Influence of Twenty-First-Century Atmospheric and Sea Surface Temperature Forcing on West African Climate

Abstract

he persistence of extended drought events throughout West Africa during the twentieth century has motivated a substantial effort to understand the mechanisms driving African climate variability as well as the possible response to elevated greenhouse gas (GHG) forcing. An ensemble of global climate model experiments is used to examine the relative roles of future direct atmospheric radiative forcing and SST forcing in shaping potential future changes in boreal summer precipitation over West Africa. The authors find that projected increases in precipitation throughout the western Sahel result primarily from direct atmospheric radiative forcing. The changes in atmospheric forcing generate a slight northward displacement and weakening of the African easterly jet (AEJ), a strengthening of westward monsoon flow onto West Africa, and an intensification of the tropical easterly jet (TEJ). Alternatively, the projected decreases in precipitation over much of the Guinea Coast region are caused by SST changes induced by the atmospheric radiative forcing. The changes in SSTs generate a weakening of the monsoon westerlies and the TEJ as well as a decrease in low-level convergence and resultant rising air throughout the midlevels of the troposphere. Experiments suggest a potential shift in the regional moisture balance of West Africa should global radiative forcing continue to increase, highlighting the importance of climate system feedbacks in shaping the response of regional-scale climate to global-scale changes in radiative forcing.