Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 78

atropical cyclone can intensify and this varies between ocean basins. In the
Coral Sea region of the southwestern Pacific this figure is approximately 890 hPa
central pressure and in the northwest Pacific it is more intense with maximum
potential intensities of approximately 860--870 hPa theoretically achievable.
In addition to the uplift of air or convection over warm tropical seas, tropical
cyclones also require atmospherically forced divergence of the uplifted air at
high altitude. If this does not occur then convection of air will cease at the
Earth's surface. The low-pressure system near the Earth's surface then requires
awind circulation system to develop. This often happens with a convergence of
winds into the monsoon trough. These winds of course tend to be southeasterlies
in the southern hemisphere and northeasterlies in the northern hemisphere. So
if the monsoon trough lies in the northern hemisphere, the southeasterlies, after
crossing the geographical equator, will tend, due to the Coriolis effect, to change
direction into southwesterly winds. The reverse occurs with the northeasterlies
when they cross the equator if the monsoon trough resides in the southern
hemisphere during the southern summer. The convergence of this air into the
trough can be the impetus for the development of a low level wind circulation.
Atropical cyclone can then develop and intensify with continued uplift of air
due to favourable conditions aloft. Shearing winds in the upper troposphere can
be a major limiting factor to any further intensification of the tropical cyclone
and indeed can cause dissipation of the system. These shearing winds are high-
altitude winds that literally cut off the top of the cyclone causing a lack of
'outflow', or divergence of air in the upper troposphere, and the system shuts
down.
Tropical cyclones typically form in tropical seas between 5 ◦ and 30 ◦ latitude.
They usually do not form over the equator due to the lack of influence of the
Coriolis effect, which is required to develop wind rotation around the system.
Tropical cyclones frequently form on the western side of tropical ocean basins
where warm waters typically accumulate because of the movement of ocean
currents and equatorial easterly winds. Such conditions are enhanced when the
Southern Oscillation is positive and at these times cyclones often occur closer
to the coasts of the southeastern USA, Gulf of Mexico, Caribbean Islands and in
the northwest Pacific near the Philippines and Taiwan. The same is true in the
southwest Pacific when cyclones tend to form closer to the northeast coast of
Australia. When the Southern Oscillation is negative, or an El Nino event occurs,
tropical cyclone (hurricane) development in the southwest Atlantic Ocean is
often reduced and the same is true in the far southwestern Pacific. However, at
these times cyclone activity can increase in the central Pacific with the passage of
thewarm sea surface temperatures further eastward. Warm waters and cyclone
development are not just restricted to the western sides of ocean basins, however,