The
Antarctic oscillation (AAO, to distinguish it from the
Arctic oscillation or AO) is a low-frequency
mode of atmospheric variability of the southern hemisphere. It is also known as the Southern Annular Mode (SAM) or
Southern Hemisphere Annular Mode (SHAM). It is defined as a belt of
westerly winds or low pressure surrounding
Antarctica which moves north or south as its mode of variability. In its positive phase, the westerly wind belt contracts towards
Antarctica, while its negative phase involves this belt moving towards the Equator.