Southern Annular Mode (SAM)

The Southern Annular Mode (SAM), also called the Antarctic Oscillation, is the leading mode of extratropical Southern Hemisphere circulation, a north-south shift in atmospheric mass between mid-latitudes and Antarctica. Its index is the zonal-mean sea-level-pressure difference between 40 and 65 degrees S. The positive phase has lower pressure over Antarctica and stronger, poleward-contracted westerlies that spin up the Antarctic Circumpolar Current and increase Southern Ocean upwelling. The SAM has trended positive in austral summer, attributed to ozone depletion and greenhouse forcing, affecting sea ice and carbon uptake.