Aerosol forcing is the second largest anthropogenic forcer on the historical climate, following greenhouse gases. This forcing shifted both temporally and spatially over the 20th century due to industrialization and then emission regulations, resulting in complex multidecadal climate impacts. One observed signal that has been partially attributed to this forcing is the drought and recovery of rainfall in the arid Sahel region of Northern Africa. Analyses of coupled global climate model (GCM) Large Ensembles (LE) show that aerosols cause drying up to the 1970s and recovery thereafter with similar timing, though lower magnitude to the observed drought.

Here, I describe work using idealized GCM simulations to study the mechanisms underlying the Sahel climate response. We focus on determining the role of atmosphere versus ocean mechanisms. First, using atmosphere GCM simulations, we determine the direct-atmospheric effect of aerosol emissions in the absence of sea surface temperature (SST) changes versus the ocean-mediated effect of aerosol-forced SST anomalies in the absence of emission changes. We then further decompose those signals into their regional components. Finally, I describe preliminary results using Slab Ocean Model (SOM) simulations in which the ocean is represented by a single mixed layer without circulation. These are targeted at determining the effect of the direct atmospheric plus thermodynamic ocean effect versus the effect of ocean circulation.