The West African dipole in rainfall and its forcing mechanisms in global and regional climate models

Abstract

The leading mode of 20th century West African rainfall variability represents a well-documented drought tendency in the Sahel and the Guinea Coast region from the 1960s onward. The following three modes describe an out-of-phase relationship between the Sahel Zone (SHZ) and the Guinea Coast region (GCR) to the south, with positive rainfall anomalies in GCR being associated with even more severe drought conditions in SHZ between 1970 and 1998. This West African dipole in rainfall (WDR) has been of high relevance to migration processes in recent decades over the entire subsaharan region. WDR is equally revealed in long-term observational data and global climate model output. There is a clear scale-dependence of the anticorrelation, SHZ rainfall changes being decoupled from GCR ones in the low-frequency range. It is found that the high pass filtered interannual WDR fluctuations are closely tied to Atlantic sea surface temperatures (SSTs), particularly in the Gulf of Guinea. Soil moisture is not a dominant player in forcing the dipole anomalies. Sensitivity studies with a regional climate model support the physical link between tropical Atlantic SST and WDR. The simulated rainfall response to changing SST is non-linear and in the same order of magnitude as in the long-term observations. The SST impact accounts for up to 40% of total rainfall variability, particularly over the southernmost part of West Africa, and is statistically significant at the 1% level even with respect to the remarkable day-to-day variations. Prescribing late 21st century warmer tropical Atlantic SST as derived from global climate model experiments under increasing greenhouse gas (GHG) concentrations, leads to increasing rainfall amount in GCR (around+300mm) and a reduction in freshwater supply in SHZ (around -150mm) during the July-August main rainy season. Thus, the SHZ-GCR contrast in rainfall amount may rise in the future, inducing ongoing north-to-south migrations in whole subsaharan West Africa.