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 55 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 (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

which moves north or south as its mode of variability. It is a climate driver for

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, 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 (meteorology), trough of Low-pressure area, low pressure. It often forms behind an extratropica ...

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

Phases and impacts

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 physical oceanography, 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 sur ...

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

ice shelf An ice shelf is a large platform of glacial ice floating on the ocean, fed by one or multiple tributary glaciers. Ice shelves form along coastlines where the ice thickness is insufficient to displace the more dense surrounding ocean water. T ...

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

Antarctic ice sheet The Antarctic ice sheet is a continental glacier covering 98% of the Antarctic continent, with an area of and an average thickness of over . It is the largest of Earth's two current ice sheets, containing of ice, which is equivalent to 61% of ...

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 lows) in south-eastern Australia (above Victoria) due to higher onshore flows from the

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

, decreases rain in the south-west, and decreases snow in the alpine 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 most frequently refers to Los Angeles, the second most populous city in the United States of America. La, LA, or L.A. may also refer to: Arts and entertainment Music *La (musical note), or A, the sixth note *"L.A.", a song by Elliott Smit ...

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

Negative

Its negative phase involves the belt moving towards the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

, 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, sometimes described as extreme heat, is a period of abnormally hot weather generally considered to be at least ''five consecutive days''. A heat wave is usually measured relative to the usual climate in the area and ...

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 by the

Great Dividing Range The Great Dividing Range, also known as the East Australian Cordillera or the Eastern Highlands, is a cordillera system in eastern Australia consisting of an expansive collection of mountain ranges, plateaus and rolling hills. It runs roughl ...

, which would act as a

rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from body of water, bodies of water (such as oceans and larg ...

. This phase will usually be more frequent with an

El Niño EL, El or el may refer to: Arts and entertainment Fictional entities * El, a character from the manga series ''Shugo Chara!'' by Peach-Pit * Eleven (''Stranger Things'') (El), a fictional character in the TV series ''Stranger Things'' * El, fami ...

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 Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...

levels and later stratospheric

ozone depletion Ozone depletion consists of two related events observed since the late 1970s: a lowered total amount of ozone in Earth, Earth's upper atmosphere, and a much larger springtime decrease in stratospheric ozone (the ozone layer) around Earth's polar ...

References

External links

* http://www.antarctica.ac.uk/met/gjma/sam.html Observation-based SAM index from 1957–present

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