Understanding changes in West African monsoon precipitation in response to increased CO2

Projections of future West African monsoon (WAM) precipitation change in response to increased greenhouse gases are uncertain, and an improved understanding of the drivers of WAM precipitation change is needed to help aid model development and better inform adaptation policies in the region. Here, we address two of these drivers: the direct radiative effect of increased CO₂ (referring to the impact of increased CO₂ in the absence of SST changes), and the impact of a uniform SST warming. Atmosphere only models are used to investigate the response, finding that these two drivers have opposing impacts on WAM precipitation. In response to the direct radiative effect, an increase in precipitation is caused by a northward shift and a weakening of the shallow meridional circulation over West Africa, advecting less dry air into the monsoon rainband. In contrast, the uniform SST warming causes a decrease in precipitation due to a strengthening of the shallow meridional circulation and enhanced moisture gradients between the moist monsoon airmass and the dry desert airmass. These changes in the shallow meridional circulation are shown to be caused by large scale temperature changes as well as the more localised impact of a soil moisture feedback mechanism over the Sahel. It is then shown that the processes discussed are relevant to the intermodel uncertainty in WAM projections across a range of CMIP6 models.