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).
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).
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;