Gulf of Mexico
parts, where surface temperatures are warmest. As the
sun's apparent motion reaches the summer equinox in
each hemisphere, surface ocean waters begin to warm
over a period of two to three months to temperatures
in excess of 26°C. Tropical cyclones, thus, develop
from December to May in the southern hemisphere
and from June to October in the northern hemisphere.
At the same time, easterly trades on the equatorial side
of agglutinated mobile polar highs intensify and blow
these warm surface waters to the westward sides of
oceans. Hence, the warmest ocean waters accumulate
to a great depth in the Coral Sea off the east coast of
Australia, in the Caribbean, and in the west Pacific
Ocean, south-east of China. A deep layer of warming
reduces the upward mixing of cold sub-surface water
that can truncate the cyclone before it fully develops.
Warm water also develops in shallow seas because
there is a smaller volume of water to heat up. Fig-
ure 3.3 presents the origin of tropical cyclones. The
majority originate either at the western sides of oceans,
or over shallow seas, where temperatures are enhanced
by one of the above processes.
Origin and number of tropical cyclones per year for the 25-year period 1952-1977 (after Gray, 1975).