Also known as the Antarctic Oscillation, this term describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the Southern Hemisphere. It is a zonally symmetric or annular structure, with a large low pressure anomaly centred on the South Pole and a ring of high pressure anomalies at mid-latitudes.

In a positive SAM event, the belt of strong westerly winds contracts towards Antarctica. This results in anomalously dry conditions over southern South America, New Zealand, and Tasmania,and wet conditions over much of Australia and South Africa. The stronger westerlies above the Southern Ocean also increase the insulation of Antarctica. As a result, there is less heat exchange between the tropics and the poles, leading to a cooling of Antarctica and the surrounding seas.