Geoscience Reference
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450 to 600m ( Figure 11.5 ). Thus, the cool eastern
tropical oceans are characterized by extensive, but
shallow, marine stratocumulus, which gives little
rainfall. Downstream the inversion weakens and
its base rises ( Figure 11.6 ) because the subsidence
decreases away from the eastern part of the
anticyclone and cumulus towers penetrate
through the inversion from time 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 disturbances 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
Specific humidity (g/kg)
4
6
8
10
12
14
700
16
DRY
AIR
T
800
T D
900
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 24 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 Equatorial 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 develop-
mental stages of tropical low pressure systems. An
incipient easterly wave formed west of Hawaii,
which, however, filled and dissipated during the
°
MOIST
SUBLAYER
T ocean
1000
10
15
20
25
30
Temperature (°C)
Figure 11.5 The vertical structure of Trade Wind
air between the surface and 700mb in the central
equatorial Atlantic, 6-12 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). Courtesy of the
American Meteorological Society.
75°W
60°
45°
30°
15°
15°E
30°N
500
15°
1500
1000
2000
500
15°
1500
30°S
Figure 11.6 The height (in meters) of the base of
the trade wind inversion over the tropical Atlantic.
Source: From Riehl (1954). By permission of McGraw Hill.
 
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