Geoscience Reference
In-Depth Information
'Gilbert'. This hurricane was generated 320 km east of
Barbados on 9 September 1988 and moved westward at
an average speed of 24 to 27 km hr -1 , dissipating off the
east coast of Mexico. Aided by an upper tropospheric
high-pressure cell north of Cuba, Hurricane Gilbert
intensified very rapidly, the pressure at its centre
dropped to 888 mb (the lowest ever recorded in the
western hemisphere), and maximum wind speeds near
the core were in excess of 55 m s -1 . More than 500 mm
of rain fell on the highest parts of Jamaica in only nine
hours. However, the most striking feature of this record
storm was its size, being some three times that of
average Caribbean hurricanes. At its maximum extent,
the hurricane had a diameter of 3500 km, disrupting the
ITCZ along more than one-sixth of the earth's equatorial
circumference and drawing in air from as far away as
Florida and the Galapagos Islands.
The main tropical cyclone activity in both hemi-
spheres is in late summer to autumn during times
of maximum northward and southward shifts of the
equatorial trough (Table 11.1). A few storms affect
both the western North Atlantic and North Pacific areas
as early as May and as late as December, and have
occurred in every month in the latter area. In the Bay of
Bengal, there is also a secondary early summer maxi-
mum. Floods from a tropical cyclone that struck coastal
Bangladesh on 24 to 30 April 1991 caused over 130,000
deaths from drowning and left over ten million people
homeless. The annual frequency of cyclones shown in
Table 11.1 is only approximate, since in some cases
it is uncertain whether the winds actually exceeded
hurricane force. In addition, storms in the more remote
parts of the South Pacific and Indian Oceans frequently
escaped detection prior to the use of weather satellites.
A number of conditions are necessary, even if not
always sufficient, for cyclone formation. One require-
ment as shown by Figure 11.8 is an extensive ocean area
with a surface temperature greater than 27°C. Cyclones
rarely form near the equator, where the Coriolis para-
meter is close to zero, or in zones of strong vertical wind
shear (i.e. beneath a jet stream), since both factors
inhibit the development of an organized vortex. There
is also a definite connection between the seasonal
position of the equatorial trough and zones of cyclone
formation. This is borne out by the fact that no cyclones
occur in the South Atlantic (where the trough never
lies south of 5°S) or in the southeast Pacific (where
the trough remains north of the equator). However, the
northeast Pacific has an unexpected number of cyclonic
vortices in summer. Many of these move westward near
the trough line at about 10 to 15°N. About 60 per cent
of tropical cyclones seem to originate 5 to 10° latitude
poleward of the equatorial trough in the doldrum
sectors, where the trough is at least 5° latitude from the
equator. The development regions of cyclones lie
mainly over the western sections of the Atlantic, Pacific
Table 11.1 Annual frequencies and usual seasonal occurrence of tropical cyclones
(maximum sustained winds exceeding 25 m s -1 ), 1958 to 1977.
Location
Annual frequency
Main occurrence
Western North Pacific
26.3
July-October
Eastern North Pacific
13.4
August-September
Western North Atlantic
8.8
August-October
Northern Indian Ocean
6.4
May-June; October-November
Northern hemisphere total
54.6
Southwest Indian Ocean
8.4
January-March
Southeast Indian Ocean
10.3
January-March
Western South Pacific
5.9
January-March
Southern hemisphere total
24.5
Global total
79.1
Note : Area totals are rounded.
Source : After Gray (1979).
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