Understanding Changes in West African Monsoon Precipitation Under Increased Levels of CO2

Abstract

Future projections of West African Monsoon (WAM) precipitation under increased levels of CO2 are uncertain. To address this, an improved understanding of the mechanisms driving WAM precipitation change is needed to shed light on inter- model differences and aid model development. By decomposing the full forcing of increased CO2 into different components, it is possible to analyse processes and mechanisms present in the full response in isolation. Here we investigate such a decomposition, analysing the impact of the direct radiative effect and the impact of a uniform ocean warming. It is shown that in response to the direct radiative effect, a northward shift and a weakening of the shallow meridional circulation leads to an increase in WAM precipitation. These circulation changes are generated by differ- ential warming patterns between the moist monsoon airmass and the drier desert airmass to the north, and through a soil moisture - surface heat flux feedback that acts to amplify the precipitation response. In response to the uniform ocean warming it is shown that a decrease in precipitation is caused by a combination of increased stability over the monsoon region (due to warming and convection over the ocean), a strengthening of the shallow meridional circulation, and enhanced meridional specific humidity gradients that lead to increased potency of dry air ad- vection into the monsoon rainband. The intermodel differences in the response to the direct radiative effect, uniform SST warming, and patterned SST change are also investigated, finding the models that project the largest changes in precipita- tion also exhibit the mechanisms identified more strongly. These results provide useful insight into the different drivers of WAM precipitation change and highlight mechanisms at the heart of the intermodel spread in WAM projections.