The Antarctic Oscillation, also known as the Southern Annular Mode, is the southern analog to the Arctic Oscillation (Northern Annular Mode). It features the alternation of strong westerly (i.e. from the west and toward the east) winds blowing farther south at roughly 65ºS latitude, and farther north, at about 40ºS latitude. It affects climate patterns in Australia and Antarctica.
The variation is caused by air pressure seesawing between the higher(~65ºS) and lower (~40ºS) latitudes. The index formula, as for the Southern Oscillation Index and others, is complex, involving historical pressure anomalies(departures from average).
When the AAO (SAM) index is positive, the belt of strong westerly winds moves south toward Antarctica. Most of Australia's winter weather is affected, with rain in the southeast and dry conditions in the southwest. The more southern westerly winds intensify the Antarctic Circumpolar Current, and possibly cause coastal upwelling around the entire continent, increasing ice shelf melt.
Positive AAO events are weakly correlated with La Niña. In recent years the AAO index has shown an increasing tendency to be positive during the southern hemisphere summer and fall months (December to May). There is a theory, not yet well demonstrated enough to become consensus,that the growing positive tendency of the AAO is correlated with global warming.
A negative AAO index means the westerlies are closer to the equator,bringing dry weather to southeastern Australia and snowy winters to the southwest and south central regions. Negative AAO events are weakly correlated with El Niño. The oscillation's rhythm is irregular, ranging from weeks to months.
Tomorrow:the Arctic Oscillation (AO).
Be brave, and be well.
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