Tropical cyclones are largely confined to South China Sea, Bay of Bengal and Gulf of Mexico. Why?

Tropical cyclones are largely confined to South China Sea, Bay of Bengal and Gulf of Mexico. Why? [10 marks, 2014]

A tropical cyclone is an intense low-pressure system that develops over warm tropical oceans by drawing energy from the condensation of moist air. Therefore, its distribution is controlled by the coincidence of four basic requirements:

Warm ocean + sufficient rotation + atmospheric disturbance + favourable winds

Tropical cyclones are concentrated, not confined, in the South China Sea, Bay of Bengal and Gulf of Mexico. They also occur in the eastern Pacific, south-west Indian Ocean and around northern Australia.

Why are these regions favourable?

1. These seas largely lie between about 5° and 30° North, where:

1. ocean waters remain warm (tropical cyclones require sea-surface temperatures of ~26.5°C or more); and
2. the Coriolis force is strong enough to produce rotation.

2. Disturbances frequently form or enter these regions:

1. the monsoon trough over South and South-East Asia;
2. tropical low-pressure systems over the western Pacific; and
3. easterly waves moving from the Atlantic and Caribbean towards the Gulf of Mexico.

3. Atmospheric conditions are seasonally favourable:

1. the atmosphere is moist and unstable;
2. wind shear becomes relatively low; and
3. rising air can remain vertically organised.

Hence, cyclones are less common in some other regions because:

1. Near the Equator: Coriolis force is too weak.
2. Eastern margins of many oceans: cold currents and upwelling reduce sea temperatures.
3. South Atlantic: cooler waters, stronger wind shear and fewer initial disturbances.
4. Arabian Sea: dry continental air and strong monsoon winds often hinder organisation, although intense cyclones have increased in recent decades.

To conclude, the concentration of tropical cyclones in the South China Sea, Bay of Bengal and Gulf of Mexico results from the combined presence of warm water, suitable latitude, abundant moisture, recurring disturbances and seasonally low wind shear.