Causes of exceptional atmospheric circulation changes in the Southern Hemisphere

Abstract

[1] 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.

Half‐century seasonal relationships between the Southern Annular mode and Antarctic temperatures

“…It might prove instructive to utilise General Circulation Models (GCMs) to examine the future stability of the relationship in a climate with increased greenhouse gases, which modelling studies have linked to a more positive SAM (e.g. Fyfe et al, 1999;Kushner et al, 2001;Marshall et al, 2004;Shindell and Schmidt, 2004).…”

Influences of ozone depletion, the solar cycle and the QBO on the Southern Annular Mode

“…The stronger winds are now thought to be responsible for increased summer temperatures in the eastern Antarctic Peninsula, which have led to widespread collapse of ice sheets (Marshall et al, 2006). The trend in SAM has led to speculation that it is responding to changed temperatures and heights in the stratosphere, caused by the ozone hole and propagating downwards to the surface ( Thompson and Solomon, 2002), and to increased greenhouse gases (Marshall et al, 2004).…”

Winter warming in West Antarctica caused by central tropical Pacific warming

“…The SAM does not readily account for warming in continental West Antarctica, however, because increased westerlies tend to cool, rather than warm, the Antarctic interior 8,9 . Furthermore, the positive trend in the SAM is restricted to summer and autumn 7 , whereas West Antarctic warming has occurred predominantly in winter and spring 2,10 .…”

“…Recent warming over the Antarctic Peninsula has been explained in part by a positive trend in the Southern Annular Mode (SAM), manifested as a strengthening of the circumpolar westerlies, and attributed to stratospheric ozone depletion and increasing greenhouse gases 6,7 . The SAM does not readily account for warming in continental West Antarctica, however, because increased westerlies tend to cool, rather than warm, the Antarctic interior 8,9 .…”