altitude) over Asia of cold dry air from the continent. Thus, much of Asia during 1997. Above average during August to October 1997. changes in the intensity of the monsoon surface pressure became established over (NOAA/CDC, USA) systems associated with the Southern much of tropical Asia, including southern Oscillation (and El Niño) will also affect the China, with the onset of the El Niño and this rainfall patterns. Year-to-year variability in was consistent with the strongly negative the accumulation of winter snow over the values of the SOI. As a consequence of the high Tibetan Plateau to the west has been higher than normal surface pressure and linked to anomalies in the following summer weakened monsoon circulation, the inflow monsoon and rainfall over agricultural areas. of warm moist air from the surrounding Significant climate anomalies over oceans was reduced. China during the 1997–98 El Niño were: The characteristics of the near surface Figure II.38 • Reduced rainfall over northern China circulation for August to October 1997 can Monntage of climate that lasted from summer of 1997 be seen in the composite sea level pressure indicators over Asia through to late winter of 1998; anomaly map (Figure II.37a) and low level during August to October 1997: a) 250 hPa • Very hot temperatures over northern wind flow (Figure II.37b). The consistent, (approximately 10.5 km China during summer of 1997 and though weak, westerly airflow over North altitude) anomaly of above normal temperatures continuing East Asia was a sufficient barrier to the vector mean wind; through winter; intrusion of warm moist tropical air over b) 250 hPa vector mean wind; c) anomaly of • Persistent winter rainfall over the north China during this period. The vertical motion at 700 southeast that saturated the soils and monsoon did not penetrate as far north as hPa (approximately 3 km raised river heights to record levels for normal and most regions north of the altitude); and d) anomaly of outgoing longwave that season; Yangtze River received significantly below radiation. • Exceptionally heavy snowfalls over the average rainfall. (NOAA/CDC, USA) Tibetan Plateau during the 1997–98 winter; (a) (m/s) (b) (m/s) 60°N 60°N • Abnormally high rainfalls following the 33 7 30 exceptional winter rains and spring 50°N 50°N 27 snowmelt produced record floods over 6 24 ° ° 40 N 5 40 N the Yangtze Basin after the El Niño 21

4 18 event; and 30°N 30°N

15 • Abnormally high rainfall over the 3 northeast during the summer of 20°N 20°N 12 2 1998. 9 10°N 1 10°N 6 In most years the atmospheric 70°E9080°E °E 100°E 110°E 120°E 130°E 140°E 70°E9080°E °E100°E 110°E 120°E130°E 140°E (c) (m/s) (d) (W/m2) 60°N 60°N convection over Asia during the summer 0.04 50 40 0.03 monsoon provides the seasonal focus of the 50°N 50°N 30 0.02 ascending branch of the Walker Circulation. 20

40°N 0.01 40°N Therefore, disruption to the Walker 10 Circulation by the eastward movement of 0 0 30°N 30°N deep atmospheric convection associated -0.01 -10 -20 -0.02 ° ° with an El Niño event will also have an 20 N 20 N -30 -0.03 effect on the organization of the monsoon -40

10°N -0.04 10°N -50 and associated rainfall. 70°E9080°E °E 100°E 110°E 120°E 130°E 140°E 70°E9080°E °E 100°E110°E 120°E130°E 140°E 48 II. The 1997–98 El Niño event

(a) (m/s)(b) (m/s) The southward shift in the jetstream 60°N 60°N 9 60 produced changes in the vertical motion

8 fields of the middle atmosphere, particularly 50°N 50°N 50 the increased tendency towards subsiding air 7

40°N 40°N north of the jetstream over central to 40 6 northern China (Figure II.38c — subsiding

30°N 5 30°N 30 air is shaded orange/green). The increased

4 outgoing longwave radiation over central to

20°N 20°N 20 northern China (Figure II.38d — positive 3 anomalies are shaded orange/green) is 10°N 2 10°N 70°E80°E 90°E 100°E 110°E 120°E 130°E 140°E 70°E80°E 90°E 100°E 110°E 120°E 130°E 140°E 10 consistent with the suppression of convection and summer rainfall over China Figure II.40 north of the Yangtze River. a) Wind anomaly; and As with other regions of southern and b) wind flow in the high eastern Asia, tropical storms, cyclones and atmosphere (250 hPa — L approximately 10.5 km) typhoons normally contribute a significant over Asia from January to L proportion of total rainfall over southern March 1998. China. Only four typhoons made landfall (NOAA/CDC, USA) over China during 1997 and this was the

Figure II.39 < 1000 lowest frequency in the past 46 years. The 700-999.9 Accumulated rainfall for 500-699.9 300-499.9 200-299.9 first landfall was not until early August 1997, winter and spring of 100-199.9 50-99.9 10-49.9 later than all previous first seasonal landfalls 1997–98 (11 November > 10 1997 to 10 March in the past 46 years. 1998) over China. (CMA/NCC, China) November 1997–March 1998 Rainfall across southern China from November 1997 through March 1998 was persistent and total accumulations were above normal (see Figure II.39). In Figure II.41 Mean temperature addition, heavy snowfalls led to an above anomalies (°C) over normal accumulation over the Tibetan China for the period Plateau. Some locations over the southeast December 1997 to February 1998. received in excess of 1 000 mm during the (°C) (CMA/NCC, China) -1.5-1.0 -.5 0 .5 1.0 2.0 3.0 period. Totals also generally exceeded previous record values and many across the region were more than twice the seasonal average. From January 1997 through March 1998, the axis of the high atmosphere jetstream continued to be further south than normal with anomalous westerly Figure II.42 winds extending from northwestern India to Total rainfall for the south of China (Figure II.40a). The axis < 1000 period June 1998 to 800–9999 600–7999 of maximum wind speed was across August 1998 over 400–5999 200–3999 100–1999 southern China and peak speeds were west China. 50–999 (CMA/NCC, China) > 50 of China (Figure II.40b). The equatorward shift of the high atmosphere jetstream is There were also critical changes to the linked to the reduced equatorial convection westerly wind flow of the high atmosphere. and weaker meridional circulation in the The subtropical jetstream shifted longitudes of East Asia associated with the El equatorward and there were stronger Niño. It is a contributing factor to the westerly winds over the Indian subcontinent enhanced winter rainfall across southern and through Myanmar and southern China; China. Over northern China, poleward of the over northwestern China the westerly winds jetstream, below average rainfall persisted were reduced (Figure II.38a). However, the into winter and early spring. Winter jetstream emanating from the region of the temperatures were more than 3°C above Asian summer monsoon remained strong normal over some parts of the region (Figure II.38b). (Figure II.41). 49 (a) (m/s) (b) (m/s) The maps of anomalous vertical motion 60°N 60°N 9 27 fields at 3 km (Figure II.43c) and anomalous

8 24 50°N 50°N outgoing longwave radiation (Figure II.43d) 7 21 have patterns over eastern China that are 40°N 40°N 6 18 consistent with the excess summer rainfall.

5 30°N 30°N 15 There is enhanced upward motion (Figure

4 12 II.43c — blue/mauve shading) over 20°N 20°N northeastern China and over southern China 3 9 on the southern exit region of the 10°N 2 10°N 6 70°E9080°E °E100°E110°E 120°E130°E 140°E 70°E9080°E °E100°E110°E120°E130°E140°E (c) (Pa/s) (d) (W/m2) subtropical jetstream. The regions of 60°N 60°N 0.04 50 reduced outgoing longwave radiation 40 0.03 50°N 50°N (Figure II.43d — blue shading) over 30 0.02 20 northeastern China and southern China

40°N 0.01 40°N 10 generally coincide with the areas of 0 0 increased rainfall. 30°N 30°N -0.01 -10 Although there was heavy rainfall over -20 -0.02 ° ° 20 N 20 N -30 southern China during the summer of 1998 -0.03 -40 the tropical storm and cyclone activity was 10°N -0.04 10°N -50 70°E9080°E °E100°E110°E120°E130°E 140°E 70°E9080°E °E100°E 110°E120°E130°E140°E again significantly below normal during the period. The first occurrence of a tropical Figure II.43 March–August 1998 storm making landfall over southern China Montage of climate during 1998 was not registered until early indicators over Asia during June to August The El Niño event was in decline after May August. This is consistent with the historical 1998: a) 250 hPa but above normal rain was recorded over record of few tropical storms forming over (approximately 10.5 km altitude) vector mean two broad regions of China during June the western Pacific Ocean and South China wind anomaly; b) 250 through August 1998 (see Figure II.42). The Sea during an El Niño event, hPa vector mean wind; c) anomaly of vertical highest rainfall totals were over the Yangtze notwithstanding that the El Niño was in motion at 700 hPa Basin in the south and the other region of rapid decline after May 1998. (approximately 3 km altitude); and d) anomaly above average rainfall was over northeast of outgoing longwave China extending into eastern Mongolia. Over Impacts radiation. the Yangtze Basin rainfall totals in the range (NOAA/CDC, USA) 700–900 mm were common and some China experienced major impacts from the regions exceeded 1 000 mm. The combined climate anomalies experienced during and effect of the heavy summer rains and those immediately following the period of the of the previous winter and the early 1997–98 El Niño event. The regions of snowmelt from the Tibetan Plateau was to climate anomaly were in the subtropics and cause many rivers to reach record levels. middle latitude and cannot be directly linked There was extensive flooding. to the longitudinal shift in equatorial Based on historical records the two convection associated with El Niño. most important factors determining the However, there is a high probability of strength of the summer monsoon over Asia causal relationships, at least into late spring are El Niño and the snow cover over the of 1998. The climate anomalies, particularly Tibetan Plateau. Both of these factors rainfall and wintertime temperatures, over contributed to the main summer rainbelt China are linked to persisting changes to the being located further south than normal over Northern Hemisphere subtropical jetstream the Yangtze River valley during the summer characteristics. Similar patterns of climate of 1998. The montage of maps of climate anomaly, particularly those over northern indicators in Figure II.43 identifies the effect China, have occurred during past El Niño of anomalous regional circulation on the events. rainfall patterns. The anomaly of high Drought occurred over parts of atmosphere wind flow (Figure II.43a) northern China because below average indicates that the axis of strongest westerly summer rainfall during 1997 persisted into winds is further equatorward than normal autumn and winter of 1998. Some areas had over eastern China. The overall effect is that their lowest precipitation for the period in the maximum winds of the jetstream extend 47 years. Most areas north of the Yangtze further eastward from the Tibetan Plateau River were affected to some extent by the and the axis of maximum winds shifts below average rainfall. southward over eastern China Winter floods are rare over southern (Figure II.43b). China but the persisting rains and above 50 II. The 1997–98 El Niño event

normal accumulations from autumn 1997 to (a) (hPa) (b) (m/s) 30°N 30°N winter 1998 caused waterlogging and 5 8 4 ° ° 7 flooding of fields and high river levels. 20 N 3 20 N

Record February levels were reached on the 2 6 ° ° 10 N 1 10 N Mingjiang River and critical flood levels 5 0 were exceeded on a number of rivers, 4 EQ -1 EQ including also the Xinjiang, Ganjiang and -2 3 Beijiang Rivers. By 16 March 1998 the level 10°S -3 10°S 2 of the Yangtze River at Hankou was 21.33 -4 20°S -5 20°S 1 90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E 90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E metres, the highest recorded for that time of (Pa/s) (W/m2) ° (c) ° (d) the year. The high levels of rivers, lakes and 30 N 0.05 30 N 50 0.04 40 reservoir storages by late winter made water ° ° 20 N 0.03 20 N 30 conservation and management very difficult 0.02 20 ° ° during the following spring. Another 10 N 0.01 10 N 10 complicating factor was the commencement 0 0 of spring runoff from Tibetan snowmelt up EQ -0.01 EQ -10 -0.02 -20 ° ° to a month earlier than normal in places. 10 S -0.03 10 S -30

There was major flooding and -0.04 -40 ° ° 20 S 20 S -50 extensive destruction over a wide area as a 90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E -0.05 90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E consequence of the heavy summer rains over the Yangtze Basin that followed record the intertropical convergence zone. South of Figure II.44 winter rains. The summer flooding was the equator there is a distinct peak in Montage of selected climate indicators over generally the worst since at least 1954 and rainfall from October to March. North of the the equatorial Asia-Pacific in parts set new all-time records. In terms of equator the peak rainfall comes with the region for August to the total area affected, the river heights southwest monsoon of the Northern October 1997: reached and the duration of flooding, the Hemisphere summer, and tropical cyclones anomalies of a) sea level pressure; b) surface 1998 summer flood achieved values rarely and storms moving across the Philippines vector mean wind; c) experienced in past records. enhance the seasonal rainfall in that region. vertical motion at 700 Overall, the 1998 summer flooding of Significant climate anomalies of the hPa (approximately 3 km altitude); and d) outgoing 2 the Yangtze River affected 212 000 km of 1997–98 El Niño event over the region were: longwave radiation. which 130 000 km2 was declared a disaster • Suppressed rainfall over the region with (NOAA/CDC, USA) area. The population affected within the drought in many parts; disaster area exceeded 223 million persons • Abnormally high temperatures and with more than 3 000 deaths attributed to drying of vegetation; the flooding. Approximately 5 000 homes • Increased frost incidence over highland were destroyed and the direct economic loss regions; and was estimated at 166 billion yuan. • Reduced tropical storm activity over the Western Pacific and South China Sea. The maritime continent is within the Equatorial Asia-Pacific ascending branch of the Walker Circulation during most years. The onset of the El Niño The equatorial Asia-Pacific region event during mid-1997 established warm sea considered here extends from Indonesia to surface temperatures across the equatorial Papua New Guinea and includes the islands central Pacific with temperatures exceeding of the Philippines. The region is also 28°C much further east than normal. Deep referred to as the “maritime continent” atmospheric convection extended eastward because of the very strong diurnal cycle of with the warm waters and the region of the the island rainfall distributions. The maritime continent came under the surrounding waters are warm all through influence of higher than normal surface air the year and daily solar heating over the pressure. Across equatorial Asia and the islands tends to establish local circulations western equatorial Pacific Ocean the normal with warm moist air from the sea pattern of ascending air associated with the converging inland and triggering deep Walker Circulation was disrupted. atmospheric convection and rainfall during the afternoons. June–November 1997 The islands of the region generally receive rainfall in all months of the year, The Northern Hemisphere summer although there are peaks of rainfall monsoon is the dominant climatic feature of associated with the seasonal movement of Asia and the intertropical convergence zone 51 moves northward over the western Pacific Ocean and South East Asia during this season. Disruption to these normal seasonal Drought was a prominent issue for Papua atmospheric circulations resulted in New Guinea during the second half of abnormal weather patterns over the 1997 and early 1998. Low rainfall is usual maritime continent during 1997. Firstly, the for the first half of the calendar year but deep tropical convection over the central by mid-1997 the effects of below normal equatorial Pacific Ocean and the changed rainfall had become a concern. Frosts in cross-Pacific Ocean sea surface temperature the highlands accompanied the dry gradient altered the atmospheric surface weather and, as forests dried, large-scale pressure patterns, with generally higher wildfires caused further damage. Farmers pressure over the maritime continent region. readying gardens for the next crop were Secondly, the deep atmospheric convection reported to have started many fires. was located over the central Pacific Ocean The developing impact of the below and tropical storms and cyclones tended to normal rainfall was for a deterioration in form further east than usual. The impact of living conditions as food crops wasted or the El Niño on the seasonal climate failed and as water quality and supplies circulation can be seen in the montage of declined. The lack of water disrupted climate indicators in Figure II.44. production at two of Papua New Guinea’s Above normal sea level pressure major mines. By the end of 1997 the persisted over the maritime continent decline in export revenue from mining (Figure II.44a) and the anomaly extended added to downward pressure on the Papua New Guinea currency exchange Figure II.45 (a) rate. There were major disruptions to Maps of duration in months during 1997 electricity production in the Port Moresby when consecutive monthly area and government operations were rainfall was a) less than affected, mainly by disruptions to water 150 mm per month and electricity supplies. (defining the “dry 10 months 8 months season”); and b) less 6 months The key food crop for about 65 per 4 months than 50 mm per month. 2 months cent of the population is sweet potato, 0 months (MGA, Indonesia) with the dependency greatest in the (b) highlands. Sweet potato is mainly grown Figure II.46 under a continuous cultivation process Cumulative rainfall over and takes between five and 12 months to the Philippines for a) July to September 1997; and mature. The frosts and drought disrupted

b) October to December 10 months the crop cultivation and ensured that food 8 months 1997, as a percentage 6 months shortages would ultimately occur. Sago is 4 months of normal for each 2 months season. 0 months the staple food for a further 15 per cent of (PAGASA, Philippines)

from the Asian land mass to Australia. The (a) ≥ 40% (b) ≥ 40% equatorial low level winds are generally 41-80 41-80 light and small changes can have significant 81-120 81-120 impact on low level convergence and the > 120 > 120 location of convection. The significant anomalies in the wind flow (Figure II.44b) are firstly, the increased strength of the easterly winds west of Sumatra and secondly, the tendency for strengthened southerly winds east of Papua New Guinea becoming westerly winds north of the equator. The pattern in the anomalies over the South China Sea and western Pacific Ocean is not strong but is consistent with a weaker than normal monsoon and reduced convergence of air over the maritime continent. The computed positive anomalies of vertical velocity in the middle atmosphere 52 II. The 1997–98 El Niño event

Papua New Guinea — a case study* the population and sago palms mainly grow 7:00 am and midday, and the other half of *Source: Report by the in the lowlands. Large quantities of water the city was supplied between 12:30 pm and Australian Agency for are required to process sago and water 6:00 pm. The lack of water forced a shift to International Deveiopment (AusAID) shortages cut back food production; some more expensive diesel-fired power plants palms were also lost in wildfires. with an overall increase in cost of A drought assessment concluded that US $10 million. by December 1997 around 260 000 people International aid to Papua New Guinea were in a critical life-threatening situation was commenced in December 1997 and with no food other than that collected from supported national efforts to assist the the bush (e.g. leaves, seeds and uncultivated estimated 1.2 million people suffering a tubers). A further 980 000 were estimated to severe, and to some a life-threatening, food have small and inadequate amounts of food shortage. Australia initiated a discrete available from gardens, sago palm, coconuts drought relief programme that was operated or freshwater fish. Water supply was also a jointly with the national government. The major problem with 47 000 people having focus was the delivery of food to the worst only limited amounts of water for drinking affected and inaccessible areas by Australian and another 363 000 carrying water of and Papua New Guinea defence personnel. questionable quality extensive distances. In total, Australia contributed approximately Inadequate food and water had an impact US $19.5 million to drought relief with other Figure II.47 on people’s health. The problems were international donors providing direct support Accumulated rainfall and spread across all provinces. of around US $5 million. The national deficit from normal for the The two largest mining ventures, Ok government provision for drought relief is period March 1997 to February1998 at Tedi and Porgera, had their operations estimated to have been more than Kavieng, Papua New disrupted between August 1997 and US $12.5 million. Guinea

February 1998 because of lack of water for 3 500 operations and river navigation. The loss of 1997-1998 mineral export revenue because of drought 3 000 Deficit could be as high as US $200 million. 2 500 Hydroelectricity normally supplies between 60 and 70 per cent of Papua New 2 000 Guinea’s electricity needs and the level is 1 500 approximately 90 per cent for Port Moresby. 1 000 Water levels in the Port Moresby system had 500

reached low levels by June 1997 and load Accumulated rainfall (mm) shedding commenced in November 1997 — 0 half of the city was supplied between Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb.

(Figure II.44c — 700 hPa is approximately occurs within the period April and 3 km altitude) show a consistent pattern of September with minimum rainfall in July and suppressed ascent or even subsiding air August. The season coincides with the (shaded yellow/green) south of the equator. outflow of air from Australia during the The tendency for reduced upward motion Southern Hemisphere winter. Consequently, also extends into the Northern Hemisphere the islands towards the eastern end of the over Kalimantan and past the southern archipelago tend to have a more marked dry Philippines into the western Pacific Ocean. season than do those to the west. The The positive anomaly of vertical motion impact of the 1997–98 El Niño event on (reduced ascent and suppression of rainfall over Indonesia was to significantly convection) and the positive anomaly extend the dry season over many parts. The pattern of outgoing longwave radiation over map in Figure II.45a shows that the duration the same region (Figure II.44d — positive of the dry season during 1997 was more anomalies green/orange) are consistent with than six months over a large part of reduced convection and rainfall over the Indonesia, and the duration was between islands of the maritime continent. eight and ten months for many islands of the The “dry season” over the islands of southeast. The map in Figure II.45b shows Indonesia (defined as consecutive months that many parts received less than 50 mm with less than 150 mm of rainfall) usually per month for extensive periods. 53 Over the Philippines there were significant breaks in the southwest monsoon and many places received below average rainfall. However, during the monsoon bursts, some localities experienced local Taiwan storms with heavy rainfall that caused 8 flooding and landslides. Predominantly the 20°N rainfall was below normal with only a few 12 9 L parts with above normal rainfall. The 3 6 southwest monsoon terminated earlier than 14 2 normal during September 1997. Maps of L 5 three-month cumulative rainfall over the 7 11 Philippines 10 Philippines, as a percentage of normal, for 13 the periods July to September 1997 and 4 October to December 1997 are in Figure II.46. 10°N Tropical cyclone frequency over the 1 western Pacific Ocean was less than normal during 1997. Overall, during the second half of 1997 tropical cyclones tended to form further east than normal and the tracks 120°E 130°E curved northward before coming into the Philippines are indicative of a weakened 1 TD Atring Jan. 21–27 Philippines region. No tropical cyclones Asian winter monsoon (Figure II.49b). 2 TS Bining May 26-29 entered the Philippines area of responsibility The anomalous subsiding air (Figure 3 T Kuring Jun. 15-18 4 TS Daling Jun. 22-26 during September, the first time in 30 years, II.49c — shaded green/yellow) and 5 T Elang Jun. 22-26 compared to the usual two to three during increased outgoing longwave radiation 6 TS Goring Jul. 30-31 that month. The tracks of all tropical (Figure II.49d — shaded green/yellow) over 7 T Huling Jul. 31-Aug.06 8 T Ibiang Aug.15-18 cyclones passing through the Philippines northeastern Indonesia, the Philippines and 9 TS Luming Aug. 21-29 area of responsibility during 1997 are shown north of the equator identify continued 10 T Miling Aug. 21-29 in Figure II.48. Rainfall associated with suppression of deep convection normally 11 T Narsing Oct. 16-22 12 T Openg Nov. 04-05 tropical cyclones is usually a significant expected over these regions. 13 T Pining Nov. 12-16 contributor to annual rainfall, and the The broad region of subsiding air that 14 T Rubing Dec. 19-23 reduced number of tropical cyclones in 1997 extends eastward from Kalimantan Figure II.48 was one reason for the generally below represents the continuing influence of the El Tracks of tropical average rainfall. Niño event over the western Pacific Ocean. cyclones that entered the Rainfall over the Philippines during the first Philippines area of half of 1998 was significantly below normal responsibility during November 1997–June 1998 1997. with most parts of the country receiving less (PAGASA, Philippines) During the Northern Hemisphere winter the than 50 per cent of normal in the January to Asian winter monsoon dominates the March period. The very dry conditions weather of the South China Sea and the persisted until the onset of the southwest Philippines. Northeast winds prevail during monsoon in late May. June was also a month the period. South of the equator, the of below average rainfall over much of the intertropical convergence zone is generally Philippines. Cumulative rainfall over the active with light winds and deep Philippines as a percentage of normal for atmospheric convection over Indonesia and the periods January to March 1998 and April Papua New Guinea, although there are to June 1998 is shown in Figure II.50. periods of west-northwest winds in response There were no tropical cyclones over to the Australian summer monsoon. A the Philippines area of forecasting montage of anomalies of climate indicators responsibility during the first half of 1998 over the region for the period January to and that was another indicator of the March 1998 is in Figure II.49. continuing influence of the El Niño over the Sea level pressure over the maritime western Pacific Ocean. continent continued to be significantly South of the equator, the vertical motion above normal during the early part of 1998 (Figure II.49c – ascending shaded blue) and (Figure II.49a). The anomalous sea level outgoing longwave radiation (Figure II.49d – pressure was consistent with the mature El negative anomaly shaded blue) were Niño event and the negative values of the consistent with a return of deep atmospheric SOI. The south-southwest surface wind convection and rainfall to western Indonesia anomalies over the South China Sea and the and eastern Papua New Guinea. 54 II. The 1997–98 El Niño event

(a) (hPa) (b) (m/s) 30°N respectively. Indonesia moved from a net 5 30°N 8 4 exporter to a net importer of food grain 20°N 7 3 20°N during the period. International food aid 2 6 averted starvation in the highlands of Papua 10°N ° 1 10 N 5 New Guinea and all governments of the 0 EQ 4 -1 EQ region had to increase spending on food

-2 3 support measures. Decreased agricultural 10°S ° -3 10 S 2 activity meant that many agricultural -4 ° workers lost jobs and moved to urban areas 20 S 20°S 1 90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E -5 90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E (c) (Pa/s) (d) (W/m2) seeking employment, often without success. 30°N 30°N 0.1 50 Drought reduced the amount of stored 0.08 40 20°N 20°N water available for drinking, industry, 0.06 30

0.04 20 irrigation and energy production. Reduced 10°N 10°N 0.02 10 hydroelectric generation in the Philippines 0 0 and Papua New Guinea meant a shift to EQ EQ -10 -0.02 -20 thermal power and a need to spend foreign -0.04 10°S 10°S -30 reserves on imported fuel oil. Water -0.06 -40

-0.08 -50 restrictions were necessary in many 20°S 20°S ° ° ° ° ° ° ° ° 90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E -0.1 90 E 100 E 110 E 120 E 130 E 140 E 150 E 160 E places, including metropolitan Manila and many water districts across the Figure II.49 Impacts Philippines. Montage of selected Extreme heat and water shortages climate indicators over The major impacts of the 1997–98 El Niño contributed to health-related problems in the equatorial Asia-Pacific region from January to event across the equatorial Asia-Pacific region many parts of the region. In the most March 1998 showing resulted from the significantly reduced rainfall severely drought affected parts people composite anomalies of over an extended period in many areas. suffered malnutrition and their natural a) sea level pressure; b) low level (approximately Agriculture, water supply, energy production, capacity to fight diseases was reduced. Lack 750 metres) wind flow; the environment and human health were all of potable water and a deterioration in c) vertical motion at affected by regional drought. sanitation and hygiene increased the 3 km; and d) outgoing longwave radiation. As a consequence of drought there was incidence of water-related communicable (NOAA/CDC, USA) decreased agricultural production over many diseases. Malaria, cholera, typhoid fever and parts of the region. In addition, frosts were dengue were reported to be more prevalent more frequent in some of the higher areas, across the region and are direct impacts of Figure II.50 particularly over Papua New Guinea, the anomalous climate conditions associated Cumulative rainfall over the Philippines for (a) because of the drier air and reduced with the El Niño event. January to March 1998 cloudiness; food crops were destroyed in The most widely reported impact of the and (b) April to June some of the highlands. In the Philippines 1997–98 El Niño event over the region was 1998 as a percentage of normal for each season. rice and corn production declined by more the problem of smoke haze from out-of- (PAGASA, Philippines) than 40 per cent and 25 per cent control fires. Virgin forests, parks, reserves and agricultural lands were caught up in the conflagrations. The extent of out-of-control fires, especially those over Kalimantan, was (a) ≥ 40% (b) ≥ 40% 41-80 41-80 closely monitored using satellite imagery but 81-120 81-120 often the necessary equipment and trained > 120 > 120 personnel were not in place to control the fires. Many fires burning in inaccessible forests were impossible to control. Not only were fires out of control but also burning continued to be used as a method for clearing the forests for future agriculture. The thick smoke and haze was a direct threat to human health through respiratory problems and skin disorders. In addition to the immediate health problems and the loss of forest resources from the fires there was significant environmental damage to flora and fauna. The land surface, often in hilly terrain, was left exposed to future erosion when monsoon rains returned. 55 (a) (hPa) (b) (m/s) Figure II.51 EQ 5 EQ 14 Maps of a) sea level 4 pressure anomaly; and b) 10°S 10°S 12 3 high atmosphere (250 2 10 hPa is approximately ° ° 20 S 1 20 S 10.5 km) wind anomaly 0 8 for June to August of ° ° 1997. 30 S -1 30 S 6 (NOAA/CDC, USA) -2 40°S 40°S -3 4 -4 50°S 50°S 2 140°E160°E 180 160°W 140°W 120°W-5 140°E 160°E 180 160°W 140°W 120°W South-West Pacific Rainfall decimal ranges The climate of the South-West Pacific Ocean Highest on record is extremely variable from year to year and 10 Very much above average the variability is linked to the El Niño/ 80 Above average Southern Oscillation. Reports from as early 47 Average as the latter half of the nineteenth century 20 Decrease/increase note the tendency for prolonged dry 1 Very much record average conditions to prevail simultaneously across Lowest on record many parts of Australia, New Zealand and the islands of the South-West Pacific Ocean to the Date Line. For example, contemporary descriptions of the droughts of 1877–78 and the very wet conditions experienced during expect to receive rainfall during the summer Figure II.52 1871 reflect the broad climate controls months. The Pacific Islands and the coastal Map of June to August 1997 rainfall deciles operating to modulate seasonal weather parts of northern Australia can receive over Australia showing patterns. East of the International Date Line rainfall during the remainder of the year as a the extensive area with there are also broad controls but the phase consequence of showers developing in the below and very much is reversed — rain and tropical storms are prevailing Southeast Trade Winds. below average rainfall. (BOM/NCC, Australia) more frequent during El Niño events but are Southeastern Australia and New Zealand suppressed during periods when the SOI is come under the influence of mid-latitude positive. cyclones and fronts during winter as the Significant climate anomalies over the westerly wind flow moves northward. region of the South-West Pacific Ocean were: • Blocking high pressure systems over May–October 1997 southeastern Australia during the Southern Hemisphere winter and spring The atmospheric circulation over the South- and abnormally persistent westerly West Pacific Ocean underwent significant winds across New Zealand; change following the onset of the El Niño • Reduced rainfall away from the equator, event during the Southern Hemisphere particularly westward of the autumn of 1997. Surface high pressure International Date Line; and systems tended to “block” over southeastern • Increased frequency of tropical storms Australia and sea level pressure was and cyclones east of the International generally lower than normal over the central Date Line. South Pacific Ocean in the region of The “wet season” of northern Australia Polynesia (Figure II.51a). The pattern of and the tropical islands of the South-West winter sea level pressure anomaly is typical Pacific lasts from approximately November for an El Niño event and is consistent with to March and corresponds to the period of the negative values of the SOI (lower warmest ocean temperatures and maximum pressure over Tahiti and higher pressure solar heating during the Southern over Darwin) that had become established Hemisphere summer. This is also the period and persisted during the period. At the same when tropical storms and cyclones can be time the subtropical jetstream in the high expected over the warm waters of the South atmosphere strengthened and moved further Pacific Ocean and the Coral Sea. eastward with a maximum wind anomaly in Southeastern Australia and New Zealand, in the region of the central South Pacific Ocean the more temperate mid-latitudes, also (Figure II.51b). 56 II. The 1997–98 El Niño event

(a) (m/s) (b) (W/m2) Figure II.53 EQ EQ 6 Maps of a) vector mean 50 surface winds; and b) 5.5 ° ° 40 10 S 5 10 S anomaly of outgoing 30 longwave radiation over 4.5 20 ° ° the South-West Pacific 20 S 4 20 S 10 Ocean from December 3.5 0 1997 to February 1998. 30°S 3 30°S -10 (NOAA/CDC, USA) 2.5 -20 -30 40°S 2 40°S -40 1.5 -50 50°S 1 50°S 140°E160°E 180 160°W 140°W 120°W140°E 160°E 180 160°W 140°W 120°W

Figure II.54 200 average (Decile range 1 — in the lowest ten Composite of monthly 150 percentile) over large areas (Figure II.52). rainfall from 21 representative sites over 100 Over the Coral Sea and North Tasman Sea New Caledonia during 50 the strengthened pressure gradient caused the period January 1997 0 the prevailing southeasterly winds to be to May 1998 relative to average. -50 stronger than normal.

(Météo-France) Precipitation (% of average) -100 During the Southern Hemisphere spring, as the El Niño was developing Jul. 97 Jan. 97 Jan. 98 Jun. 97 Apr. 97 Apr. Apr. 98 Apr. Feb. 97 Feb. 98 Oct. 97 Sep. 97 May 97 May 98 Mar. 97 Mar. Mar. 98 Mar. Dec. 97 Nov. 97 Nov. Aug. 97 towards its mature phase, sea level pressure

mm (a) remained higher than normal over Australia 14 400 Accumulated average rainfall and southeast winds were strong over the 12 400 Accumulated actual rainfall Coral Sea and the South-West Pacific Ocean. 10 400 Also, persistent west-southwest winds were 8 400 experienced along the mountainous west 6 400 coast of New Zealand. In the high 2 400 atmosphere the subtropical jetstream winds 400 continued to be located further eastward Jul. Jul. Jan. Sep. Sep. May May Mar. Nov. Nov. 1997 1998 than normal over the central South Pacific mm (b) Months 18 000 Ocean. 16 000 Despite the continued blocking high 14 000 Figure II.55 12 000 pressure systems over southeastern Australia Accumulated monthly 10 000 a number of rain systems were able to move rainfall over representative 8 000 over central and eastern Australia such that stations of a) northern and 6 000 b) western Fiji for the 4 000 much of the country received average to period May 1997 to 2 000 above average spring rainfall totals. December 1998 0 Jul. Jul. Jan. Sep. Sep. May May Mar. compared to normal. Nov. Nov. 1997 1998 (Fiji Meteorological Service) Months December 1997–May 1998 Three “off-season” cyclones affected Fiji Higher than normal sea level pressure during 1997. Tropical cyclone June, only the continued to be experienced over Australia fourth cyclone on record since 1840 to and the South-West Pacific Ocean while the threaten Fiji during the off-season, affected El Niño event was at its mature phase during the northwestern parts in early May. the Southern Hemisphere summer of Damaging winds up to 100 km/h and some 1997–98. The tendency for high pressure heavy rain were reported. Tropical cyclone systems to block was maintained but the Keli, the first cyclone reported in June, focus moved from southeast Australia to the passed about 450 km northeast of Fiji. South Pacific Ocean east of New Zealand. Tropical cyclone Lusi passed near western Enhanced subsidence associated with the Fiji during mid-October with wind gusts to blocking high pressure systems resulted in 93 km/h and widespread rain. relatively dry air, clear skies and stronger Over southeastern Australia the than normal southeasterly winds over the blocking high pressure systems pushed the subtropics and the islands of the South-West wintertime mid-latitude cyclones further Pacific Ocean. Four episodes of strong wind southward. Consequently, very few cold over Fiji, lasting in total about 20 days, fronts or cyclones passed over the region disrupted sea transport. The drier air and and winter rainfall was below average clear skies affected temperatures over the (Decile range 2 or 3) and very much below islands. A number of record low minimum 57 Figure II.56 winds, received significantly above average Rainfall as a percentage rainfall while east of the New Zealand of average over New Zealand for the period mountains rainfall was below average and October 1997 to March parts experienced drought (Figure II.56). 1998 and showing the wetter conditions in the west as a result of the Impacts stronger onshore winds, and reduced rainfall over Communities living on the coral atolls and the east in the lee of the small islands of the South-West Pacific mountains. (NIWA, New Zealand) Ocean are particularly susceptible to climate anomalies. There is generally limited water storage beyond groundwater, including the freshwater lenses of atolls, and the runoff 150% from short mountain streams. Impacts of 125% prolonged dry periods are quickly felt and become severe if there is no external relief. 100% Conversely, communities are often severely 75% affected when tropical storms and cyclones 50% strike because of deaths from wind-borne debris and storm tides, and the aftermath effects of destroyed infrastructure and stored temperatures were established across Fiji food, lost crops and disease epidemics. during the dry seasons of 1997 and 1998 but The impact of the 1997–98 El Niño record high temperatures were established event varied over the region but the most during February 1998. common issue was reduced seasonal rainfall The composite surface wind pattern and drought. This was a particular problem over the South-West Pacific Ocean for the to many of the islands of the South-West Southern Hemisphere summer (December to Pacific west of the Date Line. However, February) of 1997–98 is shown in because of the changed wind regimes some Figure II.53a. The region of light winds regions, such as the western parts of New identifying the intertropical convergence Zealand, received significantly more rainfall zone extends from the north Coral Sea than normal. There were many more tropical eastwards to the region of Polynesia. There cyclones over the central South Pacific is active deep atmospheric convection Ocean than normal causing severe damage associated with the intertropical to several island communities. convergence zone but the focus of the Over Fiji the influence of reduced convection is in the central equatorial rainfall was not felt until September 1997, Pacific Ocean (Figure II.53b — blue/mauve when the expected onset of rains did not shading is reduced outgoing longwave occur and the subsequent “wet season” radiation and signifies deeper convection was deficient over most divisions, than normal). particularly the western and northern The air associated with the above divisions. Many localities had accumulated normal sea level pressure eastward of New rainfall totals of only 50 per cent of averages Zealand was generally dry and stable and for the period October 1997 to March 1998 resulted in below average seasonal rainfall inclusive (see Figure II.56). About 80 per across the region. New Caledonia had a cent of Fiji was affected by drought and prolonged drought from August 1997 to March agriculture was devastated. The 1998 sugar 1998 (Figure II.54) and the accumulated cane crop was the lowest since 1939, when rainfall deficit of a number of representative the area under cultivation was much smaller, observing sites over western and northern and there was complete failure of the rice Fiji from August 1997 to December 1998 was crop. Overall, the loss of agriculture was about 50 per cent of normal (Figure II.55). valued at about US $65 million. In western Not all localities in the region Viti Levu and the Yasawa Group experienced significantly reduced rainfall approximately 90 per cent of the population during the El Niño event. For example, some received food and water rations from the hill slopes exposed to persisting moist wind government costing more than US $1.5 from the ocean received about average million per month, and there were rainfall. The western slopes of New Zealand, significant health problems arising from the exposed to stronger than normal westerly lack of clean water. It was not until 58 II. The 1997–98 El Niño event

November 1998 that seasonal rains returned 3 to normal. 2 Over the Cook Islands, east of the International Date Line, there were 17 1 tropical cyclones during the 1997–98 0 season, which is a record. The season -1 started early with tropical cyclone Martin -2 devastating Manihiki on 31 October 1997– Rainfall anomalies -3 1 November 1997. A storm surge swept the SOI atoll and 30 persons were washed out to -4 sea and drowned, all but four buildings 1955 1960 1965 1970 1975 1980 1985 1990 1995 were damaged beyond repair and electricity supply was stopped. At Coral bleaching was a major Figure II.57 Raratonga in early December 1997, tropical environmental impact across the South-West Standardized rainfall anomaly over Southern cyclone Pam relieved the serious drought Pacific. Sea level was lowered by more than Africa for the October to that had developed and there was as much 20 cm for many months over summer so that April rainy season and rainfall in six hours as in the previous four at low tide shallow reef water became corresponding SOI values months. abnormally warm. Also, some reefs that are from 1955–56 to 1997–98. Stronger than normal onshore normally covered became exposed at low (Drought Monitoring Centre, southwest winds prevailed over New tide. Harare) Zealand during winter and brought excess rainfall to the southwest but rainfall deficits over the east, in the shadow of the mountain East and Southern Africa range. The summer was also very warm with below average rains leading to drought. As a Rainfall over equatorial East Africa is often result of the higher temperatures and significantly below normal through the drought there were losses of pastures and a Northern Hemisphere summer during an El reduced number of animals. Production of Niño event. However, as the intertropical meat, wool, dairy and grain were convergence zone shifts southward late in US $220 million below normal. There were the year this generally changes to very significant losses of native and planted trees. heavy rainfall over eastern Kenya and the The full cost to the New Zealand economy United Republic of Tanzania. was estimated at US $530 million. The main rainy season of the subtropics The impact of below average rainfall on of Southern Africa is from October to April Australia was not as severe as for many and has a high degree of year-to-year previous El Niño events. Despite a very dry variability. The variability of rainfall is winter over most of eastern Australia there associated with the Southern Oscillation of were widespread relieving spring rains the Asia-Pacific region. The extent of the during September and October 1997 that association between southern African saved crops and provided some boost for rainfall and the Southern Oscillation can be pastures. Over the southeast, by the end of seen in Figure II.57, where standardized the 1997–98 summer, water storages were mean rainfall anomalies for Southern Africa depleted, major river flows were at low are plotted with the SOI for the levels and irrigation allocations to farmers October–April period for each year from were reduced. Over these parts the rainfall 1955–56 to 1997–98. deficiencies had commenced before the Significant climate anomalies that onset of the El Niño and the relief rains had occurred over East and Southern Africa largely missed the area. There were several during the 1997–98 El Niño were: deaths and some property destroyed by • Warmer than normal sea surface wildfires but the most significant loss was to temperature over the western forests and parklands. A brief surge in the equatorial Indian Ocean and in the Australian summer monsoon over northern eastern Gulf of Guinea; and Australia during late January 1998 brought • Intense and persisting rains over heavy rain to the tropical north and flooding equatorial East Africa from October to the region of Katherine in the Northern 1997 to April 1998. Territory. However, it was not until April During the second half of 1997, warmer 1998 that widespread rains eased the than normal sea surface temperatures generally dry conditions associated with the (higher than 28°C over a wide area) 1997–98 El Niño event. developed over the western equatorial 59 Indian Ocean and in the coastal waters (a) (°C)(b) (m/s) 2.5 10 offshore from Namibia and Angola. As 10°N 10°N 2 northeast winds became established over the 9 EQ 1.5 EQ north Indian Ocean, and the intertropical 8 convergence zone of East Africa moved 1 10°S 10°S 7 southward, the abnormally warm waters of 0.5 0 6 the western Indian Ocean provided 20°S 20°S -0.5 sufficient moisture and energy to sustain 5

° -1 ° deep atmospheric convection. Periods of 30 S 30 S 4 very heavy rains and flooding were -1.5 3 experienced over most of the East African 40°S -2 40°S 2 region from September 1997 to February 010°E3020°E010°E5040°E60°E °E -2.5 °E3020°E °E5040°E60°E °E 1998. The regional maps of sea surface

temperature anomaly and the surface winds (a) (°C)(b) (m/s) for the period October to November 1997 2.5 50 10°N 10°N are shown in Figure 58, a and b respectively. 2 40 1.5 30 The relatively strong southerly winds in EQ EQ the Atlantic Ocean offshore from Namibia 1 20 induced upwelling and cooling in the 10°S 0.5 10°S 10 surface ocean layers but abnormally warm 0 0 20°S 20°S sea surface temperatures persisted further -0.5 -10

north in the eastern Gulf of Guinea until late -1 -20 30°S 30°S February 1988. The warmer than normal sea -1.5 -30

surface temperatures over the western 40°S -2 40°S -40

equatorial Indian Ocean also persisted -2.5 -50 010°E3020°E010°E5040°E60°E °E °E3020°E °E5040°E60°E °E through February 1998 (Figure II.59a) and the eastern Gulf of Guinea and the western Indian Ocean had sea surface temperatures rainfall over the northeast of the region, Figure II.58 above the threshold for deep tropical linking to the above average rainfall over Maps of a) sea surface temperature anomaly; atmospheric convection. The map of equatorial East Africa, is consistent with and b) surface wind for outgoing longwave radiation anomaly for typical El Niño events. October to November January and February 1998 identifies the 1997. (NOAA/CDC, USA) areas of unusual deep convection over East Impacts Africa (Figure II.59b – negative anomalies associated with deep atmospheric convec- The period from July to September 1997 Figure II.59 Maps of a) sea surface tion shaded blue/mauve). These areas are was relatively dry over those areas of sub- temperature anomaly; consistent with the very much above average Saharan Africa that are normally affected by and b) anomaly of rainfall received during the period. the Northern Hemisphere summer rains. outgoing longwave radiation for January to Rainfall for Southern Africa, particularly However, very wet conditions became February 1998. the southwest of the subcontinent, is usually established during mid-September 1997 and (NOAA/CIRES, USA) significantly below average during an El persisted until February 1998. Flooding was Niño event. The event of 1997–98 was experienced over most of the East African therefore unusual in that Southern Africa, region that includes Sudan, Eritrea, Ethiopia, especially the central and eastern parts, Djibouti, Somalia, Kenya, Uganda, Rwanda, received average to above average rainfall. Burundi and the United Republic of The map of outgoing longwave radiation Tanzania. anomaly (Figure II.59b) also clearly The impacts of the heavy rain and identifies increased cloudiness over the flooding included loss of life through central and eastern parts of Southern Africa drowning, landslides and outbreaks of during this time. Whether or not this unusual disease. There was also widespread loss of rainfall (unusual for an El Niño event) is a rail and road networks causing disruption to result of Rossby wave teleconnections from transport. Food supplies and the rural the warm waters and increased convection economies were seriously disrupted through over the Gulf of Guinea will require further lost agricultural production following analysis. waterlogging of soils and rotting of crops in The total rainfall over Southern Africa the fields, and through livestock deaths from for the period 1 July 1997 to 31 March 1998, drowning and disease. and the comparison with normal, is shown Outbreak of disease, because of in Figure II.60. Only the above average stagnant and polluted surface waters and the 60 II. The 1997–98 El Niño event increased populations of disease carriers such as mosquitoes, was a particular problem for the people in the region. Increased incidences of cholera, typhoid, (a) (b) malaria and Rift Valley fever were noted during the 1997–98 El Niño event.

Figure II.60 a) Total rainfall over Southern Africa for the period 1 July 1997 to mm % 31 March 1998 and the < 200 < 65 comparison with b) 201-400 normal rainfall for the 65-74 period over Southern 401-600 Africa. 601-800 75-125 (Drought Monitoring Centre, Harare) > 800 >125

A global assessment

It is not possible to provide a truly accurate assessment of wire services, reports from United Nations agencies or the global impacts of the 1997–98 El Niño event because from governments, academic journals, the media, etc. data are not available from all countries, and when data Estimates used in the report to measure negative are available they vary in the criteria used for assessment. impacts were: direct monetary loss, mortality, morbidity, Some losses are known with a degree of accuracy, persons affected, persons displaced or made homeless, particularly insured losses, but these are not acres affected, households affected, bridges/culverts representative. For example, insured losses under- destroyed or damaged, length of road damaged or represent the total structural losses in many countries and destroyed, assistance requested, relief aid given, do not reflect non-material losses including deaths, prevention and preparedness. Beneficial effects were injuries and morbidity. described through anecdotal information but attempts to In an attempt to assess the impacts from the 1997–98 quantify them were hampered by poor reporting. El Niño event in a comprehensive way, the US NOAA Much of the analysis performed for the Office of Global Programs initiated a research study to Compendium is acknowledged to lack of rigorous advance understanding and knowledge of human-climate baseline or reference data because, as a general rule, the interactions, including losses and benefits. The output of standardization of data collection and reporting relating the research, a Compendium of Climate Variability, relies to natural disasters is nearly non-existent. Nevertheless, on reports of natural disasters in order to gain a the Compendium does provide useful global and comparable reference and baseline. Data for the regional assessments of the 1997–98 El Niño event. Compendium was compiled from a variety of news and These are reproduced in the table below.

Direct $ Acres Region Mortality Morbidity Affected Displaced loss affected

Africa 118 13 325 107 301 8 900 000 1 357 500 476 838 Asia 3 800 5 648 124 647 41 246 053 2 544 900 3 861 753 Asia-Pacific 5 333 1 316 52 209 66 810 105 143 984 7 031 199 North America 6 647 559 Incomplete 41 100 410 000 30 787 900 Note: Direct $ losses are South and Meso expressed in US$ millions, America 18 068 858 256 965 864 856 363 500 14 102 690 while all other indicators GLOBAL TOTAL 34 349 24 120 533 237 110 997 518 6 258 000 56 687 632 reported are actual numbers. 61 Part III

The Way Ahead

Predictions on seasonal Systematic observation and the timescales collection of records of the climate system have enabled the development of statistical methods for the prediction of future climate Useful empirical methods for predicting anomalies. Statistical models are generally local or regional departures from normal of easy to build, easy to apply, can be easy to seasonal conditions have been developed understand and diagnose, and require over many parts of the globe. Many of these modest computational resources. There are methods were developed prior to limitations to the use of statistical models. knowledge of the physical dimensions and Mostly these limitations relate to attempts to processes of El Niño and of its links with the use a simple linear model to explain a Southern Oscillation. As an example, over complex relationship, especially where the coastal Ecuador and Peru the abnormal model is not linked to physical processes. warming of the offshore ocean surface Historical climate records are relatively waters has long been recognized as a short and the “training” of a model is built precursor to possible heavy rain and up from a limited set of experiences. The flooding in those areas. Also, for many application is restricted to the particular decades the onset and strength of the locality and/or season, and an experience summer monsoon over India and China outside the training envelope (e.g. a new have been predicted using various forms of record event) may not be well handled. the Southern Ocsilliation Index (SOI), the There is a risk of failure when a statistical spatial extent of snowpack over the Tibetan model developed in one environment (or Plateau and a quasi-biennial oscillation. geographic location) is taken elsewhere There are many more examples of empirical without historical records for training and methods of climate prediction, especially of validation. more recent origins associated with the El Dynamic models of the climate system Niño phenomenon. have evolved in step with computing Empirical methods for climate technology and the ability to routinely prediction utilize factors associated with collect relevant data about the global climate large-scale slowly varying processes of the system. The dynamic models use non-linear atmosphere that regulate, to some extent, relationships to explain or predict the departures from the normal annual cycle of evolution of the climate system from one climate. For example, once established, a state to another. The climate system is far sea surface temperature anomaly will often more complex than can be adequately persist for many months and may continue handled by even using the most powerful to force the overlying atmosphere. How the computers. Many processes take place at sea surface temperature anomaly of the scales of space and time that are not equatorial Pacific Ocean interacts with the resolved by the model and these atmosphere to produce climate anomalies processes are parameterized, or simplified. thousands of kilometres away is not yet fully The parameterization of clouds, for understood. The value of the sea surface example, is designed to deal with the temperature of the equatorial Pacific Ocean statistics of cloud populations and not as a predictor lies in the fact that the local individual clouds. climate in various regions of the globe is Dynamic models are tuned, but very affected in approximately the same way generally and not to specific performance each time there is a similar large-scale with regard to particular variables in a anomaly. particular region, and so are less prone to 62 III. The Way Ahead biased performance through over-fitting. In although the commencement of the anomaly Figure III.1 principle, dynamic models should be decay in January 1998 was correct. Plume diagrams of the responsive to changes in the background The skill of prediction is improving with evolution of forecasts of state (changes on decadal and longer the evolution of scientifically more complex sea surface temperature timescales) because the physical laws and and technically more advanced dynamic anomalies predicted by NOAA/NCEP in the parameterizations are representative of the climate models. The evolution is carried equatorial Pacific region climate system and responsive to systematic forward both by the outcomes of scientific NIÑO 3.4 (long 170°W forcing. The major negative attribute of research and the development of more to 120°W, lat 5°N to 5°S). The black line is the dynamic models is the enormous advanced computing platforms. Future observed anomaly and infrastructure necessary to support such analysis of the 1997–98 El Niño event is the coloured lines are the systems. More detail on the status of climate expected to contribute further to the forecasts initiated in the months indicated in the prediction techniques is given in the development of predictive skill. As known upper right corner. Appendix. processes are examined with new data and (Trenberth, 1998) The performance of many coupled 3 3 ocean-atmosphere climate models (where Jan., Feb., Mar. Apr., May, Jun. outputs were made available as research or 2 2 experimental products) during the 1997–98 1 1 El Niño event has been assessed. Many were 0 0 predicting during 1996 that warming of the -1 -1 SST anomaly (w/TP) SST anomaly (w/TP) central equatorial Pacific Ocean (i.e. an El -2 -2 Niño event) would take place during 1997. Jan.00 Jan.00 Jul. 96 Jul. 97 Jul. 98 Jul. 99 Jul. 96 Jul. 97 Jul. 98 Jul. 99 Jan. 97 Jan. 98 Jan. 99 Jan. 97 Jan. 98 Jan. 99 However, the strength of the evolution was 3 3 Jul., Aug., Sep. Oct., Nov., Dec. handled well only after the event was under 2 2 way. 1 1 Two examples of El Niño forecasts are 0 0 given to indicate the level of skill achieved -1 -1 during the 1997–98 event. The forecasts are SST anomaly (w/TP) SST anomaly (w/TP) -2 -2 from full climate models, where an Jan.00 Jan.00 Jul. 96 Jul. 97 Jul. 98 Jul. 99 Jul. 96 Jul. 97 Jul. 98 Jul. 99 atmospheric general circulation model is Jan. 97 Jan. 98 Jan. 99 Jan. 97 Jan. 98 Jan. 99 coupled to a dynamical ocean general (a) (b) circulation model. The results represent the 4 4 ability of climate models to simulate the 3 3 changing sea surface temperature of the C) C)

° 2 ° 2 equatorial Pacific Ocean. 1 1 The National Centers for Environmental SST ( SST ( Forecast Forecast Prediction (NCEP) of the United States 0 Observed 0 Observed produce an ensemble of predictions for each -1 -1 month. Each prediction is for 11 months in Oct. Jan. Apr. Jul. Oct. Jan. Oct. Jan. Apr. Jul. Oct. Jan. (c) (d) advance. The “plume” diagrams in Figure 4 4

III.1 represent the evolution of sea surface 3 3 temperature from the ensembles for the C) C) NIÑO 3.4 region of the equatorial Pacific ° 2 ° 2 1 1 Ocean. Warming was predicted by the NCEP SST ( SST ( Forecast Forecast model from November 1996, although the 0 Observed 0 Observed magnitude was underestimated prior to April -1 -1 1998. Forecasts were then excellent until Oct. Jan. Apr. Jul. Oct. Jan. Oct. Jan. Apr. Jul. Oct. Jan. May 1998. Figure III.2 The European Centre for Medium-range processes previously not recognized through Plume of sea surface Weather Forecasting (ECMWF) has made lack of data are discovered, their place in temperature anomalies available predictions for the NIÑO 3 region the dynamics of the climate system will predicted by ECMWF for of the eastern equatorial Pacific Ocean. The become established. the eastern equatorial Pacific Ocean NIÑO 3 ECMWF produces 13 ensemble members for In addition to the skill of the sea surface (long 150°W to 90°W, each forecast of six months in advance and a temperature forecasts it will be necessary to lat 5°N to 5°S). sample is shown in Figure III.2. The ECMWF verify how well the models predicted Forecasts were initiated in a) November 1996; model identified modest warming in weather regimes around the globe. It is the b) February 1997; November 1996, but in February 1997 the characteristics of the weather regimes, c) May 1997; and model prediction was levelling off in May including precipitation, temperature and d) August 1997. The heavy line shows the whereas the event continued to develop. winds, that determine the socio-economic observed values. The August 1997 prediction was too warm impacts such as flooding, erosion, drought, (Trenberth, 1998) 63 conditions for wildfires, etc. Because models 15 Figure III.3 30 The format of seasonal with strong similarities in construction 10°N 55 forecasts produced by produced quite different results it is the International Research important to identify why some produced Institute for Climate 15 0° 25 Prediction (IRI). For each good forecasts while others were 60 20 30 60 region a probability is significantly in error. Separating out the roles 35 given of rainfall in the 10 45 of initial conditions, ocean physics, the 10°S wettest third, the driest atmospheric physics and the coupling is a 40 third or the third of years 35 about the climatological challenge, but essential to consistently 45 median. The example 20°S 30 improve predictions in the future. 40 shown is South America, A major advantage of dynamic climate 20 45 30 January to March 1998. 40 30 (IRI) 25 models is that a natural output of ensemble 30°S 40 predictions (repeated integrations with slightly different starting conditions in the 40°S 40 atmosphere) is probabilistic information 35 25 about likely future events. Information on Key Percentage likelihood at. changing probabilities of future events is 50°S A Above-normal rainfall very useful for risk management, particularly N Near-normal rainfall the management of low-frequency extreme B Below-normal rainfall events that are the basis of natural disasters. 60°S 90°W7080°W °W60°W50°W40°W30°W The value of probabilistic climate information for risk management is now just beginning to be appreciated across a range event and need to be included in forecast of sectors. models. An example of probabilistic forecasts is The warmest and second warmest years the “net assessment” forecasts produced by since the 1860s, when reliable instrument the International Research Institute for records began, were 1998 and 1997 Climate Prediction (IRI). The net assessment respectively. 1998 was the twentieth forecasts are a combination of model consecutive year with an annual global predictions and statistical inputs, and are mean surface temperature that exceeded the expressed in terms of probabilities of the 1961–90 average. Whether the warmth was respective season’s rainfall being in the at least partly due to the unusually high El wettest third, driest third and the third Niño activity or whether the global warming centred upon the climatological median. The is contributing to the frequency and size of probabilities express the level of confidence El Niño events is a question to be resolved. in the forecast. An example of a net The issue is a key consideration of the assessment is in Figure III.3. Intergovernmental Panel on Climate Change. Not all of the extreme weather events Research and model development within the and climate anomalies that occur during an framework of the World Climate Research El Niño event should be attributed to El Programme are fundamental to Niño. Many of the extremes and anomalies understanding and responding to human are but part of the broader natural variability influence on climate change, and adapting of the climate system. Ensemble forecasts to natural trends in climate. are being made with different boundary forcing (e.g. with and without the anomaly of equatorial sea surface temperature) in an Applications of forecasts attempt to separate El Niño teleconnections from the background variability. A forecast is an estimate of the The apparently unusual behaviour of El characteristics of a future event. The value Niño events over the past 20 years, with a from a forecast is realized when estimates of tendency for more El Niños and fewer La the characteristics of the future event are Niñas, highlights the need for more integrated into decision-making to create comprehensive models that can handle beneficial outcomes. Therefore, the potential changes in the deeper ocean circulations benefit of climate forecasts in a particular and in atmospheric composition. application relates to how the estimates of Greenhouse gases and aerosols from human future climate characteristics can be used to activity and debris from volcanic eruptions improve decisions, the outcome of which are additional forcings that are likely to are sensitive to climate variability. Realizing modify the climate response to an El Niño the potential benefit from a climate forecast 64 III. The Way Ahead is contingent both on how well the future feature that can help reduce the impacts of climate characteristics can be specified (the incorrect predictions. accuracy of the prediction) and on the Strategies that take account of the relative sensitivity of the application. operational characteristics of the forecast (as In the development of an application defined through the yes/no system) are for a climate forecast the first question that important for gaining benefit. Only limited should be asked of a forecaster is: benefit is likely to be gained from an “What can you predict?” unplanned responsive application and no This is not a trivial question because benefit (or even loss) is likely from an different characteristics of the future climate inappropriate application. Aspects of can be predicted with different skill levels. appropriate strategies that can be developed These skill levels relate to the varying may include contingencies and the performance of models in different regions integration of forecast information on and seasons and for different climate different timescales applied at different times elements. A model that has generally high during the planning and implementation of skill may have poor performance over some an activity. regions for some periods of the year, or for If the benefits from the currently limited some variable. Most climate models, for skill of climate predictions are to be example, have significantly reduced skill maximized then it is necessary for the user during the Northern Hemisphere spring, and of forecasts to clearly understand the climate generally surface air pressure and temperature sensitivity and vulnerability of the systems are predicted better than rainfall. being managed. Thus the forecaster needs to Two methods often used by scientists for ask of the user: assessing skill of forecasts are signal-to-noise “What do you want predicted?” ratio and correlation coefficients. Signal-to- Such a question should encourage the noise ratio can be used as a measure of user to better quantify loss functions in repetitiveness in an ensemble of forecasts relation to the activity being managed, an where the only differences in the ensemble exercise that will promote improved components are slight variations of initial decision-making if appropriate skill conditions. If a characteristic is repeated with information of the model for predicting key little variation in many of the members of the variables for the application is known. An ensemble of forecasts then there may be activity that is only sensitive to extreme increased confidence that the characteristic events will require different information has real meaning. A high correlation support to one that is highly responsive coefficient can be interpreted as indicating within the normal range of climate that the model explains a significant variability. An example of the first type of proportion, but not necessarily the amplitude, activity is in agriculture where a sudden cold of the variability. But the high signal-to-noise outbreak may severely impact on sensitive ratio and the high correlation coefficient may crops with subsequent loss of yield. An not be relevant to many applications. example of the second activity is power A useful approach to assessment from usage where milder winter temperatures both the modelling and applications reduce demand for power usage to provide perspectives is to estimate the ability of heating but a colder than normal winter will models to predict specific climate events encourage above average power usage; within a yes/no framework. This approach management of power generation and enables the hit and false alarm rates to be distribution will be improved through better calculated for that event for that forecast forecasts of temperature anomalies for technique. The hit rate represents the which power demand is sensitive. proportion of events that was correctly If the sensitivity and vulnerability of an predicted while the false alarm rate signifies application are well defined then that the proportion of occasions when the event application information can be used to was predicted but did not occur. Correct specify significant climate parameters and forecasts (whether the event will or will not critical threshold values to which the occur) are valuable in the planning process forecaster can give specific attention. The but benefits are inevitably offset by incorrect sensitivity and vulnerability studies are a forecasts. This yes/no approach is necessary step to initiate integration of particularly useful as it can be directly linked scientific information into decision-making to benefits gained in applications. It further and the realizing of benefits from climate assists in contingency planning, an essential predictions. This is particularly important in 65 Bridging the knowledge gulf — a case study*

* Taken from: Meinke H. It is a challenge for climate scientists and SOI are related to rainfall variability and are and Z. Hochman, 1999. activity managers alike to identify decisions useful for rainfall forecasting at a range of Using seasonal climate that can usefully be aided by climate locations around Australia. A skilful seasonal forecasts to manage dryland crops in northern forecasting. For a forecast to be effective its forecast provides an opportunity for farm Australia — experience inclusion in decision-making ultimately has managers to better tailor crop management from the 1997/98 to improve the long-term performance of decisions to the season. season. In: Hammer, G.L., N. Nicholls and C. the activity, either by increasing profits, by The essential decision tool that has Mitchell (eds), improving sustainability indicators, or by been developed specifically for northeastern Applications of seasonal reducing risk. There are many initiatives Australia is a crop simulation model that is climate forecasting to being taken in different countries and driven by daily meteorological data (i.e. agricultral and natural ecosystems — the sectors to bridge the knowledge gulf. The minimum and maximum temperature, solar Australian experience. initiative described here is therefore not radiation and rainfall) as input. The model is Kluwer Academic, The wholly unique but serves to illustrate a responsive to differences in soil water and Netherlands. 1999, in press. number of principles that will be important soil nitrogen status but does not account for in developing a wider acceptance and usage factors such as water logging, pests or of climate forecast information. disease. The simulation model can be used The Agricultural Production Systems with suitable adjustments for a variety of Research Unit is a multidisciplinary crops, including wheat, sorghum and government funded facility in northeastern chickpeas. Australia that has taken up the challenge of In April 1997, when it was first managing agricultural systems more apparent to the researchers that an El Niño effectively for climate variability. In addition event might be developing, studies were to research into identification and carried out to estimate potential impacts and quantification of sensitivity and vulnerability to evaluate alternative tactical decisions. A of different crops, the unit is also promoting detailed rainfall analysis concluded that the the usage of beneficial research on farms. chances of receiving planting rain were only Dryland farming is the typical mode of slightly reduced if the SOI was negative in agriculture over northeastern Australia and May. However, the total amount of winter high rainfall variability is the main cause of rainfall was likely to be reduced. yield fluctuation. Although most dramatic at The rainfall analysis was followed by farm level the impact of climate variability is studies using the crop simulation model set apparent throughout the entire Australian for wheat yields. Different starting soil economy. The benefits of better moisture conditions were considered and, to management for climate variability will quantify the impact of the El Niño, each year therefore extend from the farm enterprise of simulated yield was categorized according level to the scale of the national economy. to the corresponding May value and phase of Significant physically based lag- the SOI. Median yields of those years with relationships exist that link the SOI, as a negative index at the end of May were predictor, with future rainfall amount and its compared with the median yield for all years temporal distribution over eastern Australia. An (see Table III.1). El Niño event, which generally corresponds to The wheat crop simulations for various negative values of the SOI, usually lasts for locations pointed to a consistent median about a year, beginning its cycle in the yield reduction in years of negative index Southern Hemisphere autumn. Phases of the during May. However, provided there were

the area of natural disaster mitigation, come together so that each has the same recognizing that societal systems are perspective to the question: generally adapted to operate within a “What was predicted?” climatological range; disaster occurs A number of studies have been made when a critical threshold is exceeded, such that examine impediments to the more as river height, wind speed, period without widespread use of forecasts. Some issues are: rain, etc. • Forecasts are difficult to interpret; In order to close the knowledge gulf it • A lack of decision models to integrate will be essential for forecasters and users to forecast information; 66 adequate soil moisture reserves available at (a) Figure III.4 100 Potential grain yields (a) the time of planting, the simulations pointed Chickpea 80 and gross margin (b) Wheat to high chances of economically viable predicted for wheat and 60 yields even during El Niño years. Through a chickpeas in years with a detailed on-farm monitoring programme it 40 consistenly negative SOI phase by the end of 20 was established that wheat grown on good May at Jimbour, Probability of exceedence (%) 0 soil moisture reserves in 1997 across 012345Queensland. Simulations northeastern Australia performed well (b) Yield (t/ha) were conducted for a although the seasonal rainfall was 100 half-full soil water profile Chickpea at sowing. 80 substantially below average. The crops were Wheat (Meinke and assisted by timely rain at flowering. 60 Hochman, 1999) However, only few crops planted on 40 marginal stored water reserves produced 20 economically viable yields. Probability of exceedence (%) 0 -200 0 200 3 400600 800 1 000 The information from rainfall analysis, Gross margins ($/ha) crop simulation and categorization for management options and quantify the likely seasonal outlooks was used to examine effect on production and farm income risk. tactical production decisions relating to To be most effective, this approach requires Table III.1 Simulated median wheat wheat, sorghum and chickpeas, and on an understanding of the probabilistic nature yields (t/ha) for decisions about potential relative benefits of the information provided. Also, to truly six locations in the from planting wheat or chickpeas in benefit from the approach, it must be used northeastern Australian particular years depending on the prevailing consistently for many seasons. Further, it wheat belt. Shown is the median for the ‘all years’ climate outlook (Figure III.4). Chickpeas must be integrated into the whole decision- case and the median for have a shorter growing season and a later making process as one of many the years that had a planting date so that they are perceived as a management tools. Producers must not consistently negative (-ve) SOI phase by the end of potentially less risky option than wheat become disheartened or reckless by any May. Simulations were when rainfall is scarce. perceived “failure” or “win” of the forecast conducted for a 20 May Good farm managers have a rich in any given season. The development of sowing date on a profile containing either appreciation of agricultural systems such decision support models provides 180 mm of stored soil components and their interactions. For powerful tools for integrating climate moisture (‘full’) or 90 mm example, they understand the relative information and prediction into decision- of stored soil moisture (‘half full’). contributions of stored soil moisture at making and bridging the knowledge gulf (Meinke and Hochman, sowing versus the benefits of in-season between climate scientists and users. 1999) rainfall. The scientific method is valuable for farm managers. It helps to replace their Full profile Half-full profile intuition about complex systems with: a) hard Location All years -ve May SOI All years -ve May SOI data about the current state of the system; and (t/ha) (t/ha) (t/ha) (t/ha) b) probabilistic information about the way in which the unknown (the climate of the Dalby 2.85 1.75 1.19 0.58 season ahead) will affect the outcome of Emerald 1.97 1.53 0.60 0.58 alternative management decisions. Goondiwindi 3.48 3.19 1.86 1.65 Moree 3.30 2.73 1.87 0.92 Producers can use historic rainfall Narrabri 3.32 2.31 1.94 1.43 records in conjunction with dynamic crop Roma 2.21 1.87 0.96 0.62 simulation models to evaluate alternative

• Uncertainty over accuracy; These studies highlight a knowledge • Additional information is necessary, gulf that separates the forecaster from the including locally specific potential users of climate forecasts. Climate information; science has made enormous advances over • Proof of value is necessary; recent years but the full benefits are not • There is a lack of access to expertise; being realized, partly because of cognitive • Difficult to assess forecasts; and illusions that have prevented seamless • There is competing or conflicting integration of forecasts into activity-specific forecast information. decision support models. Some elements of 67 cognitive illusion that are important from the an important requirement in the application forecasters’ perspective are the framing effect of these methods. and the interpretation of the forecast, the A barrier to the more extensive availability of information and its assumed development and use of statistical seasonal importance, and the overconfidence in forecasts in many parts of the globe is lack presentation of information. From the users’ of long climate records in computer format. perspective, some elements of cognitive Climate records in computer format are illusion that are detrimental are optimism essential to build effective statistical about personal risk, asymmetry between prediction models appropriate to the losses and gains, and added information bias locality. Many countries have climate records that causes difficulty in assimilating multiple that have been maintained in manuscript information sources. form and WMO is coordinating assistance An inescapable conclusion from the through the DARE (Data Rescue) project for experience of the 1997–98 El Niño event is more robust preservation of the records. As that not enough effort has been made to a first step manuscripts are being copied to develop applications that can take microfilm for preservation and later digital advantage of the capability of climate processing. forecasts. Many scientists were WMO is also providing leadership in overconfident of their knowledge of the developing the capacities of national climatological outcomes from the event and Meteorological and Hydrological Services for this found alarmist expression in the media. data management through the CLICOM (or Both scientists and the public were quick to CLImate COMputing) project. Standardized draw parallels between the potential of the computer software for climate data 1997–98 event and the outcomes from the management, appropriate hardware and 1982–83 event, previously reckoned as the training are provided to developing countries. “El Niño event of the century”. Also, because Both DARE and CLICOM are being funded of the rapid development in capability for under the Voluntary Cooperation Programme climate prediction over the past decade, very of WMO (supported by contributions from few national Meteorological and Member countries). Progress is very slow; Hydrological Services (NMHSs) were some manuscripts remain in danger of prepared to issue specific community deterioration and some data have been guidance, or the guidance issued was lost in permanently lost. Additional funding is needed the plethora of alarmist statements in the to accelerate progress on these vital projects. media attributed to “international experts”. The variability of the climate anomaly Detailed reviews of the impacts from patterns from one El Niño event to another the 1997–98 El Niño event are necessary to reflects the non-linear coupling between the provide guidance as to which are the more ocean and the atmosphere. When integrated sensitive and vulnerable regions of the globe over months or seasons, and averaged over and sectors of societies. Multidisciplinary many events, there is a characteristic studies and activities, that bring together anomaly pattern. However, locally, and climate scientists and users, will promote the during any particular event, there can be necessary transfers to bridge the knowledge significant departure from the expected gulf and ensure more beneficial utilization of pattern. For this reasons many NMHSs are climate information and prediction services issuing predictions in terms of probabilistic for the future. outcomes and this format is increasingly finding favour with users of the services. Computer models of the climate system Climate Information and for predicting conditions a season or more Prediction Services ahead are still mainly in experimental mode. Many model developers, however, make the Prediction on seasonal and longer timescales, outputs available over the Internet with as a basis for warning, is a developing caveats as to their usage. A number of more community service. An increasing number of limited models for the prediction of El Niño national Meteorological and Hydrological (more specifically, the sea surface Services are providing seasonal predictions temperature of the central Pacific Ocean — based on statistical methods. Understanding NIÑO 3) have been functioning for a the relationship between El Niño and number of years. variability of the local climate, including the Over the past few decades scientists seasonal characteristics of the variability, is have made enormous advances in monitoring 68 III. The Way Ahead

* The TOGA (Tropical the climate system and in understanding the • Provide an international framework to Ocean Global causes of climate variability, especially the enhance and promote climate Atmosphere) project, with its emphasis on ocean role of El Niño and the overall ENSO information and prediction; atmosphere coupling and process*. However, these advances of • Encourage the development of the ENSO process, has knowledge and predictive capability have not operational climate prediction; and provided a focus and been matched by achievement of community • Facilitate the development and the stimulus for international cooperation in climate benefits, including protection of life and strengthening of a global network of research. mitigation of loss. A contributing reason is regional/national climate centres. that many communities do not have access to One expected outcome from CLIPS is regular climate updates about prevailing the strengthening of the interactions conditions relevant to their region. Often the between providers and users of climate only information available is through services. The value and benefits from CLIPS irregular, sometimes alarmist, media reports will be maximized through multidisciplinary quoting “overseas experts”. There are, studies that establish the risks and however, a number of initiatives under way sensitivities of local social and economic that aim to improve the effectiveness with sectors and go on to develop service which information on impending climate delivery and decision support systems based events or trends is transmitted to decision on well-researched response models. It is makers and the public at large. essential that new services change traditional The International Research Institute for ways and responses to produce better Climate Prediction (IRI), an initiative of the outcomes, including more robust public United States through its National Oceanic infrastructure systems, more resilient food and Atmospheric Administration (NOAA), and water supplies, or prevention and more fosters the improvement, production and rapid response to disease outbreaks. use of global forecasts of seasonal-to- The CLIPS project is expected to interannual climate variability for the explicit enhance capacity building within NMHSs benefit of society. The IRI is completing the through development of a framework of development of a workable structure to networks for regional cooperation. Existing combine worldwide research capabilities networks, such as those encompassing the with explicit operational and applications African Centre of Meteorological objectives. Its core facility, located at Applications for Development (ACMAD), the Lamont-Doherty Earth Observatory in Drought Monitoring Centres (DMCs) of East Palisades, New York, comprises social Africa and Southern Africa and the Caribbean science and physical science elements and Meteorological Council, clearly point to serves as the nucleus for partnerships within such benefits; however, their respective an international network of research and successes are constrained due to a serious modelling institutions and applications lack of resources. The IRI and a few national centres. The NOAA Office of Global centres produce global monitoring and Programs (NOAA/OGP) has provided base prediction products but there also is a need funding in support of core activities and for products relevant to the different modular funding in support of collaborative geographic regions. The identification of activities since mid-1996. regional climate centres and their support The Climate Information and Prediction through infrastructure resources, staff Services (CLIPS) project is a complementary training and other forms of capacity building initiative that has been established by WMO. will ensure the availability of monitoring and CLIPS is expected to draw on the new prediction products necessary for high- opportunities for data management and quality climate services. climate prediction and directly assist national Meteorological and Hydrological Services to deliver an enhanced range of Risk and society operational climate services at the national level, including prediction on seasonal to Meeting the needs of future generations, interannual timescales. CLIPS is intended to especially the needs for personal safety and build on the many successful data collection, for security of food and water supplies, is a data management and research programmes key principle of sustainable development. of WMO and has, as its objectives, to: Neglect of national policies for natural • Demonstrate the value and eventual disaster mitigation and risk reduction can set socio-economic benefits of climate back community development by many information and prediction services; years. This was witnessed over many regions 69 of the globe during the 1997–98 El Niño related to climate extremes, are increasingly event as accumulated family possessions more predictable and quantifiable through were lost, community reserves were the application of modern science. The exhausted and national development mitigation of loss and suffering to vulnerable initiatives were delayed in the wake of people, however, implies a need for policy disaster recovery. Without due consideration and institutional reforms at all levels. of natural disaster reduction the vulnerability One objective of the International of many social groups within a community Decade for Natural Disaster Reduction, in its can increase. programme of disaster prevention for the Recurring extreme events, or natural twenty-first century, is to shape future hazards, are at the focus of natural disasters. directions for sustained international and Droughts, floods, storms and extreme interdisciplinary commitment to disaster temperature episodes are examples of prevention, by identifying crucial programme natural hazards that occur in many parts of functions and essential institutional responsi- the globe, and their occurrence is relatively bilities. Within the programme there are five independent of human activity. In many themes for organizing activities and events: regions human activity has flourished, • Hazard, vulnerability and risk despite the presence of significant climate assessment so as to identify priority hazards because of awareness and risks faced by individual communities. preparedness to mitigate the risks. • Disaster prevention and sustainable While the presence of the hazards may development, in order to make disaster be relatively independent of human reduction an integral part of national activities, natural disasters are always rooted planning. in social processes and institutions. Such • Effective early warning, so that local disasters can strike both the rich and the communities are enabled to develop poor in developed and developing their own capabilities. countries, but they do not affect the social • Political and public policy commitment, groups equally. The rich everywhere, to ensure sustained political because of better preparation, are generally involvement in protecting natural less vulnerable than are the poor, who are resources. generally less prepared and more exposed • Shared knowledge and technological to the hazards of climate extremes. transfer, to provide the most Planning, early warning and well- sophisticated scientific basis for the prepared response strategies are major tools greatest possible protection. for mitigation of loss associated with natural In developing response strategies to the disasters. To be fully effective these tools hazards associated with climate extremes and will build on the synergies between science the potential for natural disasters, it will be and technology, decision makers and necessary to examine the risk to society from Already vulnerable planners at all levels, as well as the public at the perspective of its different dimensions. populations may disporportionally suffer large. Here, natural disasters will be examined from more in natural disasters. The incidence, locations and intensities the perspective of the economic, (IDNDR) of many natural hazards, especially those environmental, developmental and societal dimensions in the context of the 1997–78 El Niño event.

Economic dimension The experience from the 1997–98 El Niño event is that there were significant macro- economic impacts to many countries. Some of these impacts have been quantified whereas others were identified but data were not available to do more than estimate the national cost. Many economic costs related to depletion of foreign reserves and diversion of financial resources, each of which has an impact on community well- being. The unplanned economic costs of climate-related disasters, where they 70 III. The Way Ahead occurred, set back national development aspects of infrastructure loss that have lasting programmes. Some typical macroeconomic impact are: impacts were: • Loss of housing and public buildings is • Reduced hydroelectric generation a reduction in well-being that translates because of drought, and the need to to community attitudes and import fuel for additional thermal productivity. generation. Foreign reserves were • Loss of critical components of transport reduced or other imports forsaken. networks (bridges, road and rail • Reduced food production because of sections) reduces the efficiency of trade drought, and the need for new or addi- and commerce. tional imports of food. This also impacted • Contamination of water supply requires on currency reserves and reduced the expensive remedies to avoid outbreaks opportunity for other vital imports. of sickness and the spread of disease. • Reduced agricultural production • Destruction of forest by fire is a loss of because of drought or flood, or a natural resource and blight on the breakdown of transport links through visual environment, especially in areas flood damage led, in some cases, to loss promoted for tourism. of international markets. Commerce has • Damaged tourist facilities set back a preference for reliable suppliers and if opportunities in an increasingly an export market is lost the national important industry sector. and community economics can be The inclusion of climate information adversely affected until the markets are into infrastructure planning is an essential re-established or new exports and step in mitigation of natural disasters. Local markets are developed, and these may climate information on frequency and take years. magnitude of extreme events is vital to • The social and economic base of many underpin the safe design and construction of regional communities was devastated public infrastructure facilities. It is important during the 1997–98 El Niño event and that the climate information is relevant to the their very survival needed national, and particular locality because many hazards vary sometimes international, assistance. It considerably over relatively short distances. became necessary for governments to This is one reason that a robust national provide additional support in the form meteorological observing network and of emergency shelter, food assistance, associated data collection and management provision of safe water and disease system are of national importance. mitigation. Effective national expenditure for Developmental dimension A macroeconomic impact strengthening infrastructure and early of climate-related natural disasters may include preventative actions following warning are Often it is not feasible to build all reduced food production actions to reduce economic loss during infrastructures to withstand extreme natural due to flooding and soil climate extremes, such as those associated hazards. In these circumstances the erosion, and a breadkdown of with El Niño. The integration of scientific formulation of a national development transportation links. knowledge about natural hazard risk strategy requires recognition of hazard risk (IDNDR) reduction into macroeconomic policies will have beneficial outcomes at the national level in the long term.

Environmental dimension Many regional and national infrastructures were adversely affected during the 1997–98 El Niño event, particularly as a result of flooding, strong winds or wildfire. Many community resources built up over generations were destroyed and regional industries, trade and commerce were severely impaired. The aftermath of destroyed infrastructure will continue to be felt within communities for years because of lost opportunities, particularly in the areas of trade and commerce. Some 71 and a capacity to cope must be included in development policies. Such an approach emphasises preparedness planning and recognizes that appropriate community response will reduce the deleterious impact of an event. To mitigate the community impacts of El Niño it is imperative to identify the nature of the risk and which groups in society are vulnerable, including how and why some groups are particularly vulnerable. This information is the basis for devising appropriate strategies to reduce vulnerability. Disaster reduction at the community level requires supportive policies and institutions at local, national and international levels. National policies and actions will lose their effectiveness and to provide guidance as to what is the Few studies quantify the where local communities do not control the appropriate response for different sensitivity of local resources required to reduce natural circumstances. In the case of El Niño, the communities to climate variability, particularly in disasters, or institutions constrain the ability ongoing direction will include government- periods of climate of vulnerable groups to take actions. supported climate information and extremes. Consultation and active participation of each prediction services, integrated within the (IDNDR) community’s diverse social components international scientific and technological provide the opportunity to address the framework, that are available to the totality of issues. community and responsible sectoral The climate component of effective agencies. These national services are the development policies will include: authoritative guidance that underpins • Recognition that climate is variable on community actions for mitigation of loss and seasonal to interannual timescales with suffering during climate extremes. recurring extremes, and inclusion of Authoritative guidance would avoid contingency planning to mitigate its inappropriate community responses to ill- potential impacts. advised comment and alarmist media • Enhanced monitoring in sensitive and reports. vulnerable sectors, such as food An important aspect of public guidance production, clean water supplies, to the community is a need to ensure that all wildfire management and health. aspects are conveyed in terminology and • National early warning systems that vernacular that reflect a range of social formally trigger adaptive strategies and groupings. A major issue in the 1997–98 community responses appropriate to El Niño event was media reporting in stark the level of risk. terms of expected impact and disaster, rather • Public education material that is than well-founded risk assessment. People distributed in appropriate media and can develop a sense of risk even without a terminology, including in school formal understanding of probability. programmes, on risk and vulnerability Contingency planning and implementation to climate extremes such as El Niño, relate future responses to potential risk and and the appropriate actions to adapt changing perceived risk. Public awareness and respond. programmes and school education should have, as their objective, rational decision- Societal dimension making in the light of the best available advice. The inclusion of risk reduction for climate- related hazards within social policies would recognize that communities are expected to An integrating framework respond in a planned way to authoritative for action direction in times of climate extreme. If the policies are to be successful it will be The 1997–98 El Niño event received necessary for governments to ensure the widespread scientific and media attention necessary authoritative information flow, for two important reasons. Firstly, the 72 III. The Way Ahead

technological systems have not been WWW widely matched by achievement of Global Observing community benefits, including protection System Detection of (GOS) climate change of life and mitigation of loss. A contributing

Global reason is that many communities do not Atmospheric have access to adequate climate services Watch

(GAW) Seasonal and USER COMMUNITIES relevant to their region. As noted earlier, interannual often the only information available is Global climate prediction Ocean Global Climate Observing Climate Information through irregular, sometimes alarmist, System Observing and media reports about the global (GOOS) System Prediction (GCOS) Monitoring of Services phenomenon without the necessary linkage (CLIPS) Global ecosystems and Terrestrial to local vulnerability and appropriate land degradation Observing national and community response measures.

cryosphere biosphere System (GTOS) Also, there are relatively few studies that GLOBAL OBSERVING SYSTEMS World quantify the climate sensitivity of local atmosphere ocean land freshwater Hydrological Climate research Cycle communities in developing countries, Observing System including the risk to public infrastructures, (WHYCOS) food production, water supplies and of disease outbreak during periods of climate Figure III.5 1997–98 El Niño was very strong when extreme. Basic elements of an compared to previously recorded events. In Better climate information and integrated infrastructure for climate prediction many ways the 1997–98 event was prediction services at the local level are systems. comparable in strength to the major event of essential pre-requisites for improved (World Climate News, 1982–83, previously regarded as the most community preparedness and No. 15, June 1999) intense event of the century. Secondly, early responsiveness to climate extremes, and to detection of the event and its evolution were mitigate the impacts, including those linked made possible because of research to the El Niño phenomenon. However, to identifying the characteristics of the achieve these outcomes it will require a phenomenon and new and improved concerted international effort at a number of observing systems for monitoring the global levels. climate system. Flood rains, drought, unusual storm events and other local and International scientific regional climate manifestations that occurred around the globe were cast within a single infrastructure pattern of cause. Notwithstanding the improved The Climate Agenda, jointly supported by knowledge of the phenomenon and its WMO and partner agencies within the likely pattern of regional impacts, the United Nations system and by the event resulted in loss of life, destruction of International Council for Science (ICSU) and private and public infrastructure, loss of other non-governmental organizations, is a food storage and production systems, comprehensive integrating framework for all outbreaks of disease and other serious aspects of international climate-related socio-economic manifestations. The programmes. The Climate Agenda embraces accumulating impacts became of four activities — science, services, increasing concern to the global assessment and observations — that are the community. Despite the best efforts of pillars of the climate issue. An international governments and various United Nations effort to strengthen the Climate Agenda is agencies, both in terms of services and essential for building the global scientific enhanced relief operations, the event and technical infrastructure necessary to and its aftermath brought intolerable loss underpin regional and national strategies to and community suffering to many affected mitigate the impacts of climate extremes, regions. such as those associated with El Niño. Over recent decades science and Figure III.5 shows the basic elements of technology have made enormous advances an integrated infrastucture for the climate in developing capabilities to monitor the system. The World Weather Watch (WWW), climate system and to understand the causes with its Global Observing System (GOS), of climate variability and extremes. Global Telecommunication System (GTS) However, these advances in scientific and Global Data-processing System (GDPS), knowledge and development of is organized and coordinated by WMO. It 73 provides, through its Member countries, the satellite altimeter instrumentation for essential global framework for the measuring sea level. However, the need for collection, analysis and distribution of advanced technical systems for observing meteorological and related environmental climate does not decrease the value of information. The GOS comprises observing conventional systems. Implicit in the call to facilities on land, at sea, in the air and in introduce new technological systems for outer space. These facilities are climate monitoring is the requirement that implemented and operated by the Member the degradation experienced in the national countries of WMO and all countries benefit land-based observing networks of the GOS from the integrated effort. The WWW over recent years is reversed, and that provides the essential operational conventional networks essential for climate infrastructure for monitoring the global monitoring are upgraded to recognized climate system. However, in many parts of standards. the globe no data are being gathered, the There remain many uncertainties about existing networks are inadequate, or the the El Niño phenomenon and the networks and archives are becoming teleconnections that affect global weather degraded through lack of ongoing national patterns. There is also enormous scope to support. improve the global climate models for Close cooperation between the WWW prediction. The Climate Variability and and the World Climate Programme (WCP) is Predictability (CLIVAR) project has been beginning to address these issues. The ICSU, established within the framework of the WMO, the Intergovernmental WCRP to extend the investigation of climate Oceanographic Commission of UNESCO and variability and predictability to larger UNEP are cooperating in a Global Climate geographic regions and longer timescales Observing System (GCOS) to provide for the ultimate benefit of the world’s meteorological and related environmental societies. A scientific plan has been data necessary for climate research and prepared for a new 15-year research operational climate services. GCOS will programme and one component, CLIVAR- include new components but will build GOALS that addresses the Global Ocean- upon the established WWW system and Atmosphere-Land System, builds on the provide an integrated and expanded global successful TOGA programme of 1985–94. observing system to meet the needs for The objective of CLIVAR-GOALS is to study climate variability and change. International seasonal-to-interannual climate variability funding support will be necessary for and to develop predictability of the global implementation of GCOS, which is an ocean-atmosphere-land system. New essential component of the scientific and methods of observation and the technical infrastructure that needs to be development of advanced climate models developed to deliver effective climate for better prediction are two outcomes information and prediction services for the expected from CLIVAR. future. UNEP is taking the lead, within the Research into the El Niño phenomenon framework of the World Climate Impacts and has been a focus of international Response Strategies Programme (WCIRP) of cooperation within the framework of the the World Climate Programme, in the World Climate Research Programme (WCRP) assessment of sensitivity and vulnerability to for more than two decades. WMO, the IOC climate variability and change, and for of UNESCO and ICSU are co-sponsors of the developing options for socio-economic WCRP. The Tropical Ocean Global strategies to respond to climate variability Atmosphere (TOGA) project was established and change. UNEP, working closely with the within the framework of the WCRP and US National Center for Atmospheric consequently the detection and evolution of Research (NCAR), commenced its WCIRP the surface and subsurface characteristics of activities related to the assessment of the Pacific Ocean during the 1997–98 El impacts of climate variability in 1985 by Niño event were achieved with carrying out a review of the 1982–83 El Niño unprecedented accuracy and timeliness. It event. Subsequently, UNEP has maintained a will be necessary to ensure that, through Working Group on the Socio-economic funding and cooperative international Aspects of El Niño. The activities of the management, the GOS is augmented by the Working Group have included workshops proven research systems, as it was with the and publications, many of which have been TAO array of moored ocean buoys and supported by NCAR. 74 III. The Way Ahead

processes. The framework will recognize the capability for prediction, it will recognize what is important to be predicted, and it will ensure that the scientific information flow is CUBA CHINA COSTA RICA PHILIPPINES in a form that can be readily assimilated. PANAMA ETHIOPIA VIET NAM Decision makers, whether they are ECUADOR KENYA INDONESIA PERU responsible for an industry sector, a PAPUA NEW GUINEA community service or environmental PARAGUAY MOZAMBIQUE FIJI resources, all have different information needs that are specific to the sensitivity and vulnerability of their sector. The ultimate success of the Climate Agenda will depend on active support from Figure III.6 The pattern of global impacts from the governments working through national and Fifteen countries selected 1997–98 El Niño event confirms that international institutions. The Climate to participate in the communities and economies are affected Agenda could be fully implemented within UNEP/NCAR-led study of the socio-economic differently during El Niño events. However, the framework of existing international impacts of the 1997–98 it is only possible to provide generalizations climate programmes if governments: El Niño event that is over many parts of the globe because data • Increase support for national activities, aimed at improving preparedness and early on the full extent of impacts are either not as part of international programmes that warning to climate available or are held separately by different they have helped design; extremes. authorities and have not, as yet, been • Strengthen, or establish, national (NCAR) collated. Mitigation of the impacts of climate programmes for coordination El Niño will require in-depth sensitivity and and development of climate-related vulnerability studies at the regional, national infrastructure and services; and sectoral levels. These studies are • Build scientific and technical capacity at necessary to identify, at the national and the national, regional and international local levels, the scale and intensity of levels; and potential impacts from climate extremes and • Fund international coordination will provide a basis for preparation of mechanisms. appropriate response strategies. However, international assistance will UNEP, in cooperation with NCAR, has be essential to support capacity building and received an allocation through the United lifting the effectiveness of the national Nations Fund for International Partnerships scientific infrastructure and service delivery to support a project aimed at improving the in many parts of the world. Modern early warning and preparedness for El Niño instruments and sensors that measure Southern Oscillation (ENSO) events. With climate and environmental elements, and the supplemental funding and the collaboration systems for collecting and processing data, of WMO and the United Nations University, require technical expertise, careful it has been possible to include 15 countries management and ongoing maintenance. A in the preliminary list covered by the project coordinated programme of assistance and (Figure III.6). capacity building for the national The country assessments will review Meteorological and Hydrological Services of forecasts and impacts of the 1997–98 many countries will be required if they are El Niño event, as well as the early warning to be capable of contributing to and and natural disaster preparedness systems in benefiting from the international scientific the selected countries. Based on the and technical infrastructure. A fully global assessments the project will identify research climate observing and data management and policy needs and develop preliminary system will be essential to provide the data guidelines for regional and national natural to initialize climate prediction models. All disaster preparedness plans for ENSO warm countries will need to be able to effectively and cold events and their impacts. The access and apply the forecasts if they are to experts in the countries will also form an improve national disaster preparedness and international El Niño network to exchange mitigation. information and experiences. Improved climate prediction will not be Regional cooperation fully effective unless the associated services are delivered in a framework that assists in The 1997–98 El Niño demonstrated that in the various sectoral decision-making different regions around the globe many 75 neighbouring countries had similar impacts University of Hawaii, the Pacific Regional from the event. Collection and assessment of Office of the US National Weather Service, relevant data at the regional level would the US Office of Global Programs (NOAA), provide governments and communities with and the Pacific Basin Development Council. common detailed information about the The jurisdictions served include American overall extent and severity of prevailing Samoa, Commonwealth of the Northern climate anomalies and their impacts. The Mariana Islands, the Federated States of unrestricted international exchange of Micronesia, Guam, Hawaii, the Republic of climate data provides mutual benefits for the Marshall Islands, and the Republic of mitigation of impacts, as well as being Palau. PEAC also carries out research and essential for monitoring of the climate applications development. A proposal is system and initializing climate prediction under consideration to widen Pacific models. Cooperative action and joint cooperation to include countries within the development of facilities at the regional level South Pacific Regional Environment can provide an infrastructure in support of Programme (SPREP). climate services that is more effective than Governments on the Pacific Coast of either country in a group can support South America, since 1952, have cooperated independently. within the framework of the Permanent A number of regional arrangements are Commission for the South Pacific on matters already in place between cooperating relating to the marine environment. national Meteorological and Hydrological Objectives of the Commission include to Services to address common issues. Some of protect and make profitable use of the these arrangements include regional maritime environment and to promote institutions, such as the African Centre of scientific research. The ERFEN Programme, Meteorological Applications for which has been active since 1974, is multi- Development (ACMAD — Niamey, Niger) disciplinary and studies El Niño from the and the Drought Monitoring Centres (DMCs) oceanographic, meteorological and of East Africa (Nairobi, Kenya) and Southern hydrological perspective. To further Africa (Harare, Zimbabwe). These Centres strengthen cooperation on scientific, are formally constituted, supported by resource management and environmental contributing governments, and are expected activities the cooperating governments have to deliver a range of service products. Other proposed to establish a special institute in regional arrangements, such as between Guayaquil, Ecuador. The institute would the Meteorological Services of the countries undertake a programme of regional of the Association of South East Asian observations, analysis and research related Nations (ASEAN), facilitate cooperation, and to El Niño and work closely with other options are under consideration for regional institutions in countries of the region centres of operations that would provide a working on El Niño-related studies. set of common regional products in support For developing countries that are of climate monitoring and prediction affected by El Niño, there are efficiencies to services. be gained by participating in and having Government and non-governmental access to a range of centrally prepared organizations are cooperating within regional analyses and predictions, rather regional arrangements established within the than to invest solely in their own scientific framework of the ICSU International infrastructures. Cooperative management of Geosphere-Biosphere Programme (IGBP). regional networks and systems would also The Inter-American Institute (IAI) for global provide uniform standards and be likely to change and the Asia-Pacific Network (APN) lift quality. Joint access to common scientific for global change each coordinate regional databases, analyses and other products programmes, involving government and would facilitate technology transfer, non-governmental institutions and scientists, cooperative assessment and regional for which climate variability (ENSO) and research for the benefit of national change are important components. institutions. The Pacific ENSO Applications Centre During the 1997–98 El Niño event (PEAC) was established as a pilot project in there was a sequence of regional climate 1994 to provide ENSO forecasts and fora sponsored by international information products to the US-affiliated organizations, including the International Pacific Islands. PEAC was developed as a Research Institute for Climate Prediction joint venture of the University of Guam, the (IRI) and WMO. The fora were held at 76 III. The Way Ahead various locations in Africa, South East Asia, framework for coordination and to ensure Central and South America and brought that the appropriate infrastructure and together international scientists, and regional climate information and prediction services scientists and decision makers. The are operational and support preparedness objectives of the fora were, inter alia, to planning, early warning and better review the status of the El Niño and its management across a range of sectors. A regional impacts, and to develop a national climate programme should clearly consensus prediction for the region covering identify national objectives for the the months ahead. programme, assign responsibility for action The regional climate fora had two very and specify coordination arrangements positive outcomes. Firstly, they promoted between agencies. A national climate building of regional knowledge through programme establishes a shared discussion that linked global overviews with responsibility for a multidimensional regional specifics. Secondly, local and approach to identification of climate regional media publicity surrounding the sensitivity and vulnerability and to managing climate fora raised community awareness climate risk and the attendant potential for about El Niño and the likely future impacts natural disaster. that were specific to the region. Part of the There are several essential components success of the fora can be attributed to the to a national climate programme, as follows: series of El Niño Updates prepared within • Inter-agency coordination mechanisms the WMO CLIPS project with the support of to ensure that the climate information the IRI, other international partners and and prediction needs of all sectors and national Meteorological and Hydrological the community are recognized. Services. • Collection of meteorological and related The 1997–98 El Niño event has environmental observations and highlighted the existence of a global pattern management of scientific databases of climate-related natural disasters. Many necessary for research, sensitivity and countries within a region faced similar vulnerability studies, and services. impacts and experienced similar losses • Meteorological, oceanographic, because of the scale of the climate hydrological and related scientific anomalies. There will be many benefits from research to improve climate information cooperation at a regional level to build the and prediction services. scientific knowledge and technical • Multidisciplinary studies to establish infrastructure necessary to mitigate the national risk and vulnerability at the impacts and the aftermath of El Niño and community level and within sectors, other climate-related natural disasters. and to formulate appropriate response However, ultimately the management of strategies and recommend national climate risk is a national problem. policies. • Provision of information and prediction National climate programmes services to meet community and sectoral needs. National governments have the • A linkage to regional and international responsibility for protection of the lives and programmes relevant to national the well-being of the people of their objectives and capable of providing respective countries. Mitigation of climate- national benefits. related natural disasters, such as those A national climate programme is an linked to El Niño, is a major consideration in essential component of any national the overall scope of protection and well- strategy to develop preparedness for being. The management of climate risk is hazards associated with climate risk; it multidimensional and involves agencies with would also be an essential component economic, environmental, social and assisting in the management of prevailing developmental objectives. Mitigation of climate anomalies and the mitigation of climate-related natural disasters requires a their effects. A national climate programme formal framework within government to will promote the necessary synergies to develop, at all levels, the synergies between ensure that science and technology are science and technology and decision-making integrated into planning and decision- and planning. making processes related to natural Several governments have established disaster reduction at all levels and across formal national climate programmes as a all sectors. 77 Appendix

Climate Processes

The atmosphere seasonal patterns of frequency and intensity. Each location on the globe has a climate that The weather systems have their origins reflects primarily the basic geographical in the motions of the atmosphere and the characteristics of latitude, altitude, aspect oceans that are constantly acting to and continentality. Locations nearer the redistribute heat and energy around the poles have a pronounced seasonal cycle of globe, particularly in response to the heating and cooling and marked difference seasonally changing solar heating and the in day length between summer and winter. ever-present cooling to space. Primarily, the In the tropics, where day length and solar atmosphere and the oceans are acting to heating are more uniform throughout the transfer excess heat from the tropics to polar year, it is humidity and rainfall that are regions. In each hemisphere the rate of better markers of seasonality. Altitude and poleward energy transport is a maximum aspect can cause significant variations in during winter when the temperature contrast local climate over relatively short distances. between the pole and the equator is a Inland locations tend to have greater ranges maximum. of climate characteristics between summer The motions of the atmosphere are and winter then those at similar latitudes complex because of the role of water vapour closer to the coast. and latent energy, and because of In addition to the basic geographical conservation of absolute angular momentum controls over local climate there are also on the rotating Earth. In the tropics a direct the influences of the global pattern of meridional circulation in each hemisphere weather systems and their seasonally (the Hadley Cell) is driven by release of Figure A.1 varying distribution. Mid-latitude cyclones, latent heat in regions of deep atmospheric The mean meridional subtropical anticyclones, monsoon systems convective clouds. Over tropical latitudes circulation in the Hadley Cell of the tropics and tropical cyclones are a few of the warm moist air converges at the surface and, showing the main weather systems that are important in through the release of latent energy in energy exchange different locations. The influence of regions of active convection, ascends processes. Vertical scale is greatly exaggerated. weather systems at any location varies buoyantly to the high atmosphere. The air in (W.R. Kininmonth) during the year because of the characteristic the high atmosphere flowing away from the equator develops westerly momentum Radiation to space (manifest as the westerly jetstreams of subtropical latitudes) because of the Outflow conservation of absolute angular momentum Westerly winds on the rotating Earth. The main features of Jetstreams the meridional Hadley Cell are shown in Figure A.1. Release of Poleward of the Hadley Cell, it is the latent heat and generation of asymmetry of troughs and ridges of the potential energy generally westerly flow that achieves energy Subsidence transfer to higher latitudes. The net effect of warm air flowing poleward and cold air flowing equatorward is an overall transfer of heat poleward. Also, generally there is Inflow ascending air ahead of trough systems and Easterly winds Evaporation, heating sinking air in the region ahead of ridges. Tropics Subtropics Therefore, in middle latitudes the horizontal 78 Appendix. Climate Processes circulation is characterized by large-scale westerly currents but the vertical meridional circulation is indirect, with sinking air in the West winds subtropics and rising air poleward in the East winds mid-latitudes. At the surface, anticyclonic high L pressure systems are favoured in the sinking L air ahead of the upper ridges and cyclonic systems are favoured in the ascending air H ahead of upper level troughs. The H characteristic circulation of the mid-latitudes Trade winds Meridional East winds is for a westerly flow with slowly moving circulation trough and ridge systems, and transient eddies at the surface recognized as cyclones Trade winds and anticyclones. The main features of the H H H atmospheric circulation are shown in Figure A.2. L Clouds and their ever-changing L characteristics and patterns of spatial East winds distribution in the atmosphere are an important feature of the climate system. West winds Clouds reflect incoming solar energy to space and so modify the amount of solar energy absorbed locally by the earth- atmosphere system. A change in the pattern circulation. Over the oceans there is an Figure A.2 of cloudiness will also change the pattern of abundant source of latent energy and it is A schematic longwave radiation to space, including the wind speed that is an important factor in representation of the essential features of the local rate of energy loss from the earth- regulating evaporation and the rate of mean large-scale atmosphere system. Clouds with high cold energy transfer. Over land, the rate of circulation of the tops emit less radiation than clouds with low evapotranspiration is a function of both atmosphere, showing a typical pattern of surface warmer tops; regions of cloud-free air emit wind speed and the surface characteristics; atmospheric pressure maximum radiation consistent with the high soil moisture and a vegetated surface and (in greatly generally higher temperature at the surface provide a high rate of evapotranspiration exaggerated vertical scale) the zonally of the Earth. while dry soil has a very low rate. averaged meridional Clouds also provide a direct energy The processes for absorption of solar (left) and zonal source to the atmosphere. Latent energy is radiation at the Earth’s surface change circulations (right). released as water vapour condenses during depending primarily on whether the surface (Tegart and Zillman, 1993) cloud formation, and clouds generating is land or ocean. Over land, incoming solar precipitation provide a net release of latent radiation is absorbed at the surface and the energy to the atmosphere. Clouds are rate of conduction through the underlying embedded in weather systems, either within earth is slow. At a depth of a few metres layers of ascending air or in zones of there is very little temperature change convective instability. In both situations the through the year. Soil materials conduct heat released latent energy is converted to slowly and have a relatively low thermal potential energy in the rising air. The direct capacity. As a consequence, the surface release of latent energy in clouds drives warms rapidly under the influence of meridional overturning and regions of seasonally increasing solar radiation. During cloudiness have a significant influence on winter, when solar radiation is at a the atmospheric circulation. minimum, land surfaces continue to emit longwave radiation but conduction of heat from the underlying soil is relatively slow Surface exchange and the land surface temperature cools. processes By contrast, over the oceans solar radiation penetrates into and is absorbed in Solar radiation is partially absorbed by the the surface layers of the oceans. The water atmosphere but it is direct heat transfer and has a relatively high thermal capacity and evaporation of moisture (latent energy) from wind-driven mixing of the ocean surface the underlying surfaces that are the major distributes excess heat through the upper sources of energy to drive the atmospheric layer. During winter the wind-driven 79 mixing brings heat to the surface to offset (hPa) longwave radiation loss. The sea surface 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 temperature responds more slowly to 60°N seasonally varying solar radiation than does 7.5 m/s the land because of the very high thermal capacity of water and the ability to transfer 40°N heat through the mixed layer. Neighbouring land and water bodies 20°N heat and cool at different rates as a consequence of the differing processes for absorption of solar radiation and emission of EQ longwave radiation. Land will become relatively hotter in summer and cooler in 20°S winter causing temperature and pressure gradients in the overlying atmosphere across the boundaries. Air will blow inland from 50°E 100°E 150°E 160°W 110°W 60°W 10°W the relatively cooler ocean as the land heats in summer and will blow toward the sea as The upper layer of the ocean is a region Figure A.3 the land cools in winter. The relative of near-uniform temperature with depth Annual average sea level pressure (1 000 hPa) distribution of land and sea has a significant because wind blowing over the water wind field. influence on the global climate and its surface generates waves and turbulence. The (Adapted from Harrison seasonal cycle because of the different mixing action distributes heat through the and Larkin, 1996) responses of land and ocean surfaces to layer. The depth of this ‘mixed layer’ is varying solar radiation. governed by accumulation of heat and by The annual average sea level pressure the speed and resulting turbulence of the and wind field distribution over the oceans prevailing wind. It is characteristically of the is shown in Figure A.3. But the patterns do order of 100 to 200 metres in the tropics. not reflect the seasonal variability within the The thermocline is the narrow layer that annual cycle. Notwithstanding, there are separates the relatively warm mixed surface clear signals of dominant processes in the layer from the colder underlying ocean. The Asia-Pacific region. In the Pacific Ocean thermocline is characterized by a decrease there is mean high pressure in the of temperature with depth. Because of the subtropics of both hemispheres and strong temperature gradient and stability of relatively strong easterly flow (the Trade the thermocline, the characteristics of the Winds) in equatorial latitudes. Across the colder and denser deep water can be quite central and eastern Pacific Ocean the mean different from the warmer and less dense latitude of convergence of air from each mixed surface layer. The thermocline is hemisphere (the intertropical convergence important for ocean dynamics because its zone — ITCZ*) is at about 10°N. In the depth largely determines the properties of Indian Ocean there are average high the long ocean waves (internal waves**) that pressure and anticyclonic winds in the can propagate at the density boundary. Southern Hemisphere but no corresponding The ocean surface layer characteristics * ITCZ: The intertropical convergence zone is the region of mean high pressure in the can be seen in Figure A.4, a mean longitude- equatorial region where Northern Hemisphere. Easterly wind in the depth section across the equatorial Pacific winds flowing outward equatorial latitudes throughout the year is Ocean for June. The thermocline is clearly from the subtropical high not a persisting feature of the Indian Ocean. identified as the layer of closely packed pressure regions of each hemisphere meet. The isotherms or strong temperature gradient ITCZ moves northward that rises from the west to the east. and southward with the The ocean surface layer Wind blowing over the ocean surface annual march of solar heating and is furthest generates wind-driven currents. The south early in the The oceans also have characteristic anticyclonic mid-latitude gyres of the North calendar year. processes that are important for climate. Of and South Pacific Oceans, the North and ** Internal waves: In the particular relevance are processes that South Atlantic Oceans and the Indian Ocean tropics, Kelvin waves on modify the sea surface temperature and the reflect the regions of prevailing atmospheric the equator and Rossby rate of transfer of heat between the ocean high pressure and their anticyclonic wind waves away from the equator are two models and the atmosphere. Many of these circulation (see Figure A.3). The Trade of internal waves that are processes arise out of the interaction Winds blowing across the Pacific Ocean important in the dynamics between the atmosphere and oceans at the cause a generally westward directed of the ocean processes interface. equatorial wind-driven ocean current and a associated with El Niño. 80 Appendix. Climate Processes

Figure A.4 0 surface waters. The slow upward flow is (°C) Equatorial depth-section from a depth of around 300 metres, and across the Pacific Ocean showing average 28 upwelling raises the thermocline. 26 100 27 25 23 temperature for June. 24 22 21 Turbulence generated by the surface wind 20 Note the deep mixed 19 18 stress mixes colder water from the top of the 17 layer in the west (left 16 15 thermocline to the surface. The raised side), the thermocline 200 14 (layer of closely packed 13 thermocline off the South American coast in

isotherms indicating a 12 the equatorial depth-section is largely an strong vertical temperature gradient) Depth (m) 300 outcome of this upwelling. 11 and the very cold waters Across the Pacific Ocean, particularly in at depth. central and eastern longitudes, Ekman (NOAA/PMEL, USA) 400 turning also induces a drift away from the 9 equator in both hemispheres. A raising of the thermocline and upwelling of cold water 500 occur along the equator as a result of the 140°E 160°E 180° 160°W 140°W 120°W 100°W June, Latitude = EQ diverging lateral drifts in the surface current. Figure A.5a represents the sea level setup * Ekman turing: Wind piling up of warm waters in the western and thermocline variation across the blowing across water margins. As a consequence, sea level is equatorial Pacific Ocean that develops as a establishes a surface some 60 cm higher and the thermocline is consequence of the prevailing Trade Winds. current in the direction of the wind. However, pushed deeper in the western Pacific Ocean. Figure A.5 (b and c) indicates the changes in because of the rotation of In addition to the wind-driven current sea level setup and thermocline depth the Earth the current has in the direction of the wind a lateral drift is resulting from stronger and weaker Trade a lateral drift to the right of the wind direction in generated by the prevailing wind. This is the Winds respectively. the Northern Hemisphere Ekman turning* that is induced in wind- and to the left of the driven currents by the rotation of the Earth. wind direction in the Sea surface temperatures Southern Hemisphere. An In the Southern Hemisphere the lateral drift easterly wind at the is to the left of the wind direction and in the equator will cause Northern Hemisphere the drift is to the right. Seasonal winds can quickly modify sea divergence in the wind- Lateral drift is particularly important surface temperatures in the coastal margins driven surface ocean current away from the because it is a primary factor for inducing through upwelling, such as occurs annually equator in each upwelling of cold water in the surface layer offshore from the Pacific Coast of South hemishpere and induce of the ocean. For example, southeasterly America. Persisting seasonal winds over the upwelling at the equator. winds blowing parallel to the coasts of open ocean can also significantly modify sea northern Chile, Peru and southern Ecuador surface temperatures. Winds generally act to induce a westward drift away from the coast cool the ocean surface through transfer of in the northward flowing surface waters of latent energy during evaporation, and the Peruvian current. Water at depth is exchange of heat between the ocean to the drawn to the surface to replace the diverging atmosphere. A wind-driven current will have a local heating or cooling effect according to

Figure A.5 (a) the prevailing sea surface temperature AVERAGE WIND Stylized depth-section 40 cm 40 cm gradient. across the equatorial 0 0 The annual cycle of sea surface Pacific Ocean showing 50 m 50 m a) the raised sea level temperature can be seen in Figure A.6. A E OCLIN and lowered thermocline THERM number of seasonal wind effects can be in the west under the 200 m 200 m identified in different regions of the globe. influence of the Trade The sea surface temperatures of the Winds, and how the sea (b) level and thermocline +10 cm STRONG WIND northern Indian Ocean reach their maximum 40 cm 40 cm depth changes under b) -5 cm during the later part of the Northern strong Trade Winds and 0 0 c) weak Trade Winds. 50 m 50 m Hemisphere spring (especially April–May). +20 m Depths below 0 are The temperature and humidity of the air shown in metres, depths 200 m 200 m over the region rises as a precursor to the above 0 in centimetres -20 m onset of the Asian summer monsoon. With (Glantz et al., 1987) (c) the onset of southwesterly winds associated WEAK WIND + 40 cm 20 cm 40 cm +15 cm with the summer monsoon, cooling of these 0 0 waters commences during June. The cooling 50 m 50 m of the northwestern Indian Ocean is aided by upwelling as the seasonal wind blows 200 m +20 cm 200 m parallel to the Somali coast, and from 81 45°E4590°E 135°E180° 135°W90°W °W 0° 45°E4590°E 135°E180° 135°W90°W °W 0° Figure A.6 ° 60 N Monthly departure of sea 40°N surface temperature (SST) from the local annual 20°N mean. Contours are EQ shown grouped by 20°S season every 1°C and anomalies greater than ° 40 S Dec. SST Jun. SST or less than 1°C are shaded red and blue 60°N respectively. 40°N (Harrison and Larkin, 20°N 1998)

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wind-driven advection of cooler water from Figure A.3) establish a wind-driven current the south. The cooler waters of the northern to transport water westwards. As the surface Indian Ocean tend to strengthen the air water moves westward it warms by the temperature gradient between the northern absorption of solar radiation. Through Indian Ocean and the warm Asian continent Ekman turning the surface current at the and contribute to the persistence of the equator is divergent and upwelling of cold Indian summer monsoon. water is induced, particularly in the eastern The southern subtropics of the Indian equatorial Pacific Ocean where the Ocean go through an annual cycle but the thermocline is at a relatively shallow depth. equatorial waters remain warm during the The warming of the surface water as it Southern Hemisphere summer. The westerly moves west and the upwelling of cold winds generated by the Australian monsoon water in the east combine to establish a are neither as strong nor as persistent as cross-Pacific sea surface temperature those of the Asian monsoon. gradient in equatorial latitudes. Higher The easterly Trade Winds blowing surface atmospheric pressure over the across the equatorial Pacific Ocean (see cooler waters in the east and lower 82 Appendix. Climate Processes atmospheric surface pressure over the temperature to be warmer and a strong warmer water of the west maintain a source of direct heat to the atmosphere zonal pressure gradient that sustains the during the Northern Hemisphere summer. Trade Winds. The spatial and seasonal asymmetries of the In the eastern equatorial Pacific Ocean heating sources to the atmosphere are an off the coast of South America there is a important background to a consideration of marked seasonality in sea surface the monsoon systems. temperature. The annual cycle of sea surface Differential summer heating between temperature is associated with the the tropical landmasses and the surrounding strengthening and weakening of the local seas establish surface temperature and southeast Trade Winds, which wax and pressure gradients that provide the energy to wane with the annual migration north and initiate, and assist to maintain, seasonal south of the intertropical convergence zone. monsoons. The Asian monsoon is The equatorial Trade Winds weaken and particularly strong because of the geography upwelling of cold water in the coastal of the Indian and Pacific Oceans and the regions is at a minimum during the Southern topography of the Himalaya Mountains and Hemisphere summer as the intertropical Tibetan Plateau. The wind circulation of the convergent zone is furthest south. As a Asian monsoon extends westward to Africa consequence, the coastal sea surface and eastward across the Pacific Ocean. temperatures off Ecuador and Peru are at a Very cold air and high surface maximum during February and March. The atmospheric pressure forms over the Asian cooling trend returns as the intertropical continent, particularly over the Tibetan convergence zone moves northward during Plateau, during the Northern Hemisphere the Southern Hemisphere winter, causing winter. At the surface cold air flows out from the Trade Winds and upwelling to the region of high pressure, both eastwards strengthen. around a strongly formed Aleutian low Over the northern Pacific Ocean, pressure system in the North Pacific and particularly off the East Asian continent, south-westward into the South China Sea there is a strong annual cycle of seasonal and the northern Indian Ocean. The wind heating and cooling of the surface waters. circulation of south Asia is very persistent for The cooling during the Northern several months and is referred to as the Hemisphere winter is enhanced by very cold Asian winter monsoon. The scale of the airflow from the Asian continent that extracts winter monsoon is judged from the local heat from the East China Sea and the departures of surface pressure and wind northwestern Pacific Ocean. The surface flow of January from annual averages as layers warm during the Northern shown in Figure A.7a. Hemisphere summer months, as the During the Northern Hemisphere monsoon reverses and solar heating summer, rising air in deep atmospheric increases. convective clouds over the relatively hot The annual cycle of sea surface landmass of Asia draws surface air from the temperature in the South Pacific is surrounding relatively cooler waters of the significantly less than that of the North northern Indian Ocean, the South China Sea Pacific, reflecting the absence of a and the East China Sea. The southwesterly continental influence comparable to Asia airflow over the Indian subcontinent and the and its seasonal reversal of winds. south to southeasterly flow of the Indo- China Peninsula have high humidity and converge towards the region of lower Monsoon circulations pressure over the relatively hot Asian continent. This large-scale circulation is the A greater proportion of water covers the dominant summer monsoon of Asia. The Southern Hemisphere than the Northern scale of the monsoon can be judged by the Hemisphere. Consequently, evaporation departure from the annual mean of the from the oceans of the Southern Hemisphere surface pressure and wind fields over the is a stronger source of moisture and latent oceans during July shown in Figure A.7b. energy for the atmosphere than is The maps of average outgoing evaporation from the oceans of the Northern longwave radiation for January, April, Hemisphere. However, the greater July and October in Figure A.8 show the proportion of land in the Northern seasonal progress of the focus of tropical Hemisphere causes the average global convection over the Indo-Pacific region 83 (hPa) -10-9 -8 -7-6-5-4-3-2-101234567 8 9 10 New Guinea as the “South Pacific (a) 60°N 7.5 m/s convergence zone”. Tropical convection is experienced north of the equator 40°N over the Western Pacific and as a line of 20°N convection that extends eastward

EQ across the Pacific Ocean, outlining the intertropical convergence zone. 20°S There is a very marked “dry zone” (as

50°E 100°E 150°E 160°W 110°W 60°W 10°W indicated by high values of outgoing (b) longwave radiation — blue to mauve 60°N 7.5 m/s shading) over the eastern Pacific Ocean

40°N and extending westward along the equator. ° 20 N • By April (Figure A.8b) the regions of

EQ tropical convection over Africa and South America have shifted northward 20°S as they follow the seasonal movement

50°E 100°E 150°E 160°W 110°W 60°W 10°W of maximum solar heating. Also, the intertropical convergence zone of the Figure A.7 with the annual cycle of solar heating and Pacific Ocean is more active north of Departure of monthly sea the seasonally changing monsoon the equator and the South Pacific level pressure and circulations. convergence zone is weaker. monthly wind from their local annual mean for a) • During January (Figure A.8a), solar • The Asian monsoon is well developed January — Asian winter heating is strongest in the Southern by July (Figure A.8c) and extensive monsoon; and b) July — Hemisphere and deep tropical convection is experienced over the Asian summer monsoon. SLP departure is in hPa atmospheric convection (as indicated Indian subcontinent, Indo-China and with positive departures by low values of outgoing longwave the Malay Peninsula. The intertropical shaded red and radiation — yellow to red shading) is convergence zone across the Pacific negative departures blue. mainly south of the equator, especially Ocean is active north of the equator, (Harrison and Larkin, over Africa and South America. particularly over the western Pacific 1996) Convection over the Southwest Pacific Ocean, including the Philippines, and extends southeastward from Papua off Central America. Deep convection is

Monitoring of convection

Clouds, the oceans and land surfaces (including • The temperatures of the sea surface and land vegetation) emit longwave radiation and the intensity areas of the tropics and subtropics are relatively of emission is a function of the temperature of the warm (generally higher than 25°C) and these emitting surface. Wam surfaces have high emission surfaces emit high values of longwave radiation. rates and cooler surfaces have lower emission rates. In The satellite instruments measure high values of the earth-atmosphere system part of the outgoing outgoing longwave radiation over relatively longwave radiation is absorbed by water vapour and cloud-free areas of the tropics and subtropics. carbon dioxide in the atmosphere but for selected In the tropics and subtropics, therefore, high wavelengths the longwave radiation is emitted to values of outgoing longwave radiation are indicative space with little attenuation. of areas of clear sky; low values of outgoing radiation The intensity of outgoing longwave radiation are indicative of high cold clouds, including the tops from the earth-atmosphere system is measured by of deep convective clouds. satellite instruments and is a good indicator of Over periods of months to seasons, tropical areas the presence of deep atmospheric tropical with persisting higher than normal or lower than convection. normal values of outgoing longwave radiation have • The temperarures of the tops of tropical deep respectively less than or more than normal deep atmospheric convective clouds are very cold atmospheric convection and rainfall. Measurement of (–70°C to –80°C) and these cold cloud tops emit outgoing longwave radiation using satellite relatively low values of longwave radiation to instruments is an effective tool for monitoring seasonal space. The satellite instruments measure low shifts in the tropical distribution of deep atmospheric values of outgoing longwave radiation over deep convection, and of departures from the normal convective clouds. seasonal cycle.

84 Appendix. Climate Processes

60°N 60°N Figure A.8

30°N 30°N Average outgoing longwave radiation 2 EQ EQ (W/m ) across the Indo- Pacific region for a) 30°S 30°S January; b) July; c) April; and d) October. Areas 60°S 60°S less than 210 W/m2 are 0° 2 80°E 120°E 180° 120°W60°W0° 0° 2 80°E 120°E 180° 120°W60°W0° (W/m ) (W/m ) shaded orange to red 180200 220 240 260 280 180200 220 240 260 280 and in the tropics these 60°N 60°N regions indicate deep atmospheric convection. 30°N 30°N Areas shaded blue to purple have high values EQ EQ of outgoing longwave radiation, indicating 30°S 30°S relatively clear skies. (NOAA/CDC, USA from 60°S 60°S 0° (W/m2) 80°E 120°E 180° 120°W60°W0° 0° (W/m2) 80°E 120°E 180° 120°W60°W0° NCEP/NCAR reanalysis maps; after Meehl, 1987) 180200 220 240 260 280 180200 220 240 260 280

still experienced south of the equator The persisting tropical and subtropical over the Indian Ocean and as a weaker dry zones, identified by high values of South Pacific convergence zone. outgoing longwave radiation (the blue to • Increased convection is experienced mauve areas of Figure A.8), are important south of the equator over Africa and components of the atmospheric circulation South America during October (Figure because they are the descending regions of A.8d) in response to the southward shift atmospheric overturning. of maximum solar heating. Active • There is a persisting, almost continuous convection persists north of the equator band of descending air in the subtropics over the eastern Indian Ocean and over of the Southern Hemisphere. The band the western Pacific Ocean across the of descending air is broken by a period Philippines, the South China Sea and of convection as the active South Pacific into Indo-China. convergence zone expands during the Tropical regions of Africa and South Southern Hemisphere summer (Figure America exhibit quite different regional A.8a). characteristics of seasonal convection • The subtropical band of descending air patterns to Asia and Australasia. Over Africa of the Northern Hemisphere is not as and South America, the region of deep continuous as for the Southern tropical convection extends across the Hemisphere. It is only during the continent and moves north and south with northern winter that there is a degree of the regularity of seasonal maximum solar zonal continuity. Over Asia the heating. However, over the Indo-Pacific descending air is replaced by region, as the Asian summer monsoon (July) convection during the summer weakens during autumn (October), the monsoon period. However, descending region of deep tropical convection moves air persists throughout the year in a southeastward from the Asian continent to band of the subtropics extending from Indo-China and the equatorial western the eastern Atlantic Ocean across Africa Pacific Ocean. By the Southern Hemisphere to the Middle East. summer (January), the focus of deep tropical • Descending air is a persisting feature atmospheric convection is associated with over the eastern Pacific Ocean, the South Pacific convergence zone, a region particularly south of the equator and in of generally maximum sea surface a persisting tongue that extends temperature in the tropical western South westward along the equator over the Pacific. There is also a region of active deep region of upwelling cold water. tropical convection south of the equator The seasonal march of tropical deep over the Indian Ocean. The regions of atmospheric convection and the persisting tropical convection over the Southern regions of descending air are important Hemisphere oceans contract prior to the characteristics of the annual cycle of the relatively rapid onset of the summer large-scale circulations and climate over the monsoon and deep atmospheric convection Indian and Pacific Oceans. The regions of over the Asian continent. deep atmospheric convection are where 85 latent heat primarily is released to the atmosphere, defining the ascending branch of the Hadley Cell and the western lateral (ascending) branch of the Walker monsoon Circulation. The regions of tropical convection also identify the patterns of transverse monsoon tropical rainfall. The regions of persistent Walker Circulation descending air are where the earth- atmosphere system loses maximum energy to space and are generally dry zones with seasonally low rainfall totals. NH WINTER The region of ascending air that moves seasonally between Asia and the South-West Pacific forms the focus of three extensive vertical overturning circulations that are shown in Figure A.9. The Walker Circulation Walker of the Pacific Ocean has its descending Circulation branch over the dry zone of the eastern equatorial Pacific Ocean. A transverse circulation across the Indian Ocean has its descending branch over mainly desert lateral regions of North Africa and the Middle East. monsoon NH SUMMER There is a lateral circulation that reverses direction during the year as the focus of > 27°C convection moves southeastward from Asia to the South Pacific. Any disruption to the Empirical teleconnection patterns* Figure A.9 seasonal march of convection will be derived from identifying characteristic Schematic representation of the vertical overturning reflected in the larger circulations and climate responses during past El Niño atmospheric circulations weather patterns from Africa to the events have been used to describe large- associated with the Americas. scale features of the variability of the monsoons of a) Northern The basic characteristics and processes atmospheric circulation, as well as tropical Hemisphere winter; and b) Northern Hemisphere that have been described are indicative of the and extratropical precipitation and summer. During winter the complexity of the climate. In themselves they temperature relationships. Correlation and lateral circulation is are not sufficient to describe the many facets composite analysis have been tools used in driven by cold air flowing out from Asia. The cross- of climate; many important processes of the attempts to understand and document Pacific Ocean and cross- atmosphere, the oceans and their interaction modes of interannual climate variability, Indian Ocean surface have been omitted for brevity. However, the particularly in response to forcing by winds feed heat and moisture to the convection processes described have important tropical anomalies of sea surface extending from Indonesia implications for coupling of the tropical temperature. The use of teleconnection and northern Australia to circulations of the oceans and atmosphere patterns implies that there is a physical the South-West Pacific. and serve to put interannual variability reason for the simultaneous variation over During summer, surface winds blowing across the associated with the El Niño/Southern various parts of the globe in response to the Indian Ocean and the Oscillation phenomenon in context. ocean forcing. Pacific Ocean feed heat Studies based on data from 1963 to and moisture to the deep convection extending Teleconnections 1995 have identified four main across Asia. teleconnection patterns in the Northern (Webster, 1995) Hemisphere associated with sea surface Prior to the TOGA project there had been temperature changes in the tropical Pacific considerable debate about the relative roles Ocean. of extratropical and tropical sea surface • The Pacific North America pattern is temperatures in forcing characteristic one of the most prominent of all responses in the atmosphere circulation. teleconnection patterns in the winter Studies associated with the TOGA project half of the year. It has four centres of have identified that positive anomalies of alternating sign and arches from the * Teleconnection patterns tropical sea surface temperature influence tropical Pacific Ocean across North are a consistent the atmosphere through surface fluxes of America and is a good example of a climatological response heat, moisture and momentum and a wave train. The Pacific North America of large-scale features of the atmospheric readjustment of the tropical circulation in a pattern tends to be positive during El circulation to systematic direct sense. Niño events. forcing. 86 Appendix. Climate Processes

Preferred regions for deep tropical convection are able to persist on seasonal timescales because of the ability of the L atmosphere to effectively utilize the surface heat and moisture sources and develop large-scale overturning circulations. Strong H North upper atmosphere divergence over the regions of convection in the tropics and convergence in the subtropics act as a L Rossby wave source. However, if there is a change in the pattern of the surface heat source and a shift in the focus of seasonal Storm track deep tropical convection there will be a H changes tendency to modify the upper atmosphere Rossby wave source. Figure A.10 is a simplified Equator representation of one component of the teleconnection resulting from an enhanced H equatorial sea surface temperature, and it approximately corresponds to the Pacific North America pattern. Enhanced deep convection over the region of enhanced sea surface temperature strengthens the upper Figure A.10 • The Western Pacific pattern exists in all atmosphere divergence to each hemisphere. Schematic view of the months and consists of two centres of a As a consequence, there is subtropical dominant changes in the upper atmosphere, dipole orientated north-south in the convergence and an anomalous anticyclone mainly in the Northern western North Pacific. It reflects the pair straddling the equator. A wavetrain of Hemisphere, in response latitudinal variations in the subtropical alternating high and low geopotential to changes in sea surface jetstream of the upper atmosphere. anomalies (anticyclonic and cyclonic temperature, enhanced convection and upper • The North Pacific pattern also exists as respectively) results from the quasi- atmosphere divergence in a dipole in the western North Pacific stationary Rossby wave response. the vicinity of the equator but at higher latitude, and there is some In the Northern Hemisphere the Rossby (scalloped region). (Trenberth, et al., 1998) evidence for a third centre over the Gulf wave response produces a southward shift of Mexico. The North Pacific pattern in the storm tracks associated with the exhibits a preferred response during the jetstream and diminished activity to the northern spring. north associated with the first cyclonic • The Tropical-Northern Hemisphere centre. Corresponding changes may occur in pattern consists of three centres of the Southern Hemisphere. action: one off the west coast of North The climatological stationary planetary America, one near the Great Lakes, and waves and associated jetstreams, especially one to the southeast of the United in the Northern Hemisphere, can make the States. total Rossby wave sources sensitive to the The four teleconnection patterns are position of tropical heating that induces found to project well on the observed them. Also, a number of factors influence atmospheric circulation associated with the the dispersion and propagation of Rossby cold and warm phases of ENSO and in many waves through the atmosphere, including parts of the globe they account for global asymmetries of the climatological substantial percentages of the variance state. The climatological mean planetary during those events. waves create an environment favourable for Anomalous tropical forcing has generally producing internal sources of the energy that been strongest during the northern winter, can compete in magnitude with the original coinciding with the mature stage of the El perturbation. Niño events. In the southern hemisphere The non-linear dynamics of the motions observational studies have shown that a of the atmosphere and coupling with the meridional teleconnection pattern exists underlying surfaces are being better across the south Pacific Ocean and South captured as computer models of the America (sometimes called the Pacific South atmospheric general circulation are America pattern) during the warm phase of improved. One technique to identify the ENSO during the southern winter. impact of sea surface temperature forcing on 87 the atmospheric circulation and climate masking of the El Niño signal by the high characteristics is to compare the seasonal natural variability of the mid-latitudes on climatology of a number of model runs seasonal timescales. using the same sea surface temperature anomaly pattern but slightly different starting conditions in the atmosphere. The Climate prediction ensemble averages from the set of model runs can be used to identify consistent Empirical methods for climate prediction responses. The slightly different starting utilize factors associated with large-scale conditions would be expected to generate slowly varying processes of the atmosphere random variance and so any consistent that regulate, to some extent, departures of signal should reflect the sea surface seasonal climate from the normal annual temperature forcing. cycle. For example, once established a sea Analyses of ensembles of model runs surface temperature anomaly will often highlight the dominance of natural persist for many months and may continue variability generated in the mid-latitudes to force the overlying atmosphere. How the over modest sea surface temperature signals. sea surface temperature anomaly interacts However, when the anomalies of sea surface with the atmosphere above it to produce the temperature are large in the tropics a local climate anomaly may not be fully reliable teleconnection signal can be understood. However, the sea surface expected in the subtropics and middle temperature anomaly has value as a latitudes. The ensemble runs also suggest predictor because the local climates in that the influence of the tropical sea surface various regions of the globe are affected in temperature forcing depends on the time of approximately the same way each time there the year and, in the Northern Hemisphere, is is a similar large-scale anomaly. The utility much less in summer. Teleconnections in the of many empirical methods often cannot be Southern Hemisphere appear to be weaker assessed because neither the length of than in the Northern Hemisphere, and this is observations is sufficiently long nor the due to lower amplitude of standing wave description of the phenomena involved is patterns, induced by land/sea contrasts and adequately quantified. topography. Parallel ensembles of medium-range weather forecasts, with and without tropical Statistical methods sea surface temperature forcing, have been used in an attempt to identify the “synoptic Systematic observation and the collection El Niño signal”. The ensembles of medium- of records of the climate system have range weather forecasts generate a enabled the development of statistical detectable signal by about five days and methods for the prediction of future provide information about the forcing by climate anomalies. Antecedent values of a El Niño on timescales of a week or two that predictor can be mapped with a simple is not available from seasonal timescales. function to produce future values of Analysis of a set of parallel forecasts another variable. The functions or rules over the 1997–98 winter for the Pacific North (the “model”) are derived from a set of America region demonstrated that, by day historical observations and establish the 10, the medium-range forecast model was relationship between the predictor and able to reproduce the Rossby wave trains predictand. that were observed to have propagated out Statistical models have been the basis of the tropics of the Pacific Ocean. However, of early attempts to predict on seasonal the Rossby wave train was time varying, timescales because they are generally indicating that shorter timescale variations in easy to build, easy to apply, can be easy to tropical convection such as those associated understand and diagnose, and require with the Madden-Julian Oscillation might be modest computational resources. Simple important. Also, there is the suggestion that, linear regression is the most common at least on synoptic timescales, unusual model although more complex algorithms, trajectories of the Rossby wave train are the such as logarithmic and harmonic result of ducting by the concurrent large- relationships, and combinations of multiple scale flow. Variability associated with both predictors, are also used. Statistical models the time-varying forcing by tropical have been used also for the exploration of convection and ducting may contribute to possible relationships between different 88 Appendix. Climate Processes climate variables in advance of more Dynamic models detailed analysis or field studies. There are limitations to the use of Dynamic models of the climate system have statistical models. Mostly these limitations evolved in step with computing technology relate to the attempt to use a simple linear and the ability to routinely collect relevant model to explain a complex relationship, data about the climate system. The dynamic especially where the model is not linked to models use first principle physical laws to physical processes. The chosen model and explain or predict the evolution of the the set of predictors will only explain a climate system from one state to another. limited fraction of the total variance and The models, like the climate system, there will still be “error” in the prediction. conserve such properties as mass, energy The potential value of a model arises both and momentum. from the skill in predicting an outcome, and The climate system is far more complex also from narrowing the envelope of than can be adequately handled by first uncertainty about a future event. The principles, even using the most powerful limited skill of the model can be exploited if computers. Many processes take place at it redefines the risk associated with scales of space and time that are not alternative opportunities. The model resolved by the model and these processes identifies either a significantly reduced are parameterized, or simplified to a chance of a given climate event or a statistical decomposition of the process. significantly increased chance of the event. Therefore, in addition to the equations Evaluation of the changed risk of the event governing first principle physical laws there has important application to decision- are sets of equations, rules and tables for the making. parameterizations. Processes, such as Because historical climate records advection of mass and moisture, are treated are relatively short the “training” of a by first principles but processes such as model is built up from a limited set of precipitation and convection, which are experiences. The application is restricted to complex and take place on relatively small the particular locality and/or season, and an scales, are treated by parameterizations. The experience outside the training envelope parameterization of clouds, for example, is (e.g. a new record event) may not be well designed to deal with the statistics of cloud handled. If climate is not stable, for populations and not individual clouds. example if there are decadal to century Dynamical climate models used for scale background trends, then the training seasonal to interannual prediction generally envelope becomes increasingly simulate the following processes: inappropriate for the task and presumed • Advection of mass, moisture, skill may become spurious. There is also a momentum, heat, cloud water and risk in taking a statistical model developed tracers. in one environment (or geographic • Radiation transfer, including absorption, location) and attempting to apply it transmission and reflection of short- elsewhere without historical records for and longwave radiation and the training and verification. interactions with water, ground and More complex statistical models clouds. combine a group of input parameters as • Explicit treatment of diffusive fluxes predictors and attempt to address the basic and these are much more detailed in non-linearity of the climate system. The the surface boundary layer where the training associated with these models can surface exchange processes are of introduce biases because of attempts to critical importance. produce high skill, especially where the • Spatial resolution of 200–300 km in the number of degrees of freedom is high and horizontal and 15 to 40 layers in the where the use of a priori knowledge vertical define the lower limits of assumes patterns of behaviour that are not explicit resolution of processes. necessarily obeyed. If these complex • Temporal resolution of 10 to 30 minutes models are correctly used in the defines the interval between states that appropriate setting and with sufficient are explicitly defined. training data then they can deliver valuable Dynamic models have several positive insights about relationships between climate attributes that favour their continued elements, and can provide improved development in spite of limited skill at this predictive skill. time. Dynamic models potentially give 89 information concerning the internally tropical sea surface temperature are consistent and non-linear behaviour over associated with El Niño events and are time and space of a suite of interrelated driven by ocean dynamics, especially the variables. Dynamic models are tuned, but changes in currents and thermocline with very generally and not to specific depth. In the other oceans the changes in performance with regard to particular surface heat flux (the exchange between the variables in a particular region, and so are ocean and the atmosphere) appear to play a less prone to biased performance through relatively more significant role. The over-fitting. In principle, dynamic models atmospheric components of the coupled should be responsive to changes in the prediction models will have to be quite background state (changes on decadal and accurate to go beyond simulating and longer timescales) because the physical laws predicting the response to El Niño and and parameterizations are representative of accurately simulate the variability generated the climate system and responsive to by changing sea surface temperatures over systematic forcing. the Atlantic and Indian Oceans. This The major negative attribute of dynamic accuracy can only be developed and verified models is the enormous infrastructure through field studies and ongoing necessary to support such systems, monitoring across the Atlantic and Indian particularly in an ongoing operational mode. Oceans. The required infrastructure relates to: The skill of prediction is improving with • The scientific and technical staff to the evolution of scientifically more complex build, tune, maintain and operate the and technically more advanced dynamic model and the range of output systems. climate models. The evolution is carried • A high-speed computing facility and the forward both by the outcomes of scientific skilled staff to maintain and operate the research and the development of more facility. advanced computing platforms. Future • The scientific staff to monitor and analysis of the 1997–98 El Niño event is analyse the model outputs and to expected to contribute to the development interpret the behaviour and skill of the of predictive skill as known processes are model. examined with new data and processes Dynamical climate models that have previously not recognized through lack of been built are premised on the assumption data are discovered and their place in the that predictability of the climate system climate system established (Figure A.11). arises from the response of the atmospheric Dynamic climate models are moving circulation to slowly varying sea surface from a “two-tiered” system, in which a temperature changes, principally the tropical coupled ocean model is used to predict oceans and especially the Pacific Ocean. If ocean temperature which in turn is used to the models are able to approximately drive the atmospheric model. The new simulate that response then they will capture “one-tiered” system is significantly more significant predictability. However, it is complex and uses a single integrated recognized that the actual and model climate coupled model to predict both the ocean systems, especially the atmospheric and atmospheric interactions and evolutions. circulation, are chaotic and models are In each system the initial ocean conditions sensitive to small changes in initial are critical for the skilful modelling of the conditions. Thus, predictability is inherently climate evolution (see Figure A.11). limited. Validation of the performance of To obtain useful predictions from models from different institutions would dynamic models, it will be necessary to suggest that over much of the globe they build models that have: represent the large-scale changes in the • Accurate predictions of sea surface atmospheric circulation, temperature and temperature, particularly of major rainfall relatively well. Predictability seems fluctuations such as El Niño; and to be highest and model performance • Characteristic responses to sea surface appears to be best in many areas of the temperature forcing that is realistic. tropics and subtropics. Of those climate There is a key theoretical issue that elements of direct importance to human must be resolved before significant advances welfare, it is surface temperature that is can be made in the prediction of sea surface almost always better simulated than temperature over the Atlantic and Indian precipitation but this relates to the nature of Oceans. In the Pacific Ocean, changes in the the elements. Temperature varies as a 90 Appendix. Climate Processes

Figure A.11 a) TWO-TIEREDb) ONE-TIERED Two types of dynamic climate prediction scheme: a) two-tiered Ocean data assimilation model where the Ocean data assimilation atmospheric response is driven by the predicted ocean temperature; and b) one-tiered model Coupled where the ocean and model Predict ocean temperature atmosphere are fully Coupled integrated. (W.R. Kininmonth) model Predict both Atmospheric ocean and atmosphere model Predict atmospheric response

two-sided distribution fluctuating about a Regional models mean value; precipitation at a locality is a single-sided distribution taking on positive Experiments are being carried out with values only when disturbed from rest (the regional dynamic models for seasonal non-precipitating condition). On the global climate prediction. These models have a scale evaporation and precipitation are in much higher spatial resolution than global balance and can be constrained to climate models, typically from 10 to 100 km. climatological values, but the distribution They also have more detailed treatments of and intensity of individual and accumulated land surface processes such as soil moisture, precipitation events requires good runoff, vegetation and surface radiative simulation on the scale of weather systems characteristics. The horizontal boundary and sub-grid scale processes. conditions of the regional model, such as A major advantage of dynamic climate changing wind, temperature and moisture) models is that a natural output of ensemble are prescribed from global climate models. predictions (repeated predictions from The potential positive attributes of slightly different starting conditions in the dynamic regional climate models arise atmosphere) is probabilistic information because of their improved ability to resolve about likely future events. Information on topography so that they should better changing probabilities of future events is simulate small-scale effects such as very useful for risk management, particularly seabreezes and convection. There is the the management of low-frequency extreme potential to produce much more realistic events that are the basis of natural disasters. results in terms of spatial and temporal The value of probabilistic information for distributions of rainfall, temperature, wind, risk management is now just beginning to be etc. and thus better meeting many users appreciated across a range of sectors. needs. Against the potential positives there are clear negative attributes. Computing resources and scientific and technical skills are of the same order of magnitude as those required for global modelling but higher resolution observations are required to evaluate performance. At this time there is no guarantee for improvement in the predictions and regional climate modelling remains in the research domain.

91 Selected Bibliography

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94 Acronyms

ACMAD African Centre of Meteorological IPCC Intergovernmental Panel on Climate Applications for Development Change (WMO/UNEP) ADEOS Advanced Earth Observing System IRI International Research Institute for (Japan) Climate Prediction (USA) APN Asia-Pacific Network ITCZ Intertropical convergence zone ARGOS Data relay and platform location system JPL Jet Propulsion Laboratory (NASA) (Sat.) MEI Multivariate ENSO Index ASEAN Association of South East Asian Nations MGA Meteorological and Geophysical Agency AusAID Australian Agency for International (Indonesia) Development NAC National Hurricane Center (NOAA) AVHRR Advanced very high resolution NASA National Aeronautics and Space radiometer Administration (USA) BOM/NCC Bureau of Meteorology/National NCAR National Center for Atmospheric Climatic Centre, Australia Research CDC Climate Diagnostics Center (NOAA) NCEP National Centers for Environmental CIRES Cooperative Institute for Research in Prediction (NOAA) Environmental Sciences (NOAA) NIWA National Institute of Water and CLICOM Climate Computing Atmospheric Research (New Zealand) CLIPS Climate Information and Prediction NMHS National Meteorological and Services Hydrological Service CLIVAR Climate Variability and Predictability NOAA National Oceanic and Atmospheric project (WCRP) Administration (USA) CMA/NCC China Meteorological NSCAT NASA scatterometer (Sat.) Administration/National Climatic Centre OGP Office of Global Programs (NOAA) CPC Climate Prediction Center (NOAA) OLR Outgoing longwave radiation CPPS Permanent Commission for the South PAGASA Philippine Atmospheric, Geophysical Pacific and Astronomical Services DARE Data Rescue Administration DMC Drought Monitoring Centre PEAC Pacific ENSO Applications Centre ECMWF European Centre for Medium-range PMEL Pacific Marine Environmental Laboratory Weather Forecasts (NOAA) ENSO El Niño/Southern Oscillation SENAMHI Servicio Nacional de Meteorología e ERFEN Regional Study of the El Niño Hidrología (Peru) Phenomenon (CPPS) SOI Southern Oscillation Index ETNP Eastern Transitional North Pacific SPREP South Pacific Regional Environment GCOS Global Climate Observing System Programme GDPS Global Data-processing System SST Sea surface temperature GOALS Global Ocean-Atmosphere-Land System TAO Tropical Atmosphere Ocean (WCRP/CLIVAR) TOGA Tropical Ocean Global Atmosphere GOS Global Observing System TOPEX/ Ocean Surface Topography Experiment GTS Global Telecommunication System Poseidon IAI Inter-American Institute for Global UN United Nations Change Research UNEP United Nations Environment Programme ICSU International Council for Science UNESCO United Nations Educational, Scientific IDNDR International Decade for Natural and Cultural Organization Disaster Reduction VOS Voluntary Observing Ship IGBP International Geosphere-Biosphere WCIRP World Climate Impacts Assessment and Programme (ICSU) Response Strategies Programme IGY International Geophysical Year WCP World Climate Programme INAMHI Instituto Nacional de Meteorología e WCRP World Climate Research Programme Hidrología (Ecuador) WMO World Meteorological Organization IOC Intergovernmental Oceanographic WWW World Weather Watch Commission (UNESCO) XBT Expendable bathythermograph

95 Presentations made to the First Global Assessment of the 1997–98 El Niño Event, including rapporteurs for the panel sessions on Risk and Society, held in Guayaquil, Ecuador, 9–13 November 1998

Dr Michael Manton, Bureau of Meteorology Research Dr Reid Basher, National Institute for Water and Atmospheric Centre, Australia: Scientific Overview of El Niño Southern Research, New Zealand: Anomalies and impacts on Oscillation (ENSO). Australia, New Zealand and Papua New Guinea. Dr Yves du Penhoat, Laboratoire d’Études en Géophysique Mr Widada Sulistya, Geophysical and Meteorological et Océanographie Spatiale, France: Global description of Administration, Indonesia: Anomalies and impacts on the 1997–98 El Niño event. equatorial Asia. Dr Simon Mason, International Research Institute for Dr Mike Hamnet, Pacific ENSO Applications Centre, USA: Climate Prediction, USA: Monitoring and predicting the Anomalies and impacts on the islands of the Western global ENSO response. Pacific. Dr Pilar Cornejo, Escuela Superior Politécnica del Litoral, Dr Vern Kousky, NOAA National Centre for Environmental Ecuador: Climate characteristics of the southeast Pacific Prediction, USA: Anomalies and impacts on North and Ocean. Central America. Dr Argiro Ramírez, Colombia; Capt Hector Soldi, Peru; Dr Kelly Sponberg, NOAA Office of Global Programs, USA: Dr Ruben Pinochet, Chile — representing the Permanent Global Impacts Survey of the 1997–98 El Niño. Commission for the South Pacific: Anomalies and impacts Dr Nick Graham, International Research Institute for on Pacific South America. Climate Prediction, USA: Statistical and dynamical Dr Bradford Wilcox, Inter-American Institute, Brazil: methods of climate prediction. Anomalies and impacts on Atlantic South America. Dr Michael Harrison, Meteorological Office, UK: Dr Gabriel Arduino, World Meteorological Organization, Interpreting climate predictions and their limitations. Switzerland: Water resources management in South Dr Holger Meinke, Agricultural Production Systems America. Research Unit, Australia: Applying climate predictions in Dr Raymond Zaharia, Centre National d’Etudes Spatiale, practical decision making. France: Operational ocean monitoring and international Dr Michael Glantz, National Centre for Atmospheric cooperation. Research, USA: Who predicted what, when and how well? Dr Jorge Csirke, Food and Agriculture Organization of the United Nations, Italy: The El Niño and its effects on the Risk and society world and regional fisheries. Panel Sessions coordinated by the IDNDR Secretariat Dr Salvador Lluch-Cota, Centro de Investigaciones (Chairperson: Mrs Marta Dueñas) Biológicas del Noroeste, Mexico: Global ocean Economic Dimension: Risk Reduction and Globalization anomalies and impacts on fish stocks and (Rapporteur: Dr John Scott) resources. The Environmental Dimension: Protecting Natural Dr Zachary Atheru, Drought Monitoring Centre, Kenya: Resources; Ensuring Sustainable Development Anomalies and impacts on central and east Africa. (Rapporteur: Mr Rudi Slooff) Mr Brad Garanganga, Drought Monitoring Centre, The Development Dimension: Building Capacities to Cope Zimbabwe: Anomalies and impacts on southern Africa. (Rapporteur: Mrs Aase Smedler) Dr Li Weijing, Chinese Meteorological Administration, The Societal Dimension: Bringing it to the Community China: Anomalies and impacts on eastern Asia. (Rapporteur: Mr Solon Baraclough)

96 For more information about WMO, please contact: Information and Public Affairs Office World Meteorological Organization 7 bis, avenue de la Paix P.O. Box 2300 CH-1211 Geneva 2, SWITZERLAND ✆: (41 22) 730 83 14 / 730 83 15 Fax: (41 22) 733 28 29 E-mail: [email protected] Internet: http://www.wmo.ch