Geography Reference
In-Depth Information
zonal wind maxima is much smaller. Furthermore, in both seasons the core of
maximum zonal wind speed (called the mean
jetstream
axis) is located just below
the
tropopause
(the boundary between the troposphere and stratosphere) at the
latitude where the thermal wind integrated through the troposphere is a maximum.
In both hemispheres, this is about 30˚ during winter, but moves poleward to 40˚-
45˚ during summer.
That the zonally averaged meridional cross sections of Fig. 6.1 are not represen-
tative of the mean wind structure at all longitudes can be seen in Fig. 6.2, which
shows the distribution of the time-averaged zonal wind component for DJF on
the 200-hPa surface in the Northern Hemisphere. It is clear from Fig. 6.2 that at
some longitudes there are very large deviations of the time-mean zonal flow from
its longitudinally averaged distribution. In particular there are strong zonal wind
maxima (jets) near 30˚N just east of the Asian and North American continents and
north of the Arabian peninsula; distinct minima occur in the eastern Pacific and
eastern Atlantic. Synoptic-scale disturbances tend to develop preferentially in the
regions of maximum time-mean zonal winds associated with the western Pacific
and western Atlantic jets and to propagate downstream along
storm tracks
that
approximately follow the jet axes.
Fig. 6.2
Mean zonal wind at the 200-hPa level for December-February averaged for years 1958-1997.
Contour interval 10 m s
−
1
(heavy contour, 20 m s
−
1
). (Based on NCEP/NCAR reanalyses;
after Wallace, 2003.)
