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Figure 4.7. Fields of mean 500 hPa geopotential height (gpm) for the four mid-season
months over the 1970-1999 period, based on NCEP/NCAR data (by the authors).
The winter vortex, as exemplified in the January field, is strongly asymmetrical. At
60°N, the mean flow is characterized by major troughs over eastern North America
and eastern Asia and a weaker trough over western Asia (the Urals trough). A strong
ridge is located over western North America, with much weaker ridges over the
eastern Atlantic and central Asia. The lowest geopotential heights are found over
the Canadian Arctic Archipelago. These features are related to dynamical forcing
by orography (most importantly, the Rocky Mountains and the Himalayas disturb
the circulation), thermal forcing associated with the large-scale land-ocean distri-
bution (warm oceans versus cold land), and radiative forcing. From comparison
with Figure 4.2 , note the stronger winter asymmetry at 500 hPa as compared to the
stratospheric vortex and the large relative shift in the vortex center. As discussed,
the more symmetrical nature of the stratospheric winter vortex relates to the fact that
only the long planetary waves can propagate up to high levels. In the fields shown
in Figure 4.8 , the shorter, traveling waves are averaged out. The resulting pattern of
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