SST versus climate change signals in West African rainfall: 20th-century variations and future projections

Abstract

Rainfall variability is a crucial factor in food production, water resource planning and ecosystems, especially in regions with scarce freshwater resources. In West Africa rainfall has been subject to large decadal and interdecadal variations during the 20th century. The most prominent feature is the reduction in rainfall amount throughout the second half of the century with some recovery at the end. Among the conceivable mechanisms, which might induce such low-frequency variability in West African precipitation, this study is focussed on sea surface temperature (SST) variations and increasing greenhouse gas (GHG) concentrations. A tool is presented to distinguish between both impacts by means of various climate model simulations, which are found to reproduce the observed rainfall characteristics over West Africa reasonably well. Further, a multi-model approach is used to evaluate the expected future greenhouse signal in West African rainfall with respect to natural variability and intermodel variations. It is found that observed SST fluctuations, forcing two different atmospheric climate models, are able to reproduce the main features of observed decadal rainfall anomalies in the southern part of West Africa throughout the second half of the 20th century. The seasonal response to varying SST is strongest in summer when the region is undergoing intensive monsoon dynamics. Whereas both atmospheric models simulate the observed drought tendency, following the 1960s, there is some indication that the additional GHG forcing in one model induces some significantly different rainfall anomalies in recent years, re-initiating even positive anomalies relative to the climatological mean which has also been observed since the 1990s. However, this result is still subject to model uncertainty. Coupled climate model integrations with different climate change scenarios also predict that precipitation, particularly over the Guinea Coast and Sahel region, will steadily increase into the 21st century. The model-comprehensive signal is statistically significant with respect to natural variability and model uncertainty, suggesting that the observed recovery of yearly rainfall over parts of West Africa might actually reflect the beginning impact of rising anthropogenic GHG. The physical mechanism, linking the radiative forcing to the monsoonal rainfall, probably works via warming of the tropical Atlantic Ocean.