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layer, although horizontal thermal advection by subpolar air masses has also been
invoked (Van Loon,
1967
). For the Arctic, T. Chase and colleagues (
2002
) propose
that winter temperatures in the mid-troposphere are regulated by moist adiabatic
ascent from airmasses in regular contact with high latitude open ocean regions,
such as the Norwegian Sea. The idea is that airmasses move over such open water
areas, where they are convectively heated. They then move over cold snow/ice sur-
faces. They cool from the bottom upward, albeit slowly. Before the cooling can
significantly affect the mid-troposphere, the airmasses move over open ocean again,
where they are rewarmed by convective heating. The proposed mechanism requires
further investigation, but finds support in the subsequent study of M. Tsukernik
et al. (
2004
), who identify the Norwegian Sea as a key region for convective warm-
ing (see also
Chapter 3
).
4.5
Surface and Near-Surface Circulation
4.5.1
Centers of Action
Fields of mean sea level pressure (SLP) for the four mid-season months are pro-
vided in
Figure 4.8
. The mean January circulation at sea level, representative of
winter, is dominated by the three well-known subpolar “centers of action”: (1) the
Siberian High over east-central Asia; (2) the Icelandic Low off the southeast coast of
Greenland; and (3) the Aleutian Low in the North Pacific basin. The central Arctic
Ocean appears as a saddle of relatively high pressures between the eastern Eurasian
landmass and northwestern Canada.
The three centers of action are much weaker during April. The Aleutian Low
exhibits a pronounced shift to the northeast, whereas the locus of the Siberian high
has shifted slightly west. While the winter centers of action weaken, a closed high
pressure cell develops over the Beaufort Sea. By July, the Aleutian Low has dis-
appeared as a mean feature. The Icelandic Low is quite weak, and low pressures
extend over the eastern Canadian Arctic. The Siberian High is replaced by a broad
area of mean low pressure. An area of weak mean low pressure is also found near
the pole in July. The mean October field illustrates the transition back to winter
conditions.
The winter Siberian High is a cold, shallow anticyclone, driven largely by radia-
tive cooling to space. The cold air is constrained by topography. The reality of some
of the extreme high pressures reported in the region is open to question. This stems
from difficulties in reducing station pressures recorded in upland valleys (where
there are persistent cold air “lakes”) to sea level (Walker,
1967
). But the existence
of the anticyclone is not in dispute. Low 1,000- to 500-hPa thicknesses during win-
ter are consistent with a cold anticyclone structure (Sahsamanoglou, Makrogiannis,
and Kallimopolous,
1991
) and cold highs regularly move eastward from the mean
locus of the winter Siberian High (around 45-50°N, 90-110°E). The system peri-
odically intensifies and gives rise to cold air outbreaks or surges over East Asia
