The A.O. (Arctic Oscillation) is a climate variation
that has about the same characteristics as the
NAO (North Atlantic Oscillation). But the latter has a regional
character whose influence is centered on the North Atlantic while the
action of the AO is more extensive as it has an influence on the entire
northern hemisphere and therefore the pressures in the North Pacific
are also considered. Because the Arctic Oscillation index is calculated
according to the difference in atmospheric pressure between the North
Pole and the mid-latitude northern hemisphere (45° surroundings).
When the AO is positive-phase the atmospheric pressure is
lower than normal at the north pole and higher than normal at
the mid-latitude (approximately 45° North). This causes a
very strong West wind.
As with the NAO, during winters, the temperatures in Quebec,
Greenland, North Africa and the Middle East are then lower and
higher in Europe and in the west of USA. And the weather is wetter
in Alaska, Scotland and Scandinavia but drier in the west of the
USA and in the Mediterranean.
During the positive phase in Arctic the water circulation
is reduced which reduces the thickness of the ice sea. The advection
on the Canadian side is reduced and the passage in the Fram and
the Atlantic Strait is easier as there is less ice.
When the AO is negative phase then there is high pressures
on the polar region and low pressures at the mid-latitude (approximately
45° North). The atmospheric pressure which is greater in the
north pole decreases the strength of the West wind.
In winter the temperatures are higher than normal in Quebec,
Greenland, North Africa and the Middle East, but lower in Europe
and in the west of USA.
In the negative phase the circulation of waters in the Arctic
turns in the direction of clockwise "spiral of Beaufort",
which increases the thickness of the ice sea by allowing the ice
to remain in the cold of the Arctic centre.
Just like its cousin, the AO is quantified by an index.
It is according to the variation in atmospheric pressure in pole that
the AO index varies. It has been observed that when the pressure is
lower than normal over the Arctic, the index is positive. But when the
atmospheric pressure is higher than normal, the index is negative.
here to enlarge this schema that is the comparison of the
evolution between the AO and NAO index during the winters
Created and updated
according to the data of NOAA
for the NAO and of
NOAA for the AO
Comparison during a AO- and a AO+ with the atmospheric
in tires then the temperatures anomalies of the air in colour (SAT)
linear Spatial model (LP) for (a) and nonlinear (NLP) for (b)
and precipitations anomalies (PRCP) in linear Spatial
model (LP) for (c) and nonlinear (NLP) for (d).
here to see an animation of temperature, precipitation and
atmospheric pressure variation during a transition AO+ to AO-.
THE CAUSES OF AO
The coupling of the
troposphere and stratosphere
could be a cause of the AO. The Arctic Oscillation is strongly linked
to the strength of the
stratospheric polar vortex. The stratospheric circulation anomalies
are propagated up and down to the surface of the troposphere in at least
three weeks occur mainly in winter. They are the ones that have effects
on the sign of the AO index. More the polar vortex is strong then higher
is the AO positive index and more the polar vortex is low then lower
is the AO negative index..
The blue corresponds to the positive values so
a strong polar vortex while the red
corresponds to negative values that is a weak polar vortex. The horizontal
indicates the approximate boundary between the troposphere and stratosphere.
According to Baldwin and others, the
QBO could have secondary effects on the polar vortex and hence the
AO. When the QBO is in phase of the East the polar vortex would tend
to be lower and therefore the AO would more easily be in negative phase.
here for to enlarge
This diagram created and updated according to
the link between the troposphere and stratosphere coupling during winter
from 1957/1958 to the last winter. The blue
corresponds to a strong vortex while
red to a low vortex. The g
indicate the major stratospheric warmings and each
indicates a Canadian warming less severe until 1995. As it can be seen
the downward spread of positive or negative anomalies not always fall
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