Geography Reference
In-Depth Information
30
10
15
W
W
W
W
20
25
30
25
E
E
E
40
50
60
70
80
100
125
E
E
20
E
W
W
15
1 0 031
10
20
31
10
20
31
10
20
31
MAY
JUN
JUL
AUG
1963
Time-height section of zonal wind at Canton Island (3
◦
S). Isotachs at intervals of 5 m s
−
1
.
Westerlies are shaded. (Courtesy of J. M. Wallace and V. E. Kousky.)
Fig. 12.14
stations indicate that these oscillations do propagate eastward at the theoretically
predicted speed. Therefore, there can be little doubt that the observed oscillations
are Kelvin waves.
The existence of the Rossby-gravity mode has been confirmed in observational
data from the stratosphere in the equatorial Pacific. This mode is identified most
easily in the meridional wind component, as v
is a maximum at the equator
for the Rossby-gravity mode. The observed Rossby-gravity waves have s
4,
vertical wavelengths in the range of 6-12 km, and a period range of 4-5 d. Kelvin
and Rossby-gravity waves each have significant amplitude only within about 20
◦
latitude of the equator.
A more complete comparison of observed and theoretical properties of the
Kelvin and Rossby-gravity modes is presented in Table 12.1. In comparing theory
and observation, it must be recalled that it is the frequency relative to the mean
flow, not relative to the ground, that is dynamically relevant.
It appears that Kelvin and Rossby-gravity waves are excited by oscillations in
the large-scale convective heating pattern in the equatorial troposphere. Although
these waves do not contain much energy compared to typical tropospheric weather
disturbances, they are the predominant disturbances of the equatorial stratosphere,
and through their vertical energy and momentum transport play a crucial role in
the general circulation of the stratosphere. In addition to the stratospheric modes
considered here, there are higher speed Kelvin and Rossby-gravity modes, which
are important in the upper stratosphere and mesosphere. There is also a broad spec-
trum of equatorial gravity waves, which appears to be important for the momentum
balance of the equatorial middle atmosphere.
=
