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.