Geoscience Reference
In-Depth Information
aligned roughly along the Arctic Ocean coastline. Reed and Kunkel (
1960
) view the
summer Arctic frontal zone in Eurasia as distinct from the region of high frontal fre-
quencies in middle latitudes of the Pacific Basin. This separation is clearly captured
in the automated frontal analysis. The well-defined Eurasian frontal zone seen in
summer breaks down in autumn while the high-latitude feature over Alaska is still
present in a weakened form. The Alaskan feature, best expressed in summer, hence
persists throughout the year.
Development of the summer Arctic frontal zone is interpreted as a manifestation
of: (1) differential heating between the Arctic Ocean and snow-free land; and (2)
sharpening of the baroclinicity by coastal orography. These conclusions are drawn
from the observation of that in summer, one sees development of a strong temper-
ature gradient along the coast, and that the frontal zone is best pronounced where
topography can “trap” the cold Arctic Ocean air. The differential heating link was
first suggested by Dzerdzeevskii (
1945
) and later by A. Kurashima (
1968
), and
the concept of a topographic link can be traced back to Reed and Kunkel (
1960
).
Additional support for topographic controls comes from the modeling study of A.H.
Lynch, A.G. Slater, and M. Serreze (
2001b
) who examined frontal activity over
Alaska. From comparisons between
Figures 4.10
and
4.11
, there is a close corre-
spondence between the preferred areas of summer cyclogenesis over northeastern
Eurasia and Alaska/Yukon and the relative maxima in summer frontal frequencies.
Vertical cross sections of the mean temperature gradient and winds throughout
the troposphere provide further insight.
Figure 4.12
shows cross sections at lon-
gitude 140°E. This longitude cuts through the region where the summer Eurasian
frontal zone is best expressed. The zonal wind cross section for January shows a
single jet with a core velocity of about 75 m s
−1
. The tropospheric meridional tem-
perature gradient is maximized at about 300 hPa (negative values meaning tempera-
ture decreases to the north). While bearing characteristics of the subtropical jet, the
effects of statistical averaging with polar front jets are evident in the baroclinicity
at lower levels and its extension to the north. Note the high-latitude baroclinicity at
lower stratospheric levels, capturing the lower end of the polar night stratospheric
jet discussed earlier. At this longitude, the Arctic frontal zone is best expressed dur-
ing June. The June zonal wind cross section is dominated by a jet at about 35°N,
about half as strong as that for winter. Note however, the development of a weaker,
separate 300 hPa wind maximum (10 m s
−1
) at 70°N. This can be associated with a
distinct high-latitude baroclinic zone extending to 400 hPa, maximized at low levels
close to the latitude of the Arctic Ocean coast.
The cross section in
Figure 4.13
cuts through Alaska at longitude 140°W, where
the high-latitude frontal zone is again well expressed. For January, a single jet (30
m s
−1
) is found at about 30°N. The lower end of the stratospheric polar night jet is
again seen at about 70°N. By sharp contrast, June reveals a subtropical jet at about
25°N, a polar jet at about 45°N, and a third Arctic jet at about 70°N. Similar to
January, there is a fairly sharp, low-level baroclinic zone at high latitudes, but it is
shifted north to about 70°N, again close to the latitude of the Arctic Ocean coast.
