A.A.O. Or O.A.A.






The A.A.O. (Antarctic Oscillation) is a climatic variation of Southern Hemisphere similar to the AO (Arctic Oscillation) in the Northern Hemisphere. The AAO is a variation of the atmospheric pressure between the areas of mid-latitudes and high latitudes in the Southern Hemisphere, centered from 55 to 60° South.

The AAO index = P*40°S - P*65°S whose P*40°S is the atmospheric pressure at 40° South.

Evolution of the AAO index from 1979 to 2013 in Austral winter.
This schema has been created and is updated according to the NOAA data

  • AAO+

When the AAO index is positive then the westerly winds are more important to mid-latitude as during the NAO+ or AO+ in the northern hemisphere. Antarctica also tends to be colder especially in Eastern Antarctica (the Ross Sea ice and the Marie Byrd Land) with the exception of the Antarctic Peninsula and the Weddell Sea which are warmer due to intake of relatively mild air from the seas. This is caused by the strengthening of the polar vortex which occurs above the polar region and prevents warmer air to mix with the frigid polar air, which has the effect of keeping the majority of the cold in the Antarctica. Rossby waves are of low amplitude. The temperatures are often lower in the three continents of the Southern Hemisphere approximately along the band of 30° South especially in the South of Brazil and Argentina during the positive phase of the AAO.

Regarding precipitation, during the AAO+ the Peninsula of Antarctica is more humid up to 30% while West Antarctica (the Ross Ice) and East Antarctica (the Lambert Glacier Basin ) are drier.

The positive phase of the AAO corresponds to negative anomalies of precipitation to the zonal band of 40°-50° South (lower precipitations over eastern Argentina...) and greater precipitations to the band of 50°-70° South.Along 30° South, precipitations tend to be stronger, but only on several separated regions, not along the entire zonal band. From the correlations between AAO+ and precipitation, Jones and Widmann (2003) showed that precipitation is greater in Australia, South Africa and lower in South America.

During the 1990s Antarctic Oscillation has been mostly in its positive phase during that period which is the main cause of the cooling in Antarctic.

Figure (b) shows the temperature anomaly in the
Southern Hemisphere and the figure (c) those of
rainfall during the positive phase of the AAO

Above the figure (c) shows the temperature anomaly in the Antarctic surface
in Kelvin and the figure (d) the precipitation anomaly in mm during the AAO+

  • AAO-

When the AAO index is negative the climate anomalies are the opposite to the positive phase of the AAO. Therefore the Antarctic is less cold especially in the East with the exception of the Antarctic Peninsula and the Weddell Sea, which are colder. This is caused by the less important polar vortex which occurs above the polar region. Rossby waves are high amplitude unlike when the AAO is positive. Regarding precipitation, during the AAO- the majority of Antarctica is drier.

The negative phase of the AAO corresponds to more heavy rainfall in zonal band of 40°-50° South (lower precipitation on the East of the Argentina...) and lower precipitation in the band of 50°-70° South. Along 30° South, precipitation tends to be lower, but only on several separate regions not along the entire zonal band. Jones and Widmann (2003) calculated the correlations between AAO- and precipitation observations, mean that they are lower in Australia in South Africa and greater in South America.


The Antarctic oscillation phases change at the same rate as the E.N.S.O. (El Nino and La Nina). During El Nino the phase of the AAO is most often negative whereas during La Nina it's more often a positive phase as there are for the AAO. This is due to the variation of the sea surface temperature and the wind.

On the right, Figure 1 shows the anomaly of SST (sea surface temperature) in the Ross Sea during La Niña and El Niño. Between La Niña and El Niño event there is a difference of 1 Kelvin to SST.

Figure 2 shows the direction and the importance of the wind in the Ross and Amundsen sea. The size and thickness of the arrows indicate the wind strength. The different shades of red of the hot air arrow indicate relatively cooler air during La Niña than El Niño event. However, the two red arrows indicate air masses are considerably warmer than the blue arrows. The gray arrows indicate a flow of katabatic wind.

A katabatic wind is one generated by gravity caused by the weight of a cold air mass hurtling down a geographical relief. More one moves away the dome of the ice cap, more the slope increases and thus increases the speed of the wind. This wind is triggered by a depression downstream and inversions of temperature layers. This wind is very violent, more 300.km/h, is found mainly in the poles.

Figure 1 : SST anomaly in the Ross Sea

Figure 2 : direction and importance of the wind

When comparing A.A.O. index during the austral summer (DJF) with that
of J.A.M. (the surface sea temperature anomaly (SST) of the equator Pacific
ocean) we find that the two indexes are quite often the opposite. So during an
El Niño, either when the JAM is positive, we can see that the A.A.O. index is
negative and when the JAM is negative it is the reverse. This schema was created
and is updated with the NOAA data for the AAO
and the data of Center for
Ocean-Atmospheric Prediction Studies (COAPS)
for the JAM.

Click here to see a diagram that shows the links between the AAO and ENSO (El Nino and La Nina)


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