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 co ...

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. Th ...

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

Antarctic ice sheet The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost a ...

Positive

In its positive phase, the westerly wind belt that drives the

Antarctic Circumpolar Current The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise (as seen from the South Pole) from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feat ...

intensifies and contracts towards

. In winter, a positive phase increases rainfall (including East coast lows) 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 contine ...

, 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 P ...

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 ...

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

Negative

Its negative phase involves the belt moving towards the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can al ...

, 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 the ...

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 ''Blockage'' ( Persian: سد معبر, romanized: Sade Ma'bar) is a 2017 Iranian drama film directed by Mohsen Gharaie and written by Saeed Roustayi. The film screened for the first time at the 35th Fajr Film Festival and received 4 nominations. ...

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

by the Great Dividing Range, 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 water bodies (such as oceans and large lakes) is carri ...

. 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 A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), met ...

levels and later

stratospheric The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air hi ...

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