Geography Reference
In-Depth Information
Fig. 12.11
Schematic of transient wave, mean-flow interactions occurring during a stratospheric
warming. (a) Height profiles of EP flux (dashed), EP flux divergence (heavy line), and
mean zonal flow acceleration (thin line); z
0
is the height reached by the leading edge of
the wave packet at the time pictured. (b) Latitude-height cross section showing region
where the EP flux is convergent (hatched), contours of induced zonal acceleration (thin
lines), and induced residual circulation (arrows). Regions of warming (W) and cooling
(C) are also shown. (After Andrews et al., 1987.)
equatorial mesosphere. Eventually, as more of the flow becomes easterly, waves
can no longer propagate vertically. The wave-induced residual circulation then
decreases, and radiative cooling processes are able to slowly reestablish the nor-
mal cold polar temperatures. Thermal-wind balance then implies that the normal
westerly polar vortex is also reestablished.
In some cases the wave amplification may be large enough to produce a polar
warming, but insufficient to lead to reversal of the mean zonal wind in the polar
region. Such “minor warmings” occur every winter and are generally followed by
a quick return to the normal winter circulation. A “major warming” in which the
mean zonal flow reverses at least as low as the 30-hPa level in the polar region
seems to occur only about once every couple of years. If the major warming occurs
sufficiently late in the winter, the westerly polar vortex may not be restored at all
before the normal seasonal circulation reversal.
12.5
WAVES IN THE EQUATORIAL STRATOSPHERE
Section 11.4 discussed equatorially trapped waves in the context of shallow water
theory. Under some conditions, however, equatorial waves (both gravity and Rossby
types) may propagate vertically, and the shallow water model must be replaced
by a continuously stratified atmosphere in order to examine the vertical structure.
