The Antarctic oscillation (AAO, to distinguish it from the

Arctic oscillation The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The s ...

or AO), also known as the Southern Annular Mode (SAM), is a low-frequency mode of atmospheric variability of the southern hemisphere that is defined as a belt of strong westerly winds or low pressure surrounding

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

which moves north or south as its mode of variability. It is a climate driver for Australia, influencing the country's weather conditions – It is associated with storms and

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

s that move from west to east that bring precipitation to southern Australia.

Phases and impacts

FMIB 36792 Coup de Vent d'Ouest, au Sud de l'Australie (4 September 1895)

Both positive and negative SAM events tends to last for approximately ten days to two weeks, though the timeframe between a positive and a negative event is random. It is usually in the span of a week to a few months, with a negative SAM being more common in the cool months and a positive SAM being more prolonged in the warmer months. Winds associated with the Southern Annular Mode cause oceanic

upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...

of warm circumpolar deep water along the Antarctic continental shelf, which has been linked to

ice shelf An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The ...

basal melt, representing a possible wind-driven mechanism that could destabilize large portions of the Antarctic ice sheet.

Positive

In its positive phase, the westerly wind belt that drives the Antarctic Circumpolar Current intensifies and contracts towards

. In winter, a positive phase increases rainfall (including

East coast low Australian east coast lows (known locally as east coast lows, maritime lows and sometimes as east coast cyclones) are extratropical cyclones or low-pressure systems on the coast of southeastern Australia that may be caused by both mid-latitude an ...

s) in south-eastern Australia (above

Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, Seychelle ...

) due to higher onshore flows from the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

, decreases rain in the south-west, and decreases snow in the

alpine Alpine may refer to any mountainous region. It may also refer to: Places Europe * Alps, a European mountain range ** Alpine states, which overlap with the European range Australia * Alpine, New South Wales, a Northern Village * Alpine National Pa ...

areas. In spring and summer, a positive phase reduces the chance of extreme heat and increases humid onshore flows, therefore making spring and summer wetter than normal. A positive phase would usually occur more frequently with a

La Niña La Niña (; ) is an oceanic and atmospheric phenomenon that is the colder counterpart of as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern. The name ''La Niña'' originates from Spanish for "the girl", by an ...

event.Southern Annular Mode
Bureau of Meteorology, 12 June 2019

Negative

Its negative phase involves the belt moving towards the equator, whereby decreasing rainfall in the southeast of Australia in the summer and as well as raising the possibility of spring

heatwave A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary, a heat wave is usually measured relative to the usual climate in th ...

s. Moreover, winters will usually be wetter than normal in the south and southwest with more snowfall in the alpine areas, but drier in the east coast due to less moist onshore flows from the east and blockage of

cold fronts A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

by the Great Dividing Range, which would act as a rain shadow. This phase will usually be more frequent with an

El Niño El Niño (; ; ) is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date ...

event.

Research

In 2014, Nerilie Abram used a network of temperature-sensitive ice core and tree growth records to reconstruct a 1000-year history of the Southern Annular Mode. This work suggests that the Southern Annular Mode is currently in its most extreme positive phase over at least the last 1000 years, and that recent positive trends in the SAM are attributed to increasing greenhouse gas levels and later stratospheric ozone depletion.

References

External links

* http://www.antarctica.ac.uk/met/gjma/sam.html Observation-based SAM index from 1957–present * http://hurricane.ncdc.noaa.gov/pls/paleox/f?p=519:1:0::::P1_STUDY_ID:16197 1000-year Southern Annular Mode reconstruction
Monthly Southern Hemisphere Annular Mode (SAM) Index or Antarctic Oscillation (AAO) Index 1850 - 2013

Daily Southern Hemisphere Annular Mode (SAM) Index or Antarctic Oscillation (AAO) Index 1948 - 2013
* http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/aao.loading.shtml * https://web.archive.org/web/20081216220433/http://stratus.astr.ucl.ac.be/textbook//pdf/Chapter_5.pdf (chapter 5.2.3)
SOUTHERN ANNULAR MODE IN AUSTRALIA – What is it?

Introduction to climate dynamics and climate modelling, Université catholique de Louvain {{Climate oscillations Climate of Antarctica Regional climate effects Environment of Antarctica Climate of Australia Climate oscillations