A Quantification of GHG and SST Forcing in West African Climate Change

The persistence of extended drought events throughout West Africa during the 20th Century has motivated a significant effort to understand the mechanisms driving African climate variability. This study uses the Community Climate System Model version 3 (CCSM3) to examine the relative roles of future changes in GHG forcing and the associated SST feedback in shaping potential future climate change over West Africa. We are particularly interested in determining the cause of simulated changes in summer precipitation. To analyze the dynamical response to GHG forcing and SST feedbacks, we conduct twenty-year time-slice experiments (1980-1999 and 2080-2099) using CAM3 (the atmospheric component of CCSM3) and the CCSM3-generated SSTs. The atmospheric model realistically simulates both observed precipitation distribution and atmospheric circulation patterns. Changes in future atmospheric radiative forcing are found to be the primary driver of precipitation increases projected throughout the Sahel region (poleward of 12°N). In turn, the direct atmospheric forcing has a cooling effect throughout portions of the central Sahel. Dynamically, these changes in atmospheric forcing generate a slight northward displacement and weakening of the African easterly jet (AEJ) and a strengthening of the tropical easterly jet (TEJ). Alternatively, the future SST feedback is primarily responsible for the 21st century drying projected over much of the Guinean Coast (equatorward of 12°N). This decrease in precipitation is associated with both a weakening of the monsoon westerlies and the TEJ as well as a decrease in rising motion throughout the mid-levels of the troposphere. These findings confirm previous suggestions of a potential increased role of GHG forcing on future rainfall variability in West Africa, particularly in the Sahel, and a continuation of the highly influential role of SST variations seen throughout the 20th century.