Geoscience Reference
In-Depth Information
4
The Atmospheric Circulation
Overview
Some aspects of the atmospheric circulation of the Arctic were introduced
in
Chapters 2
and
3
. The present chapter begins with a historical
perspective. We then make use of data from the NCEP/NCAR reanalysis
to provide a top-down analysis, starting with the stratosphere and moving
to the tropopause, the troposphere, and the surface. The focus is largely
on the mean state, aspects of seasonality, and important modes of large-
scale atmospheric variability.
The stratospheric circulation during winter is dominated by a strong
polar cyclonic vortex; along its margin is the polar night jet. The polar
night jet decays through spring, to be replaced in summer by a weak
circum-Arctic easterly flow. The winter vortex sometimes rapidly breaks
down because of sudden stratospheric warmings, the effects of which
can impact the troposphere and the surface. At 500 hPa, a circumpolar
vortex is present year-round. The vortex is strongly asymmetric during
winter, with major troughs over eastern North America and eastern Asia,
and a weaker trough over western Asia. A prominent ridge is located over
western North America. The lowest winter pressure heights at 500 hPa are
found over the Canadian Arctic Archipelago. This asymmetry is related to
orography, large-scale land-ocean distributions, and radiative forcing. The
mid-tropospheric vortex weakens during spring and summer and becomes
more symmetric than its winter counterpart.
The mean winter circulation at sea level has three prominent features:
the Icelandic Low off the east coast of southern Greenland, the Aleutian
Low south of Alaska, and the Siberian High in central Eurasia. The mean
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