Geoscience Reference
In-Depth Information
to time, spreading moisture into the dry air above.
Easterly waves tend to develop in the Caribbean when
the trade wind inversion is weak or even absent during
summer and autumn, whereas in winter and spring
subsidence aloft inhibits their growth, although distur-
bances may move westward above the inversion. Waves
in the easterlies also originate from the penetration of
cold fronts into low latitudes. In the sector between two
subtropical high-pressure cells, the equatorward part
of the front tends to fracture generating a westward-
moving wave.
The influence of these features on regional climate is
illustrated by the rainfall regime. For example, there
is a late summer maximum at Martinique in the
Windward Islands (15°N) when subsidence is weak,
although some of the autumn rainfall is associated with
tropical storms. In many trade wind areas, the rainfall
occurs in a few rainstorms associated with some form
of disturbance. Over a ten-year period, Oahu (Hawaii)
had an average of twenty-four rainstorms per year, ten
of which accounted for more than two-thirds of the
annual precipitation. There is quite high variability of
rainfall from year to year in such areas, since a small
reduction in the frequency of disturbances can have a
large effect on rainfall totals.
In the central equatorial Pacific, the trade wind
systems of the two hemispheres converge in the equa-
torial trough. Wave disturbances may be generated if the
trough is sufficiently far from the equator (usually to the
north) to provide a small Coriolis force to begin cyclone
motion. These disturbances quite often become unstable,
forming a cyclonic vortex as they travel westward
towards the Philippines, but the winds do not necessarily
attain hurricane strength. The synoptic chart for part of
the northwest Pacific on 17 August 1957 (Figure 11.7)
shows three developmental stages of tropical low-
pressure systems. An incipient easterly wave has formed
west of Hawaii, which, however, filled and dissipated
over the next twenty-four hours. A well-developed wave
is evident near Wake Island, having spectacular cumulus
towers extending above 9 km along the convergence
zone some 480 km east of it (see Plate 26). This wave
developed within forty-eight hours into a circular
tropical storm with winds up to 20 m s
-1
, but not into a
full hurricane. A strong, closed circulation situated east
of the Philippines is moving northwestward. Equatorial
waves may form on both sides of the equator in an
easterly current located between about 5°N and S°. In
such cases, divergence ahead of a trough in the northern
Figure 11.5
The vertical structure of trade wind air between
the surface and 700 mb in the central equatorial Atlantic, 6 to
12 February 1969, showing air temperature (T), dew-point
temperature (T
D
). The specific humidity can be read off the upper
scale.
Source
: After Augstein
et al
. (1973, p. 104), by permission of the
American Meteorological Society.
Figure 11.6
The height (in metres) of the base of the trade wind
inversion over the tropical Atlantic.
Source
: From Riehl (1954).
