Precipitation changes in a GCM resulting from the indirect effects of anthropogenic aerosols

Abstract

An atmospheric GCM coupled to a mixed‐layer ocean model is used to study changes in rainfall due to the indirect effects of anthropogenic aerosols. The model includes treatments of both the first and second indirect effects. The most striking feature of the equilibrium rainfall response to the indirect effects of anthropogenic aerosols is a southward shift of the equatorial rainfall. We hypothesize that this is caused by a hemispheric asymmetry in the cooling of the sea surface. This is supported by another pair of experiments, in which the model is run with prescribed present‐day SSTs. In one experiment the standard model is used, and in the other the SSTs in the Northern Hemisphere are increased by 1 K in the calculation of the surface fluxes, to mimic the effect of a return to pre‐industrial conditions. Compared to the run with increased NH SST, the standard run shows a southward shift of equatorial rainfall, similar to that obtained as a response to anthropogenic aerosols.