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

An atmospheric GCM coupled to a mixed-layer ocean model is used to study ch anges in rainfall due to the indirect effects of anthropogenic aerosols. Th e model includes treatments of both the first and second indirect effects. The most striking feature of the equilibrium rainfall response to the indir ect effects of anthropogenic aerosols is a southward shift of the equatoria l rainfall. We hypothesize that this is caused by a hemispheric asymmetry i n the cooling of the sea surface. This is supported by another pair of expe riments, 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 No rthern Hemisphere are increased by 1 K in the calculation of the surface fl uxes, to mimic the effect of a return to pre-industrial conditions. Compare d to the run with increased NH SST, the standard run shows a southward shif t of equatorial rainfall, similar to that obtained as a response to anthrop ogenic aerosols.