Understanding the Uncertainty in the West African Monsoon Precipitation Response to Increasing CO₂

The West African monsoon (WAM) precipitation response to increased CO₂ is uncertain, with both large increases and decreases predicted by CMIP6 models. To address this, the full impact of increased CO₂ has been decomposed into several drivers, three of which are shown to contribute most to the uncertainty in the WAM precipitation response: the direct radiative effect of increased CO₂, the impact of a uniform sea surface temperature (SST) warming, and the impact of a patterned SST change. Much of the uncertainty associated with the response to the direct radiative effect and uniform SST warming is shown to be related to differing changes in 700-hPa moisture flux divergence associated with the shallow meridional circulation over West Africa as well as differences in a soil moisture–surface heat flux feedback over the Sahel. For the SST pattern effect, the difference between North Atlantic SSTs and tropical mean SSTs as well as interhemispheric gradients in surface temperatures are key drivers of intermodel spread. Suggestions for how to reduce uncertainty in the response are discussed.