Geoscience Reference
In-Depth Information
7.4.5
The Arctic Dipole Anomaly
Again looking back to Chapter 4 , recall that the Arctic Dipole Anomaly has been
defined as an atmospheric circulation pattern which in its positive mode features above
average sea level pressure centered north of the Beaufort Sea (essentially a strong
Beaufort Sea high) and below average pressure centered over the Kara Sea. Resulting
southerly winds between the pressure anomaly centers at the surface favor transport
of ice away from the coasts of Siberia and Alaska toward the North Pole, as well as
strong melt in the East Siberian and Chukchi seas. The pattern also favors a strong
sea level pressure gradient across the Fram Strait (between Greenland and Svalbard)
that enhances the wind-driven transport of sea ice out of the Arctic Ocean and into
the North Atlantic. The negative phase has a broadly opposing pressure anomaly pat-
tern. The positive pattern was very well-developed throughout the summer of 2007
and was a key factor in this year having the second lowest September sea ice in the
satellite record (based on data through 2013). It is widely viewed that because of the
general thinning of the ice cover over the past few decades, making large areas more
vulnerable to melting out in summer, the effects of the dipole anomaly circulation pat-
tern on promoting ice loss in the summer of 2007 was much greater that would have
been the case twenty years ago, when the ice was thicker and more resilient.
7.4.6
Ocean Heat Transport
Recall from Chapter 2 that, under the Arctic halocline, there is a warm, salty layer
arising from the inflow of Atlantic waters. Although the strong stability associ-
ated with the cold Arctic halocline inhibits mixing of this warm water upward to
affect the ice cover, there is growing evidence that some of this heat may indeed
be having an effect. Evidence first arose from a series of oceanographic studies
in the late 1990s, which indicated that in comparison with earlier climatologies,
the Arctic Ocean in the 1990s was characterized by a more widespread influence
and warming of Atlantic-derived waters. The Atlantic-derived sub-layer warmed
1-2°C compared with Russian climatologies of the 1940s-1970s and the level of
the subsurface temperature maximum extended (shoaled) upward (to about 200 in
from some observations). The Atlantic water influence also spread west. Evidence
also arose of a weakening of the cold halocline in the Eurasian Basin (Steele and
Boyd, 1998 ), largely attributed to eastward diversion of Russian river inflow in
response to changes in the atmospheric circulation. The Atlantic layer changes were
ascribed to increases in the Atlantic inflow through the Fram Strait and the Barents
Sea region, and some warming of this inflow. The changing inflow appeared to have
links altered surface winds as the winter NAO/AO shifted toward a predominantly
positive phase (Dickson et al., 2000 ). Modeling studies (Maslowski et al., 2000 )
suggested that the Atlantic layer changes promoted a stronger upward ocean heat
flux, contributing to the sea ice losses. Whether weakening of the cold halocline
played an additional role is not clear. Work by W. Maslowski, W. Marble, and W.
Walezowski ( 2001 ) also pointed to an influence of a warmer inflow of Pacific waters
through the Bering Strait.
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