ClimateChangeScience Information Paper
Australian rainfall — past, present and future
A product of the Australian Climate Change Science Program Copyright and Disclaimer n Australia’s climate is © 2011 CSIRO and the Bureau of naturally variable Meteorology. To the extent permitted n Australia’s east coast, south-east by law, all rights are reserved and no and south-west have become part of this publication covered by drier over recent decades, copyright may be reproduced or copied in any form or by any means except from a wetter period during with the written permission of CSIRO the mid-twentieth century and the Bureau of Meteorology. n the north-west has become CSIRO and the Bureau of Meteorology wetter over the past century advise that the information contained in this n research is helping to explain publication comprises general statements the factors that influence based on scientific research. The reader is advised and needs to be aware that such our rainfall and how they are information may be incomplete or unable being affected by human- to be used in any specific situation. No induced climate change reliance or actions must therefore be made n relative to the long-term average, on that information without seeking prior Australia is likely to become expert professional, scientific and technical advice. To the extent permitted by law, drier in southern areas during CSIRO and the Bureau of Meteorology winter, and in southern and in (including each of its employees and eastern areas during spring. consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.
National Library of Australia Cataloguing-in-Publication entry:
Author: Holper, Paul N.
Title: Climate change, science information paper [electronic resource]: Australian rainfall : past, present and future / Paul Holper.
ISBN: 9781921826481 For further information: (Electronic document : pdf) CSIRO Enquiries ISBN: 9781921826474 Phone: 1300 363 400 (Printed copy) +61 3 9545 2176 Notes: Includes bibliographical Email: [email protected] references. Web: www.csiro.au Subjects: Rain and rainfall--Australia. Climatic changes--Australia.
Other Authors/Contributors: CSIRO.
Dewey Number: 551.577294 ClimateChangeScience Information Paper Australian rainfall — past, present and future
1. Introduction
Australia has a highly variable Changes to water management also climate. It is not uncommon for include the Murray-Darling Basin Plan one region to be in drought while to provide water for the environment and sustainable irrigation activities. flood waters cause damage Climate change presents a threat elsewhere. Superimposed to Australia’s environment and our onto this climatic variability are lifestyle. Higher temperatures are longer-term changes to rainfall. likely to have exacerbated the impact Large swathes of eastern of recent droughts. Global warming may be affecting seasonal patterns and south-eastern Australia of rainfall, leading to decreases in have experienced a drying rainfall and runoff in important food trend over recent decades, producing regions of Australia, such as has the south-west of as the Murray-Darling Basin and south-western Western Australia. Western Australia. Conversely, north-western Australia has This paper summarises what we know about Australian rainfall, including become considerably wetter. historical trends, regional changes, Rainfall changes have important policy patterns of climate variability, and implications. State and Commonwealth projections for the remainder of this governments have begun to respond to century. It draws on extensive data growing populations, rainfall reductions and information collected over the past and likely future changes via a number century by the Bureau of Meteorology, of adaptive measures. These measures on detailed climate change projections include water restrictions, campaigns produced by CSIRO and the Bureau, to lower water consumption, and and on peer-reviewed published construction of new infrastructure research from Australia and overseas. including pipelines, improvements to stormwater and wastewater management, and desalination plants.
1 2. Rainfall observations
Collecting weather data Australia has the lowest average The Bureau of Meteorology maintains an annual rainfall of all inhabited extensive network of weather stations throughout continents. Half of the country Australia. The stations record data including rainfall, temperature, humidity, pressure, receives an average of less than sunshine and wind. In addition to stations run 300 mm per year. Our rainfall by Bureau staff, there is an important volunteer pattern is strongly seasonal, with rainfall network with about 6000 observers. large year-to-year variations. Monitoring of annual to multi-decadal climate Much of the continent is variability is facilitated by the construction dominated by the influence of of high-quality rainfall datasets, developed by selecting high-quality sites, and, where dry, sinking air of the sub-tropical necessary, adjusting long, continuous high pressure belt. When high records to remove artificial shifts caused pressure systems move north by changes in station site characteristics, during winter, westerly winds instrumentation and weather exposure. drive rain-bearing cold fronts Most high quality instrumental climate records across southern Australia. As a extend back only to the late 19th century. Researchers can track earlier climate from a result, much of Victoria, South variety of sources, such as tree rings, fossils Australia and Western Australia and coral. For example, Lough (2003) used experiences wet winters and north-east Queensland coral records to relatively dry summers. More determine rainfall variations from 1754 to 1985. uniform rainfall occurs in southern Tasmania, parts of eastern Victoria and much of New South Wales. In contrast, summers are very wet in tropical northern Australia due to the monsoon. Occasional tropical cyclones bring additional rainfall. Northern Australian winters are typically dry, except for regions near the tropical east coast, leading to the widespread use of the terms ‘wet’ and ‘dry’ season rather than ‘summer’ and ‘winter’ over northern Australia. The first consistent rainfall observations date back to 1832 and demonstrate great year-to-year variability as well as variability over longer timescales, particularly in southern Australia (Power et al., 1999). During the period of modern observations, there have been three major droughts in the south: the Federation drought of 1895-1905, the World War 2 drought of 1936- 1945 and the recent drought that started in 1997 and ended in 2010.
2 Interspersed with droughts have DARWIN been relatively ‘wet’ periods, with Weipa Kalumburu 1600 Millimetres 1950-1990 of significance as this Katherine 1200 Kowanyama 3200 was when water was allocated for 1000 Cairns 2400 3200 Broome Halls Creek Normanton 2000 major irrigation projects in areas 800 2400 Tennant 600 2000 1600 Townsville Port Creek Mount Isa such as the Murray-Darling Basin. Hedland 500 1200 400 Mackay Telfer 1000 300 800 The recent 13-year drought in the Longreach Newman Alice Springs Rockhampton southern Murray-Darling Basin and Giles 200 600 Birdsville 500 Carnarvon Charleville Wiluna 200 Victoria is unprecedented compared Oodnadatta 400 BRISBANE 300 with other recorded droughts since Marree 200 Geraldton Cook Bourke Kalgoorlie- 0 1900 (CSIRO, 2010). (The previous Coffs 300 Boulder Port Harbour Ceduna Dubbo Augusta PERTH 300 record drought lasted 10 years from 800 400 Mildura 500 ADELAIDE 1936 to 1945 (CSIRO, 2010).) The 1000 Esperance Port 400 SYDNEY Lincoln CANBERRA 1200 Albany recent drought was largely restricted Horsham MELBOURNE Orbost to southern Australia and was Average rainfall Warrnambool Annual characterised by lower year-to-year Cape Grim St. Helens rainfall variability and an absence of Strahan Based on a standard 30-year Projection: Lambert conformal with climatology (1961-1990) standard parallels 10oS, 40oS. c years with above average rainfall. HOBART Commonwealth of Australia, 2009 Figure 1 shows Australia’s average Figure 1. Average annual rainfall from 1961 to 1990. Rainfall generally increases annual rainfall from 1961 to 1990. towards the coast, with mountainous areas and the far north receiving higher rainfall. Much of inland Australia is very dry. (1961-1990 represents the World Meteorological Organization standard climatological reference period.) (Source: Bureau of Meteorology)
DARWIN a. b. Weipa Kalumburu 1600 Millimetres Katherine 1200 Kowanyama 3200
1000 Cairns 2400 3200 Broome Halls Creek Normanton 2000 800 2400 Tennant 600 2000 1600 Townsville Port Creek Mount Isa Hedland 500 1200 400 Mackay Telfer 1000 300 800 Longreach Newman Alice Springs Rockhampton Giles 200 600 Birdsville 500 Carnarvon Charleville Wiluna 200 Oodnadatta 400 BRISBANE 300
Marree 200 Geraldton Cook Bourke Kalgoorlie- 0 Coffs 300 Boulder Port Harbour Ceduna Dubbo Augusta PERTH 300 800 400 Mildura 500 ADELAIDE 1000 Esperance Port 400 SYDNEY Lincoln CANBERRA 1200 Albany Horsham
MELBOURNE Orbost Average rainfall Warrnambool Annual © Commonwealth of Australia 2010, Australian Bureau of Meteorology (January, 2010) Cape Grim St. Helens Figure 2. Trends in annual mean rainfall during a.Strahan 1900-2009 and Basedb. on1950-2009. a standard 30-year Rainfall changes since 1950 Projection: Lambert conformal with climatology (1961-1990) standard parallels 10oS, 40oS. c have been significant in many regions. Units are mm perHOBA RTdecade. (Source:Commonwealth of Australia, 2009Bureau of Meteorology)
Figure 3 Rainfall from May to July over south-west Western Australia, 1890 – 2006. (Source: Hope and Ganter, 2010)
3 The rainfall trends across Australia since entered an extended dry spell. From to more than 30 mm per decade across 1950 have been large, with vast areas 1997 to 2009, virtually the entire the north-west third of Australia and experiencing reductions or increases area experienced rainfall below exceeding 50 mm per decade over (CSIRO and Bureau of Meteorology, the long-term average, with some parts of the north-west coast. The wet 2007). Most heavily populated regions places recording the lowest totals on season has become wetter, with only have experienced reductions in annual record. Most of the rainfall decline small rainfall changes occurring during rainfall since the mid-1950s (Figure 2). up to 2006 occurred in autumn, with the rest of the year (Shi et al., 2008). 2006 through 2008 also experiencing Regional changes relatively poor rainfall in winter and Rainfall extremes spring. The rainfall decline affected the South-western Western Australia East coast lows, tropical cyclones, southern area of the Murray-Darling monsoon troughs, severe During the first half of the 20th Basin and the whole of Victoria, thunderstorms, cut-off lows and century, south-west Western Australia with impacts on the environment, mid-latitude fronts can all lead experienced a number of wet winters irrigation industries and the cities of to high rainfall and floods. and few extremely dry winters. This Adelaide, Melbourne and Canberra. changed in the 1970s when winters Trends in extreme rainfall across much Potter et al. (2010) show that a 13% became drier; translating to a significant of Australia have been consistent with reduction in rainfall in the southern reduction of inflows to dams (Power mean rainfall trends, although trends in Murray-Darling Basin from 1997-2006 et al., 2005; Cai and Cowan, 2006). the extremes have typically been greater led to a streamflow decrease of 44%. The 1970s rainfall reduction has been than the mean trend (Alexander et al., exacerbated by a further decline Australian east coast 2007). That is, where average rainfall is since the 1990s (Figure 3). Most increasing, the extremes have tended Rainfall over much of the populous of the reductions have been from to increase at a faster rate. In some eastern coastal region has declined May to July (Hope et al., 2009). regions, rainfall totals have decreased since the 1950s (Figure 2b). The largest but extreme rainfall has still increased. South-eastern Australia drying exceeds 50 mm per decade There are fewer rainfall days but when (CSIRO and Bureau of Meteorology, Rainfall across south-eastern Australia it does rain, it tends to be heavier. 2007). The trend partly reflects a is highly variable on inter-annual and very wet period around the 1950s. Gallant et al. (2007) found that the decadal time-scales (Figure 4). The proportion of total rainfall stemming first half of the 20th century was North-western Australia from extreme events has increased markedly drier than the second half, North-western Australia has seen a since the 1950s in south-western with a significant increase in the late substantial increase in annual rainfall Australia and along the eastern coast, 1940s (Murphy and Timbal, 2008). since the 1950s (Figure 2b), amounting and since the 1970s in the south-east. In the late 1990s, most of the region
900 850 800 750
( mm ) 700
l l
f a 650 n i a
r 600 550 mean
l 500 a u
n 450 n
A 400 350 300 250 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 19 0 19 0 19 1 19 1 19 2 19 2 19 3 19 3 19 4 19 4 19 5 19 5 19 6 19 6 19 7 19 7 19 8 19 8 19 9 19 9 20 0 20 0 Year
Figure 4. Mean annual rainfall over mainland south-eastern Australia (see inset map) from 1900 to 2009. Also shown is the 11-year running mean (solid black). (Source: Timbal et al., 2010)
4
Influences on Australia’s rainfall Numerous factors affect 3. Factors that influence Australia’s rainfall Australia’s climate (Figure 5). There is the mid-latitude Researchers have made The Hadley Cell: The process by jet stream (strong upper- advances in identifying the which heat is transported in the atmosphere westerlies) and atmosphere from equatorial zones factors that influence our the succession of high and to higher latitudes. Due to the low pressure systems that climate. Described here rotation of the Earth, the Hadley move across southern are elements that affect Cell does not extend to the poles. Australia from west to Australia’s rainfall. Many of Rather, the downward return flow east, bringing fronts, occurs along the sub-tropical these elements fluctuate in troughs, warm northerlies, ridge around 30°S and 30°N. cold southerlies, rain, strength, timing and impact. Indian Ocean Dipole (IOD): An and fine weather. None of them acts in isolation. irregular see-sawing of surface water Add a host of regional temperatures across the Indian Ocean. and global oceanic and When the dipole is in a positive phase, Blocking: High pressure cells atmospheric influences, sea water off the Sumatra-Java coast can remain stationary at middle to and it’s little surprise that north-west of Australia tends to be high latitudes, stopping the normal climate — especially cooler than normal, and there are eastward progression of high- and rainfall — changes, fewer rain-bearing systems that extend low-pressure systems. This ‘blocking’ from year to year, to central and southern Australia. A happens most often in winter near decade to decade and negative phase of the dipole produces south-eastern Australia. Cold fronts over the centuries. the opposite effect, with warmer than approaching the blocking region from Australia’s climate is normal waters bringing increased rain. the west often weaken and travel naturally variable. south-east, carrying rain away from Interdecadal Pacific Oscillation/ the continent. However, small cyclonic Pacific Decadal Oscillation: components sometimes form on the El-Niño-like variability that occurs northern side of the blocking high on decadal and longer time scales, pressure cell. The resulting ‘cut-off’ lows linked to decadal changes in (that is, cut off from the mid-latitude ENSO and Australian climate. westerlies) make a major contribution to south-eastern Australian rainfall. Blocking in the Great Australian Bight induces rainfall in western Australia. El Niño – Southern Oscillation (ENSO): There are large-scale atmospheric changes driven by ocean surface temperature differences between waters in the western and eastern Pacific Ocean. The irregular see-sawing of warmer and cooler water across the Pacific gives rise to the cycle of El Niño and La Niña events. El Niños usually reduce rainfall over eastern Australia; La Niñas usually increase rainfall over most of Australia.
Figure 5. Large-scale features that affect Australia’s rainfall (Source: Bureau of Meteorology)
5 Detection and attribution of climate change Identifying a trend in rainfall in a region is one thing; establishing the reason for the trend is more difficult. Climatic variability causes changes on decadal Madden-Julian Oscillation (MJO): Changes to climatic influences A band of low atmospheric pressure time scales. There are changes Since the Industrial Revolution, the originating off the east coast of central caused by variations in atmospheric atmospheric concentrations of greenhouse Africa travelling eastward across the aerosols, through stratospheric gases such as carbon dioxide, methane Indian Ocean and northern Australia ozone depletion, and changes in and nitrous oxide have risen. The increase every 30 to 60 days. The state of the land use. Rising concentrations is contributing to temperature increases MJO can help establish the timing of of greenhouse gases may cause in the troposphere (the approximately rainfall across much of tropical Australia. long-term changes. There is a 10km deep lower atmosphere where In summer, the MJO can intensify and strong motivation for understanding weather occurs) and to changes in other even establish the monsoon as well why climate is changing as this climatic components including rainfall. information will help planners and as help trigger tropical cyclones. There have also been increases in human- water managers know what to Southern Annular Mode (SAM): A generated aerosols and tropospheric expect in future to avoid surprises ring of atmospheric pressure variations ozone, depletion of stratospheric ozone, and to respond adequately. that encircles Antarctica and extends and substantial changes in land use. All of these factors have the potential An important objective of to Australian latitudes. There are to affect regional climate and rainfall. climate science programs alternating changes in windiness and such as the Australian Climate storm activity between the middle El Niño events have become more Change Science Program latitudes (40-50°S) and higher latitudes, frequent and La Niña events less (ACCSP), the Indian Ocean over the southern oceans and Antarctic frequent since the late 1970s (Power Climate Initiative (IOCI) and sea ice zone (50-70°S). In its positive and Smith, 2007). This change is linked the South-Eastern Australian phase, the SAM brings light winds and to a weakening of the trade winds and Climate Initiative (SEACI) settled weather to far southern Australia. a slowing of the Walker Circulation. is to determine the extent Sub-tropical ridge: A belt of high However, in 2010-11, one of the strongest to which changes can be pressure around 30°S (roughly recorded La Niña events led to record attributed to natural climatic straddling a line running through high rainfall across much of northern and variability or other natural Adelaide and Canberra) that separates eastern Australia. This caused widespread causes, and to greenhouse the trade winds of the tropics from flooding. Australia experienced its wettest gas increases and other the westerlies of the mid-latitudes. September on record, the wettest winter human-induced changes. The ridge is characterised by light on record in northern and central Australia, winds, fine and sunny weather and the wettest summer on record in and an absence of storms. It Victoria (Bureau of Meteorology, 2011). moves southwards as summer The SAM has exhibited a pronounced approaches, bringing fine weather positive trend during summer over the past and sunshine to southern Australia. 30 years, corresponding to a decrease Tropical cyclones: These intense in surface pressure over the Antarctic tropical low-pressure systems generally and Southern Ocean and a pressure hold large amounts of moisture. When increase over mid-latitudes, including tropical cyclones hit the coast, they southern Australia (Nicholls, 2007). often bring extensive rainfall. Rainfall Since 1975, there has been a weakening can extend inland, including to southern of the intensity of winter storms (low latitudes, as the cyclone weakens pressure systems) in southern Australian to become a tropical depression. latitudes. This is related to a drop in the strength of upper tropospheric jet stream winds across mid-latitudes of the Southern Hemisphere (Frederiksen et al., 2010). These ‘storm track’ changes are consistent with the observed reduction in rainfall over southern Australia, and in particular, south-west Western Australia. They are also consistent with findings that the recent decline in rainfall in south-western Western Australia is strongly associated with a decrease in the number of low pressure systems (Hope et al., 2006) and increased persistence of high pressure systems (Hope and Ganter, 2010).
6 4. Attribution of observed changes South-western Western Australia no role in the winter rainfall reduction, events (Cai and Cowan 2008) and in and that other factors, such as multi- the Indian Ocean Dipole (Ummenhofer Hope et al. (2006) found that the decline decadal variability or land-use changes, et al., 2009). The Indian Ocean Dipole in rainfall in south-western Western must be invoked to fully account for in spring has been trending upwards Australia from the period 1958–1975 the observed winter rainfall reduction. (Cai et al., 2009a), at least in part due to 1976–2003 was strongly associated to human-induced climate change with a reduction in the number of low A negative value of SAM can have a (Cai et al., 2009b) so it may account pressure systems crossing the region. drought-breaking influence on south- for some of the decline of rainfall in More recent changes, including an western Western Australia. However, spring across south-eastern Australia. apparent further drop in rainfall in 2010 marked the highest positive However, the Indian Ocean Dipole is the 1990s, are associated with the winter SAM value on record (Cai et al., unlikely to explain the autumn-winter persistence of high pressure systems. in press). The positive phase of SAM is rainfall decline because then it has For example, 2006 was a record associated with above-average surface little impact in the region (Timbal and breaking year, with very low rainfall totals pressure in the vicinity of southern Murphy, 2007; Nicholls 2009). and mean sea-level pressure values Australia. Climate models project an well above normal across southern increasing SAM trend as warming Recent work by Watterson (2010) Australia (Hope and Ganter, 2010). continues, meaning that the long-drying indicates that sea-surface temperature trend is expected to continue through anomalies to the north and west Since the 1970s, upper level winds have the 21st century (Cai et al,. in press). of Australia are also unlikely to be weakened, reducing storm development contributing to the autumn rainfall (Frederiksen and Frederiksen, 2007). South-eastern Australia decline as they have limited influence IOCI has concluded that it is likely that Numerous studies have examined on south-east Australian rainfall during the serious rainfall decline in south- recent rainfall changes across south- autumn-winter. In addition, much of the west Western Australia is, in part, due eastern Australia. Launched in 2006, observed relationship between south- to the enhanced greenhouse effect SEACI has provided a strong focus for eastern Australia rainfall and these (Timbal et al., 2006), and unlikely this work. There are many potential sea-surface temperatures results from to be a result of natural climatic causes of the autumn-winter rainfall the large-scale atmospheric circulation variations alone (Ryan and Hope, decline that commenced in the late that is both driving the rainfall and 2006). Regional land cover changes 1990s, including changes in the driving the sea-surface temperature. are a possible additional contributor frequency of El Niño and La Niña Nicholls (2009) states that sea surface to the rainfall reduction (Pitman et al., temperatures around northern Australia 2004). Individual contributions of the are tending upwards and so should different drivers are difficult to quantify, have led to increased rainfall rather but one modelling study concluded than the observed overall decline. That that increasing concentrations of was confirmed by Smith and Timbal greenhouse gases have caused half (2010) and quantified by Timbal and of the winter rainfall reduction (Cai Hendon (in press), who found that the and Cowan, 2006). The study also influence of the tropics during the recent concluded that ozone-depletion plays rainfall deficiency across south-eastern Australia has been positive overall (hence reducing the magnitude of the deficit) with a small negative contribution in spring. Despite positive sea-surface temperature trends, the influence of El Niño and La Niña variations is also limited because there is not a pathway for these phenomena to affect rainfall in autumn and winter (Cai et al., 2011). The autumn rainfall decline has been associated with increasing intensity of the subtropical ridge over eastern Australia (CSIRO, 2010). Nicholls (2009) accounts for a similar proportion of the rainfall decline through the increase in surface pressure, which he ascribes to the upward trend in the SAM.
7 A climate modelling study showed conditions in 2010-11 highlights that the observed intensification of the importance of La Niña events the high pressure ridge could only in balancing the impact of El Niño be achieved when anthropogenic ‘droughts’. An imbalance between greenhouse gases are included La Niña and El Niño events in recent (CSIRO, 2010) consistent with the decades is a part explanation for notion that enhanced greenhouse the apparent drying trends. gases force the SAM into its high As noted earlier, a particularly strong phase (e.g. Cai and Cowan, 2006). La Niña event led to record high rainfall These changes do not fully account across much of eastern Australia, for the lengthy drought and it remains causing widespread flooding. likely that natural variability was North-western Australia also a contributing factor (CSIRO, 2010). Nicholls (2008) suggests the There remains substantial uncertainty possibility that climate change is about the cause of the large rainfall increasing the severity of Australian increases in north-west Australia. droughts, by raising temperatures Climate model simulations driven only and increasing evaporation. Cai and by increased greenhouse gases do Cowan (2008) similarly suggest that not reproduce well the rainfall increase rising temperatures have dramatically over north-western and central reduced runoff in the Murray-Darling Australia (Cai et al., 2011). Rotstayn Basin. However, these findings are et al. (2007) report that inclusion of not supported by the rainfall-run- human-induced aerosol changes in off modelling studies by Potter et model simulations results in a better al. (2010), who showed that only a match with the increasing rainfall small fraction of the decreased run- observed over north-western Australia. off response to reduced rainfall is Aerosol from Asia largely stays in linked to increased temperature. the Northern Hemisphere; they are East coast not transported over Australia. The Asian haze may alter the north-south The general drying trend over eastern temperature and pressure gradients Queensland since 1950 (Figure 2b) is over the tropical Indian Ocean, thereby consistent with increased frequency increasing the tendency of monsoonal and severity of El Niño events since the winds to flow toward Australia, 1970s (Gallant et al., 2007). There have bringing rainfall to the north-west. been more than twice as many El Niño Thus, researchers have suggested events as La Niña events in this period that human-generated aerosols and El Niño has been more dominant in from Asia may have contributed recent decades than in any other period to the observed rainfall increases on record (Power and Smith, 2007). in the northwest. However, due to Kang et al. (2011) found that including the complexity of the underlying polar ozone depletion in climate models processes, limitations of the model led to a poleward displacement of and the effects of natural variability, the Southern Hemisphere westerly it is difficult to draw firm conclusions jet, accompanied by a poleward shift (Rotstayn et al., 2007; Shi et al., 2008). of mid-to high-latitude precipitation. Wardle and Smith (2004) have argued The most noticeable consequences that warming of the Australian continent in Australia is a precipitation might have strengthened the monsoon, increase across eastern Australia resulting in increased rainfall. However, in summer, therefore suggesting they left open the question of whether that the observed decline has been recent increases in greenhouse somewhat mitigated by a tendency gases are sufficient to have caused toward the positive phase of SAM. the necessary amount of warming. The extent to which global warming is responsible is being investigated through the ACCSP. The record wet
8 5. How will rainfall change in future? Climate Change in Australia errors that are important to Australian median and the majority of individual (CSIRO and Bureau of projections, including in temperature model results, (Figure 6), caused and precipitation patterns that affect by the contraction in the rainfall belt Meteorology, 2007) presents model representation of the seasonal towards the higher (more southern) extensive information about cycle and ENSO. Cai and Cowan (2006) latitudes. Future changes in summer likely future rainfall changes. report that models simulate well the tropical rainfall in Northern Australia seasonality in the relationship between were uncertain. Figure 7 shows the Reliability of projections SAM and rainfall in south-western probability of annual rainfall change Regional precipitation variations Western Australia. However, Cai et al. exceeding various thresholds. are sensitive to small differences in (2010) state that most models perform It is likely that the most intense rainfall atmospheric circulation and other poorly in simulating the relationship events in most locations will become processes, as is evident from Australia’s between ENSO and the rainfall decline more extreme, driven by a warmer, large natural rainfall variability. It is in south-eastern Queensland. wetter atmosphere. The combination difficult to make definitive statements Models generally simulate too much of drying and increased evaporation about likely future rainfall changes for light rain, and underestimate the means soil moisture is likely to decline particular places. CSIRO and Bureau contribution and frequency of heavy over much of southern Australia. of Meteorology (2007) acknowledge rain (CSIRO and Bureau of Meteorology, that projections of global and regional CSIRO has undertaken high-resolution 2007). CSIRO (2010) reports that the climate change contain a number of climate change projections for Tasmania seasonal pattern of rainfall deficits uncertainties. Predictability is limited (Grose et al., 2010), revealing a pattern for 1997-2006 across south-eastern by factors such as human behaviour of increased rainfall over the coastal Australia did not resemble the and the uncertainties inherent in regions, and reduced rainfall over patterns of rainfall changes under complex systems. However, global central Tasmania and in the north-west. global warming projected by climate climate models continue to improve in models. However, the seasonal pattern Models show an increase in daily rainfall their ability to represent current global of rainfall deficits across the region intensity and in the number of dry days. and regional patterns of temperature, for 2006-2009 does more closely Thus, we will need to wait longer for precipitation and other variables. resemble the projected seasonal rain, but when it does fall, it will be Global rainfall patterns strongly reflect patterns of change (CSIRO, 2010). heavier. Extreme daily rainfall tends to the general atmospheric circulation. increase in many areas but not in the Likely future changes Climate models represent overall south in winter and spring when there features well, including subtropical CSIRO and Bureau of Meteorology is a strong decrease in mean rainfall. (2007) project a reduction in rainfall dry zones and the position of mid- There is no consensus among climate for southern areas of Australia, latitude storm tracks (CSIRO and models used for the Intergovernmental especially in winter, and in southern and Bureau of Meteorology, 2007). There Panel on Climate Change (IPCC) Fourth eastern areas in spring (based on the are, however, some systematic Assessment on how global warming
Figure 6. Best estimate of projected percentage rainfall change for 2030 relative to 1990 under a mid-range emissions (A1B) scenario. Stippling indicates where a decrease is likely. (Source: CSIRO and Bureau of Meteorology, 2007)
9 Figure 7. Probability (%) of annual rainfall change, relative to 1990, exceeding various thresholds based on the spread of climate model results. The mid, low and high emission scenarios are A1B, B1 and A1FI (IPCC, 2000). (Source: CSIRO & Bureau of Meteorology, 2007)
(CSIRO and Bureau of Meteorology, 2007). Positive values of SAM are associated with decreased winter rainfall over south-east and south- west Australia (Hendon et al., 2007). In recent years, south-eastern Australia and south-western Western Australia have suffered rainfall declines more severe than those projected by climate models. These declines may be in part due to natural climate variability. If this is the case, rainfall may return to levels closer to those experienced before the current dry periods. In regions where rainfall reductions are projected, such reductions may be from a base level that is higher than will influence ENSO variability in the over the coming century are likely to recently experienced. Thus, it may Pacific Ocean, except that ENSO intensify, and replenishing rainfall that be some decades before the impact will remain a major cause of climatic often occurs during La Niña years to of human-induced climate change variability over much of Australia and be somewhat reduced. Furthermore, overcomes natural variability and long- beyond. Rainfall over southern Australia rainfall deficiencies during El Niño term drier conditions are experienced. is projected to decline in response to years will be accompanied by higher The fact that in many places rainfall global warming, even if ENSO variability temperatures, which may exacerbate from 1961-1990 was markedly above in the Pacific does not change. If water shortages in some places. the long-term average is a reminder this rainfall decline is evenly spread SAM is likely to shift towards its that using rainfall trends starting throughout the ENSO cycle then rainfall positive phase (weaker westerly winds within this period is unwise. This deficiencies during El Niño events over southern Australia and stronger anomalous wet period exaggerates westerly winds at higher latitudes). All the magnitude of the subsequent climate models exhibit a trend in the rainfall decline (Timbal, 2010). SAM towards its positive phase as greenhouse gas concentrations rise
10 6. Gaps in our knowledge The ACCSP has made strong climate. Increasingly, these data of extreme events including tropical contributions to understanding are supplied publicly via web sites. cyclones, storms, drought, heatwaves, Developments in database and bushfire and floods and may increase and better modelling of web technology will permit more their impact. Nicholls (2008) has factors responsible for rainfall sophisticated data products to be identified research needs involving variability and trends. Research made available for research and improvements to data and modelling, topics have included ENSO, to support decision making. as well as improved understanding of the causes of climatic extremes. the influence of the Indian Already we are witnessing decreases Ocean, aerosols, the monsoon in surface atmospheric pressure Regional climate modelling and at high latitudes and increases in downscaling regional-scale atmospheric and tropical cyclones. the sub-tropics. These and other variables to local scales will help to large-scale changes are bound determine the likely local impacts to affect Australia’s rainfall. of climate and climate change, Since 1998, IOCI has been providing especially those associated with information about the climate of Identifying and quantifying the extent climatic extremes. Outputs from these Australia’s south-west. It has yielded of human impact on climate change downscaling techniques are dependent insights into the nature and causes is important. Nicholls (2007) listed the on the quality of the larger-scale of the rainfall decline experienced following priority research areas that global climate model simulations. during the past decades. Building are relevant to rainfall in Australia: Climate science has progressed on the experience gained, SEACI n influence of aerosol in north- dramatically in the past few decades. was launched in 2006. This program western Australia; explores climate, and climate variability However, there is still much to learn and change in the south-east of n impact of stratospheric ozone about atmospheric and oceanic Australia, including the Murray Darling declines and greenhouse gas influences on Australia’s climate, and Basin, and the whole of Victoria. increases on summer rainfall the changes to these influences caused There is still much to be learned on over parts of Queensland and by rising atmospheric concentrations both sides of the continent about how autumn-winter rainfall across of greenhouse gases. Researchers rainfall is likely to change in future. New South Wales and Victoria; need to better quantify, understand and evaluate climate change feedbacks, There are strong arguments for n circulation changes such as such as those due to water vapour, similar investigations of rainfall on ENSO, SAM and the position clouds, lapse rate and surface the eastern seaboard, where east- of the sub-tropical ridge; albedo, and establish the impacts coast lows and the trade winds n changes to seasonal cycles such of aerosol on climate. The benefits have a considerable impact, and as the onset of the monsoon; will include better climate models, also in tropical Australia. There are narrowing the range of climate change important questions to be answered n tropical cyclone intensity projections where possible, and helping for Australia’s north, such as on the and frequency; and quantify projection uncertainties. timing and strength of the monsoon, n extreme rainfall events. and on the likely future frequency and strength of tropical cyclones. Climatic extremes are a regular feature of the Australian climate, often causing Fundamental to any assessment considerable disruption, loss of life of climatic trends is the quality and and expense. Climate change is likely distribution of observations. These to alter the frequency and intensity observations must continue, and in some cases, be expanded if we are to comprehend the precise nature and impacts of changing
11 7. Conclusions 8. Credits Australia’s rainfall is highly variable. This paper has been prepared by Paul We experience this variability in many Holper on behalf of the ACCSP, funded ways — through dry spells, downpours, jointly by the Department of Climate drought and flood. Superimposed Change and Energy Efficiency, the upon this variability are longer-term Bureau of Meteorology and CSIRO. trends, with large increases in tropical Trends in Australian rainfall and their Australia and below-average rainfall causes and likely future changes experienced for more than 30 years in represent an area of great importance the south-west, and for more than a and some uncertainty. The Australian decade in the east and south-east. Climate Change Science National Framework and the ARC Centre for Extrapolation of recent trends is not Climate System Science will provide a reliable way to predict the future. valuable opportunities for further Australia’s climate will continue collaboration and advances in this field. to be variable. Overlaid upon this variability is human-induced Thank you to the following for their climate change caused by rising valuable contributions: Rob Colman, concentrations of greenhouse gases. Harry Hendon, Doerte Jakob, David Jones, Brad Murphy, Scott Power Effective planning demands the best and Bertrand Timbal from the Bureau available information about rainfall of Meteorology; Imogen Jubb, Karen changes, trends and projections. Pearce and Kathy Pullman from Research is helping understand the ACCSP; Helen Cleugh, Mary the factors that influence our Mulcahy, Meg Rive, Leon Rotstayn, rainfall and the variations that Simon Torok and Penny Whetton climate change is likely to bring. from CSIRO; and Will Steffen from the Australian National University. .
12 9. References Alexander, L., P. Hope, D. Collins, B. Cai, W., P. van Rensch, S. Borlace Trewin, A. Lynch and N. Nicholls and T. Cowan (in press) Does (2007) Trends in Australia’s the southern annular mode climate means and extremes: contribute to the persistence of A global context. Australian the multidecade-long drought over Meteorological Magazine, 56, 1-18. southwest Western Australia? Geophysical Research Letters. Bureau of Meteorology (2011) The 2010-11 La Niña: Australia Cai, W., P. van Rensch, T. Cowan and soaked by one of the strongest H.H. Hendon (2011) Teleconnection events on record. http://www. pathways for ENSO and the IOD bom.gov.au/climate/enso/ and the mechanism for impacts feature/ENSO-feature.shtml. on Australian rainfall. Journal of Climate, 24, 2556-2564. doi: Cai, W. and T. Cowan (2006) SAM and 10.1175/2011JCLI4129.1. regional rainfall in IPCC AR4 models: can anthropogenic forcing account CSIRO (2010) Climate variability and for southwest Western Australian change in south eastern Australia - A Hendon, H., D. Thompson and winter rainfall reduction? Geophysical synthesis of findings from Phase 1 of M. Wheeler (2007) Australian Research Letters, 33, L24708. the South Eastern Australian Climate rainfall and surface temperature Initiative (SEACI). CSIRO, 30 pp. Cai, W. and T. Cowan (2008) Dynamics variations associated with the of late autumn rainfall reduction over CSIRO and Bureau of Meteorology Southern Annular Mode. Journal south-eastern Australia. Geophysical (2007) Climate change in Australia. of Climate, 20, 2452-2467. Research Letters, 35, L09708, Technical report, 140 pp. www. Hope, P., W. Drosdowsky and N. doi:10.1029/2008GL033727. climatechangeinaustralia.gov.au Nicholls (2006) Shifts in the Cai, W., T. Cowan and A. Sullivan Frederiksen, J. and C. Frederiksen synoptic systems influencing (2009a) Recent unprecedented (2007) Inter-decadal changes in southwest Western Australia. skewness towards positive Southern Hemisphere winter storm Climate Dynamics, 26, 751-764. Indian Ocean Dipole occurrences track modes. Tellus, 59, 599-617. Hope, P. and C. Ganter (2010) Recent and its impact on Australian Frederiksen, J., C. Frederiksen, S. and projected rainfall trends in south rainfall. Geophysical Research Osbrough and J. Sisson (2010) west Australia and the associated Letters, 36, L11705. Causes of changing Southern shifts in weather systems. In: Cai, W., T. Cowan and A. Sullivan Hemisphere weather systems. In: Proceedings of GREENHOUSE (2009b) Climate change contributes Proceedings of GREENHOUSE 2009 Conference, [Jubb, I., P. to more frequent consecutive 2009 Conference, [Jubb, I., P. Holper and W. Cai (eds)]. CSIRO positive Indian Ocean Dipole Holper and W. Cai (eds)]. CSIRO Publishing, Melbourne. events. Geophysical Research Publishing, Melbourne. Hope, P., B. Timbal and R. Fawcett Letters, 36, L23704. Gallant, A., K. Hennessy and J. Risbey (2009) Associations between Cai, W., P. van Rensch, T. Cowan (2007) Trends in rainfall indices rainfall variability in the southwest and A. Sullivan (2010) Asymmetry for six Australian regions: 1910- and southeast of Australia in ENSO teleconnection with 2005. Australian Meteorological and their evolution through regional rainfall, its multidecadal Magazine, 56, 223-239. time. International Journal of variability, and impact. Journal Grose M.R., I. Barnes-Keoghan, S.P. Climatology, 30, 1360-1371. of Climate, 23, 4944-4955. Corney, C.J. White, G.K. Holz, J.B. IPCC (2000) Emissions Scenarios. Cai, W., T. Cowan, A. Sullivan, J. Ribbe Bennett, S.M. Gaynor and N.L. Special Report of the and G. Shi (2011) Are anthropogenic Bindoff (2010) Climate Futures for Intergovernmental Panel on aerosols responsible for the Tasmania: General climate impacts Climate Change. Nakicenovic, N. northwest Australia summer rainfall technical report, Antarctic Climate & and R. Swart (eds). Cambridge increase? A CMIP3 perspective and Ecosystems Cooperative Research University Press, UK. 570 pp. implications. Journal of Climate, Centre, Hobart, Tasmania. Kang,S., L. Polvani, J. Fyfe and doi: 10.1175/2010JCLI3832.1 M. Sigmond (2011) Impact of polar ozone depletion on subtropical precipitation. Science, doi:10.1126/science.1202131
13 Lough, J. (2003) Lessons from the Power, S., F. Tseitkin, V. Mehta, S. Timbal, B., J. Arblaster, K. Braganza, past: climate from corals. In: Torok and B. Lavery (1999) Decadal E. Fernandez, H. Hendon, B. Stone R and Partridge I (eds), climate variability in Australia during Murphy, M. Raupach, C. Rakich, Science for Drought: proceedings the 20th century. International I. Smith, K. Whan and M. of the National Drought Forum, Journal of Climate, 19, 169-184. Wheeler (2010) Understanding Brisbane, April 2003, Queensland the anthropogenic nature of the Queensland Government (2007) Department of Primary Industries. observed rainfall decline across QLD AS1.01 Annual average and South Eastern Australia, CAWCR Murphy, B. and B. Timbal (2008) A percentile rainfall. Technical Report No. 026. http:// review of recent climate variability www.epa.qld.gov.au/soe-online/ www.cawcr.gov.au/publications/ and climate change in south-eastern SOWEB300.jsp?IndicatorId=380. technicalreports/CTR_026.pdf Australia. International Journal Rotstayn, L., W. Cai, M. Dix, G. of Climatology, 28, 859-879. Timbal B. and H. Hendon (in press) Farquar, Y. Feng, P. Ginoux, M. The role of tropical modes of Nicholls, N. (2007) Detecting, Herzog, A. Ito, J. Penner, M. variability in the current rainfall understanding and attributing climate Roderick and M. Wang (2007) Have deficit across the Murray-Darling change: A background report Australian rainfall and cloudiness Basin. Water Resources Research. on research priorities prepared increased due to the remote for the Australian Greenhouse effects of the Asian anthropogenic Timbal, B. and B. Murphy (2007) Office.Australian Greenhouse aerosols? Journal of Geophysical Observed climate change in Office, Canberra, 29 pp. Research, 112, D09202. the South-East of Australia doi:10.1029/2006JD007712. and its relation to large-scale Nicholls, N. (2008) Australian climate modes of variability, BMRC and weather extremes: Past, Ryan, B. and P. Hope (eds.) (2006) Research Letters, 6, 6-11. http:// present and future. A report on Indian Ocean Climate Initiative Stage www.bom.gov.au/bmrc/pubs/ research for the Department of 2: Report of Phase 2 activity. Indian researchletter/reslett_06.pdf Climate Change. Department of Ocean Climate Initiative, 46 pp. Climate Change, Canberra, 26 pp. Ummenhofer C.C., M.H. England, Shi, G., W. Cai, T. Cowan, J. Ribbe, P.C. McIntosh, G.A. Meyers, M.J. Nicholls, N. (2009) Local and remote L. Rotstayn and M. Dix (2008) Pook, J.S. Risbey, A. Sen Gupta causes of the southern Australian Variability and trend of north west and A.S. Taschetto (2009) What autumn-winter rainfall decline, Australia rainfall: observations and causes Southeast Australia’s 1958–2007. Climate Dynamics, coupled climate modeling. Journal worst droughts? Geophysical doi: 10.1007/s00382-009-0527-6. of Climate, 21, 2938-2959. Research Letters, 36, L04706, Pitman, A., G. Narisma, R. Pielke Smith, I. and B. Timbal (2010) doi:10.1029/2008GL036801. and N. Holbrook (2004) The Links between tropical indices Wardle, R. and I. Smith (2004) impact of land cover change on and southern Australian rainfall. Modeled response of the Australian the climate of south west Western International Journal of Climatology, monsoon to changes in land Australia. Journal of Geophysical doi: 10.1002/joc.2251. surface temperatures, Geophysical Research, 109, D18109. Timbal, B. (2010) A discussion Research Letters, 31, L16205. Potter, N., F. Chiew, and A. Frost (2010) on aspects of the seasonality Watterson, I.G. (2010) Relationship An assessment of the severity of of the rainfall decline in South- between southeastern Australian recent reductions in rainfall and Eastern Australia. CAWCR rainfall and sea surface temperatures runoff in the Murray-Darling Basin. Research Letters, Issue 4, July examined using a climate Journal of Hydrology, 381, 52-64, 2010. http://www.cawcr.gov.au/ model. Journal of Geophysical publications/researchletters.php Power, S., B. Sadler and N. Nicholls Research, 115, D10108. (2005) The influence of climate Timbal, B., J. Arblaster, and S. doi:10.1029/2009JD012120. science on water management Power (2006) Attribution of late- in WA: lessons for climate twentieth-century rainfall decline scientists. Bulletin of the American in southwest Australia. Journal Meteorological Society, 86, 839-844. of Climate, 19, 2046-2062. Power, S. and I. Smith (2007) Weakening of the Walker Circulation and apparent dominance of El Niño: both reach record levels, but has ENSO really changed? Geophysical Research Letters, 34, L18702.
14 15 July 2011 The Australian Climate Change Science Program undertakes research to improve our understanding of the causes, nature, timing and consequences of climate change for Australia and our region. The Program is administered by the Department of Climate Change and Energy Efficiency, with research undertaken by the Bureau of Meteorology and CSIRO.