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

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.

Global Meteorological Drought: A Synthesis of Current Understanding with a Focus on SST Drivers of Precipitation Deficits

“…concludes that the hydroclimatic response of East Africa to anthropogenic climate change remains an open question and that more research is needed to better understand the physical processes associated with the rainfall variability of the region across multiple time scales. -Fonseca et al (2015) focus on rainfall variability across multiple time scales in West Africa and the Sahel. They conclude that SST variations are largely responsible for rainfall variability in the region.…”

“…6. Rodríguez-Fonseca et al (2015) discuss a unique aspect of the West African rainfall variability at interannual time scales: its links with the variability of tropical SSTs have shown nonstationary features (see also Rodríguez-Fonseca et al 2011 andMohimo et al 2011b). Pacific cold events and Atlantic warm events tend to appear simultaneously after the 1970s (Rodríguez-Fonseca et al 2009).…”

“…Rodríguez-Fonseca et al (2015) discuss a unique aspect of the West African rainfall variability at interannual time scales: its links with the variability of tropical SSTs have shown nonstationary features (see also Rodríguez-Fonseca et al 2011 andMohimo et al 2011b). Pacific cold events and Atlantic warm events tend to appear simultaneously after the 1970s (Rodríguez-Fonseca et al 2009). AGCM experiments demonstrate that, during this period, the impacts of simultaneous SST anomalies in the IndoPacific and Atlantic on Sahel rainfall tend to cancel each other such that the north-south dipole in rainfall anomalies over West Africa expected from Atlantic SST anomalies alone does not appear in the observations (Losada et al 2012).…”

Growth and decay of the equatorial Atlantic SST mode by means of closed heat budget in a coupled general circulation model

“…Tropical Atlantic Variability (TAV) have important impacts over Africa and South America, where rain-fed agriculture is an important part of their economies (Xie and Carton, 2004;Chang et al, 2006a;Kushnir et al, 2006;Rodríguez-Fonseca et al, 2015). However, historically major efforts have been done in understanding the Tropical Pacific which holds the most important inter-annual variability mode in terms of the global impacts: El Niño and the Southern Oscillation (ENSO, Philander, 1990).…”

“…The Atlantic Niño mode is therefore tightly associated with the WAM inter-annual variability (Janicot et al, 1998;Ward, 1998;Giannini et al, 2003;Polo et al, 2008;Losada et al, 2010;Rodriguez-Fonseca et al, 2011;Rodríguez-Fonseca et al, 2015): positive (negative) SST anomalies influence the rainfall over the Gulf of Guinea through a decrease (increase) of the local surface temperature gradient, weakening (strengthening) the monsoon flow and the surface convergence over the Sahel (Vizy and Cook, 2001;Losada et al, 2010).…”