Geoscience Reference
In-Depth Information
Figure 6.4.
Longitudinal distribution of summer frontal frequency (fronts per day) and
the fraction of annual precipitation falling in summer, averaged from 62.5 to 67.5°N.
The precipitation data include bias adjustments (from Serreze et al.,
2001
, by permission
of AMS).
As outlined in
Chapter 4
, summer sees development of a band of high frontal fre-
quencies along Eurasia from about 60-70°N, best expressed over the eastern half of
the continent. A similar band is found over Alaska. Although most strongly devel-
oped in summer, the Alaskan feature is present year-round. The high frequency of
summer frontal activity is linked to heating contrasts between the Arctic Ocean
and snow-free land, with the coastal baroclinicity sharpened by coastal orogra-
phy. Cyclogenesis is common in preferred areas over eastern Eurasia, Alaska, and
Canada where the frontal zone is best expressed.
The hydrologic impact of the frontal zone is evident when plotting the lon-
gitudinal distribution of summer frontal frequencies (fronts per day) and the
fraction of annual precipitation that falls in summer, averaged from 62.5°N
to 67.5°N (
Figure 6.4
). At 140°E, where the Eurasian frontal zone is well
expressed, nearly 60 percent of annual precipitation falls during summer. Where
the Alaskan frontal zone is well-expressed, more than 50 percent of annual pre-
cipitation falls during summer. The late summer and early autumn precipitation
maximum over the Arctic Ocean occurs as cyclones from the weakened, but still
present North Atlantic cyclone track and systems generated over Eurasia (many
along the frontal zone) migrate into the region. From Yang's (
1999
) analysis of
the North Pole records (
Figure 6.1
), mean precipitation over the central Arctic
Ocean peaks in September at about 30 mm, roughly three times the total for
April, the driest month.
The October field in
Figure 6.3
illustrates the transition back toward the winter
pattern. For land areas except west-central Eurasia, precipitation has declined from
its summer maximum. This is also true for most of the Arctic Ocean (again, see
Figure 6.1
). However, precipitation picks up sharply in the Atlantic and Pacific sec-
tors as the primary storm tracks gain strength.
