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Competing roles of the fast and slow response in the total coupled West African precipitation response to anthropogenic aerosol forcing

The response of the climate to external influences such as greenhouse gases or aerosol particles can be conceptually divided between the “fast” response directly due to the external forcing and the “slow” response caused by subsequent changes to ocean temperatures. In this study, we have investigated how the total climate response to anthropogenic aerosol emissions between the 1950s and the 2000s can be divided into its “fast” and “slow” components. We test this breakdown by using estimates of the sea surface temperature and sea ice concentration response to aerosols from two large sets of coupled atmosphere and ocean simulations from CanESM2 and CESM1-CAM5. These estimates are used to modify the surface conditions in a set of atmosphere-only simulations in CanAM4 and CAM5 respectively. We focus on the response of West African Monsoon rainfall to anthropogenic aerosols and find that in this region, the slow response increases precipitation and the fast response decreases it. These overall effects are spatially offset in the region, resulting in a total coupled response that is a net northward shift of precipitation. These results demonstrate that the oceans have an important role in mediating the monsoonal response to aerosol forcing over West Africa.