Aerosol versus greenhouse gas impacts on Southern Hemisphere general circulation changes

Abstract

This study re-examines the possible impacts of anthropogenic aerosols (AERs), which are primarily concentrated in the Northern Hemisphere extratropics, on atmospheric general circulation changes in the Southern Hemisphere (SH). The long-term trends of the SH Hadley Cell edge and Southern Annular Mode (SAM) are evaluated for the historical and single forcing experiments of 12 models archived for the Fifth Coupled Model Inter-comparison Project (CMIP5). The AER-induced temperature changes are typically opposite in sign to that of the greenhouse gas (GHG)-induced changes. However, the zonal-mean circulation changes driven by the AERs are not simply opposite to those due to the GHGs. Depending on the analysis period, they can be either opposite or the same as the GHG-induced ones. It is also found that the inter-model spread of AER-driven circulation changes is primarily dependent on the uncertainty of tropospheric temperature changes, particularly static stability changes in the subtropics, whereas that of GHG-induced changes in the austral summer is influenced by the uncertainty of both the tropospheric and polar lower-stratospheric temperature changes. This accentuates that the AER-induced SH circulation changes are not simply mirroring the GHG-induced ones.