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An Examination of El Ni~No and La Ni~Na Teleconnections to Sahel

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An Examination of El Ni~No and La Ni~Na Teleconnections to Sahel Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2017 An Examination of El Niño and La Niña Teleconnections to Sahel and Guinea Coast Rainfall in the Context of the 1968 Rainfall Regime Change Thomas Ashley Vaughan Follow this and additional works at the DigiNole: FSU's Digital Repository. For more information, please contact [email protected] FLORIDA STATE UNIVERSITY COLLEGE OF ARTS AND SCIENCES AN EXAMINATION OF EL NIÑO AND LA NIÑA TELECONNECTIONS TO SAHEL AND GUINEA COAST RAINFALL IN THE CONTEXT OF THE 1968 RAINFALL REGIME CHANGE By THOMAS ASHLEY VAUGHAN A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science 2017 Copyright © 2017 Thomas Ashley Vaughan. All Rights Reserved. Thomas Ashley Vaughan defended this thesis on July 10, 2017. The members of the supervisory committee were: Sharon Nicholson Professor Directing Thesis Philip Sura Committee Member Guosheng Liu Committee Member The Graduate School has verified and approved the above-named committee members, and certifies that the thesis has been approved in accordance with university requirements. ii ACKNOWLEDGMENTS First, I would like to express my deepest gratitude to Dr. Sharon Nicholson for affording me the opportunity to study under her guidance and for all of the hard work and time she has dedicated to me and all of her students in the Nicholson Climatology Lab over the years. When I first entered the master’s program at Florida State, I never thought I would end up researching rainfall in Africa. However, as fate would have it, I ended up in the Nicholson Lab studying African climate and it could not have been a more positive experience. I would also like to thank Douglas Klotter, who served as an invaluable resource when I needed data or had technical issues (of which there were many). In addition to Dr. Nicholson and Doug, many other faculty members have mentored and advised me during my graduate school career. Gratitude is owed to my committee members, Dr. Philip Sura and Dr. Guosheng Liu, for all of their input, guidance, and advice on this thesis and in classes. Additionally, I am grateful to Dr. Robert Hart and Dr. Jeff Chagnon for their friendship and unwavering support of me and all of my fellow graduate students in the department. I also want to thank all of my present and former colleagues in the Nicholson Lab (Adam Hartman, Sid King, and Amana Hosten) for the many discussions and for providing ideas for my project. Thank you to Aaron Swearingen and Enoch Jo for the many hours they spent helping me with computer programming and fine-tuning my Matlab codes. Thank you to my family in North Carolina and all the new friends I have made at Florida State for their loving support and patience with me throughout this process. Finally, special gratitude is owed to Matthew Dawson and Thomas McKenzie, two of my fellow graduate students whom have impacted my life more than they will ever know. I doubt I would have finished my master’s degree had it not been for Matt and Tom, and I will be forever grateful for their life advice, support, friendship, and brotherhood. iii TABLE OF CONTENTS List of Tables . vi List of Figures . vii Abstract . xii 1 INTRODUCTION 1 1.1 The West African Sahel and the Guinea Coast . 1 1.1.1 Geographical Overview . 1 1.1.2 Climatological Overview . 3 1.2 Motivation . 5 2 PRIOR RESEARCH 10 2.1 Characteristics of Sahel Rainfall . 10 2.1.1 The West African Monsoon . 10 2.1.2 Atmospheric Circulations and Jets . 11 2.1.3 Seasonal Rainfall Cycle and Intraseasonal Variability . 15 2.1.4 Spatial Variability and the Dipole Pattern . 17 2.1.5 Rainfall Trends in the Twentieth Century . 18 2.1.6 The 1968 Rainfall Regime Change . 19 2.1.7 Global Teleconnections . 20 2.2 El Niño/Southern Oscillation (ENSO) and West African Rainfall . 22 2.2.1 Introduction to ENSO . 23 2.2.2 El Niño and African Rainfall Teleconnections . 24 2.2.3 La Niña and African Rainfall Teleconnections . 28 2.3 Research Questions and Objectives . 28 3 METHODOLOGY 41 3.1 Time Frame of Study . 41 3.2 Rainfall Data . 42 3.2.1 Standard Departures . 42 3.2.2 Rainfall Regions . 43 3.2.3 Data Adjustments . 44 3.3 El Niño and La Niña Events . 45 3.3.1 Historical Definitions of ENSO . 45 3.3.2 Climate Prediction Center (CPC) Methodology . 48 3.3.3 ENSO Events in the Twentieth Century . 49 3.4 Rainfall Calculations . 51 3.5 An Examination of Atmospheric Circulations . 51 iv 4 RESULTS AND DISCUSSION 60 4.1 Rainfall Anomalies During El Niño and La Niña Years . 60 4.1.1 Sahel Regions . 60 4.1.2 Guinea Coast Region . 65 4.1.3 Sahel vs. Guinea Coast: Question of the Dipole . 67 4.2 Consistency of the ENSO Signal and Further Discussion . 67 4.3 Atmospheric Circulation Patterns and ENSO . 71 5 CONCLUDING REMARKS 92 5.1 Conclusions . 92 5.2 Implications . 95 5.3 Future Work . 95 References . 97 Biographical Sketch . 105 v LIST OF TABLES 1.1 Adopted from Nicholson (1981). Approximate climatic characteristics of four veg- etation zones in West Africa; mean annual rainfall (mm), coefficient of variation (CV%), length of the rainy season (months). 6 2.1 Adopted from Diatta and Fink (2014). Correlation coefficients between remote in- dices of climate variability and the West Sahel (WS), Central Sahel (CS), and Guinea Coast (GC). See text for further information on abbreviations. Correlation coeffi- cients in bold are significant at the 95% significance level, while those in bold and with an asterisk are significant at the 99% level according to the F-test using the standard degree of freedom. Unless otherwise indicated, the time period 1921- 2009 was analyzed. 34 3.1 A list of rainfall regions utilized in this study, the number of gauge stations in each region, and the coordinates and country of the geographic center point of each re- gion. The color labels correspond to the color of the combined region in the bottom portion of Figure 3.1 that contains that individual region. 53 3.2 Calculated ONI values before 1968 (1921-1967). Values of 0.5 or greater are high- lighted in red and values of -0.5 or less are highlighted in blue. El Niño episodes are defined when there are 5 red ONI values in a row, while La Niña events are classified as years with five consecutive blue values. In cases when an episode spans multiple years, the first year (year of onset) is considered the ENSO year. 57 3.3 Same as Table 3.2, except for the years after 1968 (1968-2012). 58 3.4 List of El Niño and La Niña Years from 1921 to 2012 that are used in this study. These years are determined based on the Oceanic Niño Index (ONI) values that are shown in Tables 3.2 and 3.3. Note that all analyses conducted in this study will consider an ENSO event as the 24-month period commencing in the July-September season of the year prior to its onset (JAS -1) and continuing until the April-June season of the year following the first year of onset (AMJ +1). For this reason, El Niño and La Niña events spanning more than 2 years have been excluded to maintain consistency. 59 4.1 Seasonal evolution of standard departures during El Niño years. Negative values are highlighted in red; positive values are highlighted in blue. 80 4.2 Seasonal evolution of standard departures during La Niña years. Negative values are highlighted in red; positive values are highlighted in blue. 81 vi LIST OF FIGURES 1.1 Adopted from Nicholson (2013). Annual mean precipitation over Northern Africa (in mm). The location of the Sahel is labeled. 6 1.2 Adopted from Nicholson (2011). Vegetation zonation of West Africa (five zones): the Sahelo-Sahara is desert steppe with widely spaced grass clusters, the Sahel is semi-desert grassland, the Soudan is a savanna grassland, the Soudano-Guinean zone is woodland, and the Guinean zone is primarily forest. 7 1.3 Adopted from CILSS (2016). The Sahel landscape is made up of scattered small trees, shrubs, and grasses. Vegetation cover varies from season-to-season and from year-to-year based on the amount of rainfall the area receives. 7 1.4 Adopted from CILSS (2016). The Guinea Coast landscape is primarily made up of forest with a discontinuous upper canopy and dense lower canopy. The region receives the most annual rainfall of all of West Africa. 8 1.5 Adopted from Nicholson (2011). Length of the rainy season (months) and month of maximum rainfall across Africa. 8 1.6 Adopted from Nicholson (2011). Temperature (°C) and rainfall (mm) at typical stations over West Africa. The adjacent map gives the location of these stations in the context of the African continent as a whole. 9 2.1 Adopted from Nicholson (2009). Classic picture of the ITCZ over Africa. In recent years, a revised view of the ITCZ has been developed. 30 2.2 Adopted from Nicholson (2009). Revised view of the West African Monsoon. 30 2.3 Adopted from Nicholson et al. (1988). Schematic of boreal winter (top) and bo- real summer (bottom) atmospheric circulation patterns (including winds and surface pressure) over the African continent. Arrows depict relative wind speed and di- rection, thin dashed circles depict various jet streams, the dotted line indicates the location of the ITCZ, and solid lines indicate surface pressure contours (in mb).
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