Geography Reference
In-Depth Information
the winter hemisphere. Above about 30 hPa the temperature decreases uniformly
from summer pole to winter pole, in qualitative accord with radiative equilibrium
conditions.
Mean zonal wind climatologies for the middle atmosphere are usually derived
from the satellite observed temperature field. This is done using the geostrophic
winds on an isobaric surface in the lower stratosphere (obtained from conventional
meteorological analyses) as a lower boundary condition and integrating the thermal
wind equation vertically. January and July mean zonal wind cross sections are
shown in the lower panels of Figs. 12.2 and 12.3, respectively. The main features
are an easterly jet in the summer hemisphere and a westerly jet in the winter
hemisphere, with maxima in the wind speeds occurring near the 60-km level. Of
particular significance are the high-latitude westerly jets in the winter hemispheres.
These polar night jets provide wave guides for the vertical propagation of quasi-
stationary planetary waves. In the Northern Hemisphere the EP flux convergence
due to such waves occasionally leads to rapid deceleration of the mean zonal
flow and an accompanying
sudden stratospheric warming
in the polar region, as
discussed in Section 12.4.
The zonal mean flow in the equatorial middle atmosphere is strongly influenced
by vertically propagating inertia-gravity waves and by equatorial wave modes,
especially the Kelvin and Rossby-gravity modes. These waves interact with the
mean flow to produce a long period oscillation called
quasi-biennial oscillation
.
This oscillation produces large year-to-year variability in the mean zonal wind in
the equatorial middle atmosphere, which is not shown in the long-term means of
Fig. 12.3.
12.2
THE ZONAL-MEAN CIRCULATION OF THE MIDDLE
ATMOSPHERE
As discussed in Section 10.1, the general circulation of the atmosphere considered
as a whole can be regarded to a first approximation as the atmospheric response to
the diabatic heating caused by absorption of solar radiation at the surface. Thus, it
is reasonable to say that the atmosphere is driven by differential diabatic heating.
For an open subregion of the atmosphere, such as the middle atmosphere, it is not
correct, however, to assume that the circulation is driven by diabatic heating. It is,
rather, necessary to consider the transfer of momentum and energy between that
subregion and the rest of the atmosphere.
In the absence of eddy motions, the zonal-mean temperature of the middle
atmosphere would relax to a radiatively determined state in which, except for a
small lag due to thermal inertia, the temperature would correspond to an annually
varying radiative equilibrium following the annual cycle in solar heating. The
circulation in such a situation would consist only of a zonal-mean zonal flow in
thermal wind balance with the meridional temperature gradient. Neglecting small
