The Sahel is a region of Africa defined by a band of arid climate located on the southern edge of the Sahara desert. During the 20th century, the region saw large multidecadal rainfall changes with substantial impacts on the region’s population, marking one of the starkest climate change signals in the recent record. We contribute to this much studied problem by assessing the mechanisms underlying the response of Sahel rainfall to anthropogenic aerosol forcing, a major component of the human influence on climate which has complex multidecadal changes. We build off of a Large Ensemble of simulations conducted in the Community Earth System model version 1 (CESM1) with which we can isolate the effect of aerosol forcing. We find that the aerosol forcing causes drying between the 1950s and 1970s, followed by a recovery between the 1970s and 2000s.
We then conduct a set of atmosphere global climate model (AGCM) Community Atmosphere Model version 5 (CAM5) simulations targeted at determining the role of the “ocean-mediated” and “direct-atmospheric” responses in the overall coupled response to aerosol forcing. The direct-atmospheric response is the effect of the forcing in the absence of surface temperature change, while the ocean-mediated response is the effect of forced ocean temperature change feeding back onto the atmosphere. We find that the 1970s -1950s drying is predominantly direct-atmospheric and the 2000s - 1970s recovery is largely ocean-mediated.
In more recent work, we have conducted further simulations that divide the direct-atmospheric and ocean-mediated responses into effect of regional emissions change and regional SST anomalies respectively. The 1970s-1950s direct-atmospheric drying is mostly due to North American emissions and the weak ocean-mediated effect is due to opposing responses to Atlantic and Pacific ocean anomalies. The 2000s-1970s direct-atmospheric drying is due to Asian and African emissions, while the ocean-mediated recovery is due to complementary responses to Atlantic and Pacific ocean anomalies, with some drying due to Indian ocean anomalies.