Causes of exceptional atmospheric circulation changes in the Southern Hemisphere

We demonstrate that recent observed trends in the annual and austral summer Southern Hemisphere Annular Mode (SAM) are unlikely to be due to internal climate variability, since they exceed any equivalent-length trends in a millennial General Circulation Model (GCM) control run with constant forcings. In contrast we show that observed trends in the SAM are consistent with the combined effects of anthropogenic and natural forcings in GCM simulations. As these trends begin prior to stratospheric ozone depletion we challenge the assertion that this process is primarily responsible for changes in the SAM. Moreover, anthropogenic forcings have a larger effect on the austral summer SAM in combination with natural forcings than when acting in isolation.