Geography Reference
In-Depth Information
between the two wavy material lines shown in Fig. 12.13. Thus, by considering the
stresses acting across a material surface corrugated by the waves, it is possible to
deduce the mean-flow acceleration caused by the waves without explicit reference
to the EP fluxes of the waves.
How such a mechanism can cause a mean zonal flow oscillation when equal
amounts of easterly and westerly momentum are transferred upward across the
tropopause by the waves can be seen qualitatively by considering Fig. 12.16. If
initially, as shown in Fig. 12.16a, the mean zonal wind is weak and westerly,
the eastward propagating waves will be preferentially damped at lower altitude
and produce a westerly acceleration, which will move downward in time as the
mean westerlies intensify and the wave-driven acceleration is concentrated at ever
lower altitudes. The westward propagating waves, however, initially penetrate to
higher altitudes, where they produce easterlies, which also move downward in time.
Eventually the westerlies are damped out as they approach the tropopause, and the
stratosphere is dominated by easterlies so that the eastward propagating waves
can penetrate to high altitude and produce a new westerly phase. In this manner
the mean zonal wind is forced to oscillate back and forth between westerlies and
easterlies with a period that depends primarily on the vertical momentum transport
and other properties of the waves, not on an oscillating external forcing.
Fig. 12.16
Schematic representation of wave-driven accelerations that lead to the zonal wind QBO.
Eastward and westward propagating gravity waves of phase speeds
+
c and
−
c, respec-
tively, propagate upward and are dissipated at rates dependent on the Doppler-shifted
frequency. (a) Initial weak westerly current selectively damps the eastward propagating
wave and leads to westerly acceleration at lower levels and easterly acceleration at higher
levels. (b) Descending westerly shear zones block penetration of eastward propagating
waves, whereas westward propagating waves produce descending easterlies aloft. Broad
arrows show locations and direction of maxima in mean wind acceleration. Wavy lines
indicate relative penetration of waves. (After Plumb, 1982.)
