Geoscience Reference
In-Depth Information
Greenland-Iceland-Norwegian seas promotes deep oceanic convection in the central
Greenland Sea in winter, associated with intense vertical heat fluxes to the atmo-
sphere. This causes positive anomalies in surface air temperature and intensification
of the Icelandic Low (a transition to the positive phase on the NAO). As a result of
these processes, there is a growth of air temperature and sea level pressure gradients
between the two basins. The gradients build to a point where they cannot be main-
tained and the basins then interact. An intensified horizontal heat flux to the Arctic
Ocean weakens the atmospheric anticyclone and Beauport Gyre. The accumulated
freshwater in the Beaufort Gyre spills into the Greenland-Iceland-Norwegian seas,
which become fresher, suppressing deep convection. With a fresher surface, sea ice
is easier to form. Both processes reduce the vertical heat flux to the atmosphere (sea
ice insulating the ocean from the atmosphere), acting to weaken the Icelandic Low
(a transition back to the negative phase of the NAO). After several years, interaction
between the basins fades. Gradients then rebuild and the cycle repeats itself.
4.7.2
The Arctic Oscillation (AO)
In 1998, a paper appeared in Geophysical Research Letters by D. Thompson, then
a Ph.D. student at the University of Washington, Seattle, and his advisor, J. Wallace
(Thompson and Wallace,
1998
). Follow-on papers were published shortly thereaf-
ter (Thompson and Wallace,
2000
; Thompson, Wallace, and Hergel,
2000
, Wallace,
2000
). Thompson and Wallace (
1998
) argued that NAO should be considered as a
regional manifestation of a more basic mode of SLP variability, which has come to
be known as the AO, or NAM. They defined the AO as the leading mode of SLP var-
iability from an EOF analysis for the Northern Hemisphere winter (using data from
1900-1997). The major characteristics (
Figure 4.21
) are a primary center of action
over the Arctic, focused in the area of the Icelandic Low, and opposing, weaker
centers of action in the North Atlantic and North Pacific. The interpretation is that if
pressures are low (high) over the Arctic, they are high (low) over the North Atlantic
and North Pacific. Thompson and Wallace (
1998
) were not the first to perform a
Northern Hemisphere analysis. As far as we are aware, the first arguments for more
a fundamental mode of variability than the NAO come from the studies of E. Lorenz
(
1951
), J. Kutzbach (
1970
) and Wallace and Guzler (
1981
).
However, as is obvious from
Figure 4.21
, the Arctic and Atlantic AO centers of
action correspond closely to the NAO winter centers of action. Looking back at
Figure 4.17
, the time series of the AO (NAM) and NAO are also very similar. Not
surprisingly, the spatial pattern of correlations between the winter AO and surface
air temperature (
Figure 4.22
) is also quite similar to the spatial pattern based on the
NAO index; notable differences include stronger positive correlations over northern
Eurasia and a more strongly expressed region of negative correlations centered over
Alaska.
Thompson and Wallace (
1998
) interpret the winter AO as the surface signature of
modulations in the strength of circumpolar stratospheric vortex. Recall from earlier
discussion that the mean winter circulation in the stratosphere is primarily zonally