Geoscience Reference
In-Depth Information
(
1960
) of a “separate” summer Arctic frontal zone (
Section 4.1
). The study used
six-hour, 850-hPa temperature fields from the NCEP/NCAR reanalysis.
Figure 4.11
shows derived frontal frequencies for winter and summer, expressed
as the mean number of fronts per day (a frequency of 0.10 means a front was present
on 10 percent of the days). Also shown are fields of the mean 850-hPa temperature
gradient in K (100 km)
−1
based on the twenty-year period. The frontal analysis
routine is inherently sensitive to strong temperature gradients in areas of extreme
topography, the boundaries of high plateaus, and where there are strong land-ocean
contrasts. Although it turns out that topography and land-ocean contrasts are impor-
tant for understanding high-latitude frontal activity, this sensitivity can also cause
problems. This issue was addressed simply by masking out known problem areas.
The winter frontal frequency field is rather noisy. High frequencies are found
over most of North America except northeastern Canada, where the mean temper-
ature gradient is weak. Note the band of high frequencies over the Atlantic basin
that is associated with the eastern North American jet and its attendant storm track.
There is an analogous feature (extending beyond the latitude bounds of the figure)
over the Pacific Basin that is associated with the East Asian jet and the Pacific storm
track. From comparison with the mean temperature gradient field, one can see how
the maxima in frontal frequency are placed on the warm side of these mean baro-
clinic zones. For example, the frontal zone over the Atlantic basin lies south of the
maximum temperature gradient for this area. The winter gradient fields also exhibit
features associated with topography and land/ocean contrasts. Consistent with the
analysis of cyclone activity, there is a high frequency of fronts east and northeast
of Greenland. There is separation between this area of frequent frontal activity
and the relative maximum in fronts further south associated with the eastern North
American jet. One could hence consider the area east and northeast of Greenland in
the context of a separate wintertime Arctic frontal zone.
Spring (not shown) exhibits less frontal activity over the Atlantic basin, and
northward migration of the Pacific basin feature. Frequencies are high over north-
western North America, with a general shift in activity toward the interior. A local
maximum is found centered at about 65°N, which at the resolution of the NCEP/
NCAR data is essentially colocated with the Brooks and Mackenzie ranges. There
is some separation between the frontal activity over Alaska and that to the south in
the Pacific Basin.
In summer, the relative maximum in frontal frequencies over Alaska is well-
expressed. Of particular interest is the development of a pronounced, zonally ori-
ented band of high frontal frequencies over northern Eurasia, which essentially
extends across the continent. Frontal frequencies are highest over eastern Eurasia,
corresponding to the Yana, Indigirka and Kolyma valleys, which are bounded to
the east, west and south by fairly high topography. The Eurasian and Alaskan fea-
tures are broadly in agreement with the regions of high summer frontal frequency
shown by Reed and Kunkel's (
1960
), with the orientation of the Eurasian feature
similar to the summer Arctic frontal zone plotted by Krebs and Barry (
1970
). The
Eurasian and Alaskan frontal zones are associated with mean baroclinic zones
