Changes in West African Summer Monsoon Precipitation Under Stratospheric Aerosol Geoengineering

Stratospheric aerosol geoengineering (SAG) is suggested as a potential way to reduce the climate impacts of global warming. Using simulations from the Geoengineering Large Ensemble project that employed stratospheric sulfate aerosols injection to keep global mean surface temperature and also the interhemispheric and equator-to-pole temperature gradients at their 2020 values (present-day climate) under Representative Concentration Pathway 8.5 scenario, we investigate the potential impact of SAG on the West African Summer Monsoon (WASM) precipitation and the involved physical processes. Results indicate that under Representative Concentration Pathway 8.5, during the monsoon period, precipitation increases by 44.76%, 19.74%, and 5.14% compared to the present-day climate in the Northern Sahel, Southern Sahel, and Western Africa region, respectively. Under SAG, relative to the present-day climate, the WASM rainfall is practically unchanged in the Northern Sahel region but in Southern Sahel and Western Africa regions, rainfall is reduced by 4.06% (0.19 +/- 0.22 mm) and 10.87% (0.72 +/- 0.27 mm), respectively. This suggests that SAG deployed to offset all warming would be effective at offsetting the effects of climate change on rainfall in the Sahel regions but that it would be overeffective in Western Africa, turning a modest positive trend into a negative trend twice as large. By applying the decomposition method, we quantified the relative contribution of different physical mechanisms responsible for precipitation changes under SAG. Results reveal that changes in the WASM precipitation are mainly driven by the reduction of the low-level land-sea thermal contrast that leads to weakened monsoon circulation and a northward shift of the monsoon precipitation.