Geography Reference
In-Depth Information
Fig. 12.13
Longitude-height section along a latitude circle north of the equator showing pressure,
temperature, and wind perturbations for a thermally damped Rossby-gravity wave. Areas
of high pressure are shaded. Small arrows indicate zonal and vertical wind perturbations
with length proportional to the wave amplitude. Meridional wind perturbations are shown
by arrows pointed into the page (northward) and out of the page (southward). The large
shaded arrow indicates the net mean flow acceleration due to the wave stress divergence.
phase propagation (m < 0) just like an ordinary westward propagating internal
gravity wave. The resulting wave structure in the x, z plane at a latitude north of
the equator is shown in Fig. 12.13. Of particular interest is the fact that poleward
moving air has positive temperature perturbations and vice versa so that the eddy
heat flux contribution to the vertical EP flux is positive.
12.5.3
Observed Equatorial Waves
Both Kelvin and Rossby-gravity modes have been identified in observational data
from the equatorial stratosphere. The observed stratospheric Kelvin waves are
primarily of zonal wave number s
1 and have periods in the range of 12-20 d.
An example of zonal wind oscillations caused by the passage of Kelvin waves at a
station near the equator is shown in the form of a time-height section in Fig. 12.14.
During the observational period shown in Fig. 12.14, the westerly phase of the
quasi-biennial oscillation (see Section 12.6) is descending so that at each level
there is a general increase in the mean zonal wind with increasing time. However,
superposed on this secular trend is a large fluctuating component with a period
between speed maxima of about 12 days and a vertical wavelength (computed
from the tilt of the oscillations with height) of about 10-12 km. Observations of the
temperature field for the same period reveal that the temperature oscillation leads
the zonal wind oscillation by 1/4 cycle (i.e., maximum temperature occurs prior
to maximum westerlies), which is just the phase relationship required for upward
propagating Kelvin waves (see Fig. 12.12). Additional observations from other
=
