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

Alberto Arribas,Michela Biasutti,Aurore Voldoire,Edward K Vizy,Serge Janicot,David P Rowell,Yongkang Xue,Carlos R Mechoso,Irene Polo,Fred Kucharski,Belén Rodríguez‐Fonseca ,Bernard Fontaine,Lisa Goddard,Andrew Colman,Cyril Caminade,Teresa Losada,Marco Gaetani,Francisco J Doblas‐Reyes ,Leonard M Druyan ,Jürgen Bader ,Elsa Mohíno ,William K M Lau ,Michael Vellinga ,Javier Garcı́a-Serrano ,Kerry H Cook

doi:10.1175/jcli-d-14-00130.1

Abstract

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.

Highlights

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)
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

Summary

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). 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 tickborne 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. A final section will summarize the most remarkable results, remaining questions, modeling issues, and future directions (section 8)

SST influence at interannual time scales

Variability at decadal time scales

Seasonal forecasting of drought in Africa

Decadal prediction of the WAM

Findings

Future projections