Variability and Predictability of West African Droughts: A Review on the Role of Sea Surface Temperature Anomalies

Belen Rodríguez-Fonseca Departamento de Física de la Tierra, Astronomía y Astrofísica-I, Facultad de Ciencias Físicas, and Instituto de Geociencias, CSIC, and Universidad Complutense de Madrid, Madrid, Spain

Search for other papers by Belen Rodríguez-Fonseca in
Current site
Google Scholar
PubMed
Close
,
Elsa Mohino Departamento de Física de la Tierra, Astronomía y Astrofísica-I, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid, Spain

Search for other papers by Elsa Mohino in
Current site
Google Scholar
PubMed
Close
,
Carlos R. Mechoso Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California

Search for other papers by Carlos R. Mechoso in
Current site
Google Scholar
PubMed
Close
,
Cyril Caminade School of Environmental Sciences, Institute of Infection and Global Health, University of Liverpool, Liverpool, United Kingdom

Search for other papers by Cyril Caminade in
Current site
Google Scholar
PubMed
Close
,
Michela Biasutti Lamont-Doherty Earth Observatory, Columbia University, New York, New York

Search for other papers by Michela Biasutti in
Current site
Google Scholar
PubMed
Close
,
Marco Gaetani Consiglio Nazionale delle Ricerche, Istituto di Biometeorologia, Rome, Italy

Search for other papers by Marco Gaetani in
Current site
Google Scholar
PubMed
Close
,
J. Garcia-Serrano Institut Català de Ciències del Clima, Barcelona, Spain

Search for other papers by J. Garcia-Serrano in
Current site
Google Scholar
PubMed
Close
,
Edward K. Vizy Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas

Search for other papers by Edward K. Vizy in
Current site
Google Scholar
PubMed
Close
,
Kerry Cook Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas

Search for other papers by Kerry Cook in
Current site
Google Scholar
PubMed
Close
,
Yongkang Xue Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California

Search for other papers by Yongkang Xue in
Current site
Google Scholar
PubMed
Close
,
Irene Polo NCAS-Climate, Department of Meteorology, University of Reading, Reading, United Kingdom

Search for other papers by Irene Polo in
Current site
Google Scholar
PubMed
Close
,
Teresa Losada Instituto de Ciencias Ambientales, Universidad de Castilla-La Mancha, Toledo, Spain

Search for other papers by Teresa Losada in
Current site
Google Scholar
PubMed
Close
,
Leonard Druyan Center for Climate Systems Research, Columbia University, and NASA Goddard Institute for Space Studies, New York, New York

Search for other papers by Leonard Druyan in
Current site
Google Scholar
PubMed
Close
,
Bernard Fontaine Centre de Recherches de Climatologie, CNRS/Université de Bourgogne, Dijon, France

Search for other papers by Bernard Fontaine in
Current site
Google Scholar
PubMed
Close
,
Juergen Bader Max Planck Institute for Meteorology, Hamburg, Germany

Search for other papers by Juergen Bader in
Current site
Google Scholar
PubMed
Close
,
Francisco J. Doblas-Reyes Institut Català de Ciències del Clima, and Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain

Search for other papers by Francisco J. Doblas-Reyes in
Current site
Google Scholar
PubMed
Close
,
Lisa Goddard International Research Institute for Climate and Society, Columbia University, New York, New York

Search for other papers by Lisa Goddard in
Current site
Google Scholar
PubMed
Close
,
Serge Janicot IRD, LOCEAN/IPSL, UPMC, Paris, France

Search for other papers by Serge Janicot in
Current site
Google Scholar
PubMed
Close
,
Alberto Arribas Met Office Hadley Center, Exeter, United Kingdom

Search for other papers by Alberto Arribas in
Current site
Google Scholar
PubMed
Close
,
William Lau Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland

Search for other papers by William Lau in
Current site
Google Scholar
PubMed
Close
,
Andrew Colman Met Office Hadley Center, Exeter, United Kingdom

Search for other papers by Andrew Colman in
Current site
Google Scholar
PubMed
Close
,
M. Vellinga Met Office Hadley Center, Exeter, United Kingdom

Search for other papers by M. Vellinga in
Current site
Google Scholar
PubMed
Close
,
David P. Rowell Met Office Hadley Center, Exeter, United Kingdom

Search for other papers by David P. Rowell in
Current site
Google Scholar
PubMed
Close
,
Fred Kucharski Abdus Salam International Centre for Theoretical Physics, Trieste, Italy

Search for other papers by Fred Kucharski in
Current site
Google Scholar
PubMed
Close
, and
Aurore Voldoire Centre National de Recherches Météorologiques/Groupe d’Etude de l’Atmosphère Météorologique, Météo-France, CNRS, Toulouse, France

Search for other papers by Aurore Voldoire in
Current site
Google Scholar
PubMed
Close
Full access

Abstract

The Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decadal forecasting. The agreement among future projections has improved from CMIP3 to CMIP5, with a general tendency for slightly wetter conditions over the central part of the Sahel, drier conditions over the western part, and a delay in the monsoon onset. The role of the Indian Ocean, the stationarity of teleconnections, the determination of the leader ocean basin in driving decadal variability, the anthropogenic role, the reduction of the model rainfall spread, and the improvement of some model components are among the most important remaining questions that continue to be the focus of current international projects.

Corresponding author address: Belén Rodríguez-Fonseca, Departamento de Física de la Tierra, Astronomía y Astrofísica I, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Ciudad Universitaria, Plaza Ciencias, 1, 28040 Madrid, Spain. E-mail: brfonsec@ucm.es

This article is included in the GDIS Drought Worldwide Special Collection.

Abstract

The Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decadal forecasting. The agreement among future projections has improved from CMIP3 to CMIP5, with a general tendency for slightly wetter conditions over the central part of the Sahel, drier conditions over the western part, and a delay in the monsoon onset. The role of the Indian Ocean, the stationarity of teleconnections, the determination of the leader ocean basin in driving decadal variability, the anthropogenic role, the reduction of the model rainfall spread, and the improvement of some model components are among the most important remaining questions that continue to be the focus of current international projects.

Corresponding author address: Belén Rodríguez-Fonseca, Departamento de Física de la Tierra, Astronomía y Astrofísica I, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Ciudad Universitaria, Plaza Ciencias, 1, 28040 Madrid, Spain. E-mail: brfonsec@ucm.es

This article is included in the GDIS Drought Worldwide Special Collection.

1. Introduction

West Africa is the westernmost region of the northern tropical African continent. The region is primarily characterized by distributions of rainfall and vegetation that are primarily zonal with strong north–south gradients, and it is considered as an entity in the meteorological context (Nicholson 2013). A monsoon season [the West African monsoon (WAM)] occurs every year, lasting from four to five months (May–September) near the Guinean coast and three months [July–September (JAS)] in the Sahel. Enhanced precipitation is associated with the seasonal northward migration of the intertropical convergence zone (ITCZ), where the northeasterly harmattan winds converge with the moisture-laden flow from the colder eastern equatorial Atlantic Ocean. Nicholson (2013) recently conducted an extensive review of rainfall variability over the Sahel and documented novel features of the region’s storm circulations. Figure 1 (from Huffman et al. 2007) shows a latitude–time plot that illustrates the seasonal cycle of rainfall in West Africa. Rainfall rates increase along the Guinean coast of Africa (approximately 4°–6°N) throughout May, and precipitation remains high in this region through June. In the early summer [7 July, according to climatology compiled with the Tropical Rainfall Measuring Mission (TRMM)], rainfall decreases along the coast of the Gulf of Guinea and the rainfall maximum becomes established over the Sahel (about 10°–15°N). This sudden jump in latitude of the precipitation maximum represents the onset of the West African monsoon (Le Barbe et al. 2002; Sultan and Janicot 2003).

Fig. 1.
Fig. 1.

Mean precipitation climatology (mm day−1) for 1998–2012 from TRMM 3B42 V6 product (Huffman et al. 2007): (a) daily values and (b) with a 10-day smoothing applied. Values are averaged from 12°W to 6°E to avoid the precipitation maximum over the Cameroon highlands, where seasonal variations are different from those in regions to the west.

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00130.1

The number of scientific papers motivated by different aspects of Sahel rainfall has increased exponentially since the 1950s (see Fig. 2), from around 150 to more than 5000 entries in the period from January to May 2013. Droughts are major natural disasters for the largely rain-fed agriculture of most African countries. Particularly in the Sahel, a weak rainy season can create dramatic situations for millions of people (according to the International Federation of Red Cross and Red Crescent Societies; http://www.ifrc.org). The Sahel drought during the 1970s and 1980s was the most significant climate event at the continental scale during the twentieth century, and is arguably among the largest climatic changes worldwide (Trenberth et al. 2007). The event was associated with changes in the intensity, spatial distribution, and temporal evolution of the WAM and associated circulation features, such as the trade winds, African easterly jet (AEJ), and tropical easterly jet (TEJ) (Le Barbe et al. 2002; Sultan and Janicot 2003; Xue et al. 2004a; Dezfuli and Nicholson 2011).

Fig. 2.
Fig. 2.

Evolution of the number of papers published in relation to “Sahelian rainfall” from the 1950s (5 papers in the 1950s and 154 in the 1960s). In the last 15 years, the papers sum 63 840. From 1950 to 1997 data are plotted as averages over each decade; from 1998 onward data are yearly plotted. Source: Google scholar (http://scholar.google.com). Before 1990 units are in number of papers per decade, and after 1990 the units are number of papers per year. The dates of some of the most remarkable international projects studying the Sahelian climate variability are marked in the figure.

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00130.1

The rainy season in the Sahel has large interannual and decadal variations. A substantial part of this variability is due to the influence of slowly varying climate subcomponents, such as sea surface temperatures (SSTs) and land surface conditions. The importance of oceanic influences at interannual and decadal time scales has been supported by the results of several studies (Folland et al. 1986; Palmer 1986; Rowell et al. 1992; Ward 1998; Camberlin et al. 2001; Giannini et al. 2003; Lu and Delworth 2005; Cook 2008; Caminade and Terray 2010; Losada et al. 2010; Rodríguez-Fonseca et al. 2011; Mohino et al. 2011a; Rowell 2013; Nicholson 2013). Other studies have addressed the effects of land–atmosphere interactions (Xue, 1997; Zeng et al. 1999; Nicholson 2000; Giannini et al. 2003; Yoshioka et al. 2007) and aerosol–radiative forcings (Kim et al. 2010). These effects can potentially interact with each other. For example, the variability of land surface conditions can affect the circulation over the ocean, which in turn can modify the SSTs and indirectly affect conditions over land (Ma et al. 2013).

The existence of significant impacts on WAM rainfall of slowly varying climate subcomponents indicates the potential for useful long-range forecasts (Vellinga et al. 2013; Gaetani and Mohino 2013; García-Serrano et al. 2013). To realize this potential with climate models, these must successfully reproduce the important characteristics of the WAM precipitation and circulation. Despite continuous model improvements in the models, a skillful simulation and prediction of the WAM, including its variability at different time and spatial scales and its association with external forcings, remains a daunting task.

The present paper surveys the literature on drought in West Africa and the Sahel with particular emphasis on recent work on these subjects. The text discusses the dynamical mechanisms linking anomalies in West African rainfall with those in SST over the World Ocean, the time dependence of these relationships, their predictability, and future projections. It is appropriate to acknowledge that many results presented in the following were obtained under the sponsorship of coordinated international research projects (see Fig. 2). The African Monsoon Multidisciplinary Analysis program (AMMA; http://amma-international.org/) has coordinated an ambitious program aimed to improve the knowledge and understanding of the WAM’s variability and predictability on daily-to-decadal time scales, including climate change (Redelsperger et al. 2006; AMMA 2010; Ruti et al. 2011). The West African Monsoon Modeling and Evaluation (WAMME; Druyan et al. 2010) is a community initiative designed to evaluate the performance of state-of-the-art GCMs and regional climate models (RCMs) in reproducing WAM precipitation, drought scenarios, and their relevant processes. WAMME applies recently available observational and assimilation data for model evaluation and improvement (Boone et al. 2010; Xue et al. 2010b). The Coordinated Regional Climate Downscaling Experiment in Africa (CORDEX-Africa) has led to a coordinated evaluation of RCM skill, spread and uncertainties for simulating the West African monsoon mean climate and, to a lesser extent, its simulated onset and variability (Nikulin et al. 2012; Hernández-Díaz et al. 2013). The European Commission Seventh Framework Programme (EC FP7) Quantifying Weather and Climate Impacts on Health in Developing Countries (QWECI) project has aimed to understand, on a more fundamental level, the climate drivers of the vector-borne diseases of malaria, Rift Valley fever, and certain tick-borne diseases, all of which have major human and livestock health and economic implications in Africa (Cash et al. 2013; Tompkins and Ermert 2013; Ermert et al. 2013; Caminade et al. 2014). Some of these international projects have focused on the impact of SST anomalies on the WAM at interannual and decadal time scales. The collective findings from research sponsored by these projects have contributed significantly to progress, particularly in cases that occurred in the last few decades. Finally, phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5) have addressed outstanding scientific questions in the Intergovernmental Panel on Climate Change (IPCC) Fourth and Fifth Assessment Reports (AR4 and AR5) process, improving understanding of climate and providing estimates of future climate change that will be useful to those considering its possible consequences.

The text is organized as follows. We start in section 2 and 3 by surveying the state-of-the-art knowledge of the SST influence on Sahel rainfall at interannual to decadal time scales, at which the variability of the ocean is the main driver of that in the atmosphere. Next, we summarize the progress in seasonal (section 5) to decadal predictability (section 6) and its skill in West Africa, following with an update of the future projections. A final section will summarize the most remarkable results, remaining questions, modeling issues, and future directions (section 8).

2. SST influence at interannual time scales

This section is dedicated to review recent findings on the influence of SST anomalies in different ocean ba