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
This schema has been created and is
updated according to the
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
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.
RELATIONSHIP BETWEEN AAO AND THE ENSO
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
El Niño, either when the JAM is positive, we can see that the A.A.O.
negative and when the JAM is negative it is the reverse. This schema
and is updated with the
NOAA data for the AAO and the data of Center
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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