Abstract

Sahel precipitation has undergone substantial multidecadal-timescale changes during the 20th century which have had severe impacts on the region’s population. Using initial condition Large Ensembles (LE) of coupled general circulation model (GCM) simulations from two institutions, forced multidecadal variability, in which Sahel precipitation declines from the 1950s to 1970s then recovers from the 1970s to 2000s, is found. This forced variability has similar timing to, but considerably smaller magnitude than observed Sahel precipitation variability. Isolating the response using single forcing simulations within the LEs reveals that anthropogenic aerosol (AA) is the primary driver of this forced variability.

The roles of the direct-atmospheric and the ocean-mediated atmospheric responses to AA forcing are determined with the atmosphere-land GCM (AGCM) components of the LE coupled GCMs. The direct-atmospheric response arises from changes to aerosol and precursor emissions with unchanged oceanic boundary conditions while the ocean-mediated response arises from changes to AA-forced sea surface temperatures and sea ice concentrations diagnosed from the AA-forced LE. In the AGCMs studied here, the direct-atmospheric response dominates the AA forced 1970s-1950s Sahel drying. On the other hand, the 2000s-1970s wetting is mainly driven by the ocean-mediated effect, with some direct atmospheric contribution. Though the responses show differences, there is qualitative agreement between the AGCMs regarding the roles of the direct-atmospheric and ocean-mediated responses. Since these effects often compete and show non-linearity, the model dependence of these effects and their role in the net aerosol-forced response of Sahel precipitation need to be carefully accounted for in future model analysis.

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