<p>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 CO<sub>2</sub> (referring to the impact of increased CO<sub>2</sub> 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.</p>

<p>The Monsoons produce some of the largest levels of uncertainty in projected precipitation change across the globe, and addressing this uncertainty is a key issue that must be faced in order to allow correct adaptation policy to be put in place.</p><p> </p><p>A set of CMIP6 GCM experiments, that allow the full effect of CO<sub>2</sub> forcing to be decomposed into individual components, highlight the leading factors that produce changes in monsoon precipitation. The results reveal a high spatial variability in these factors, with changes in the Indian Monsoon dominated by the effect of sea surface temperatures and the direct radiative effect of increased CO<sub>2</sub>, and changes in the South American Monsoon governed by the plant physiological effect and the direct radiative effect of increased CO<sub>2</sub>. The processes behind these precipitation changes are also investigated by looking at variations in atmospheric circulation and surface temperature. Results of the patterned sea surface temperature experiment demonstrate a slow-down of the Indian Monsoon circulation possibly driven by an anomalously warm Indian Ocean.</p><p> </p><p>This analysis has been performed for all land monsoon regions, decomposing the full CO<sub>2</sub> forcing into; uniform and patterned sea surface temperature change, the plant physiological effect, the direct radiative effect and the impact of sea-ice melt. These results can help identify emergent constraints, as well as indicate which aspects of climate models need to be improved in order to reduce model uncertainty.</p>

Projections of West African Monsoon (WAM) precipitation 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. The full forcing of increased CO2 can be decomposed into different components such as the impact of ocean warming, or the direct radiative effect of increased CO2. This paper investigates such a decomposition, analysing the effect of a uniform 4K ocean warming whilst keeping atmospheric CO2 concentrations constant. The analysis highlights several mechanisms acting to decrease WAM precipitation over a range of timescales, from days after the abrupt ocean warming, to the long-term equilibrium response. The initial decrease in WAM precipitation is caused by warming and enhanced convection over the ocean, stabilising the atmosphere inland and disrupting the monsoon inflow at low levels. Later in the response (after about 5 days), the WAM precipitation is reduced through a strengthening of the shallow circulation over West Africa, associated with changes in the large-scale temperature gradients and a local warming of the atmosphere related to a soil moisture feedback mechanism over the Sahel. Finally, from around 20 days after the SST increase, the WAM precipitation is also reduced through changes in specific humidity gradients that lead to increased potency of dry air advection into the monsoon rainband. The analysis concludes by demonstrating that the processes affecting precipitation in the early stages of the response are also relevant to the long-term equilibrium response.