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

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Ghissu Editorial · Accepted Answer

Tropical cyclones concentrate in the South China Sea, Bay of Bengal and Gulf of Mexico because warm water above 26.5°C, adequate Coriolis force, ready disturbances and low wind shear coincide there. Cold eastern-ocean currents, South Atlantic shear and weak equatorial Coriolis force deny cyclones elsewhere.

**Introduction**

Tropical cyclones need warm water, adequate Coriolis force, a pre-existing disturbance and calm upper winds together, and the South China Sea, the Bay of Bengal and the Gulf of Mexico combine all four.

**Conditions These Three Seas Supply**

FIGURE: geography-p30-0 | Major tropical cyclone basins of the world

1. **Warm water pools**: These semi-enclosed tropical seas stay above 26.5°C to about 50 m.
2. **Right latitude band**: Lying mostly between about 5°N and 30°N, they get enough Coriolis force to spin storms while staying warm.
3. **Ready-made disturbances**: The monsoon trough seeds Bay of Bengal and South China Sea depressions, easterly waves from Africa feed the Gulf, and typhoons like Haiyan (2013) cross into the South China Sea.
4. **Low wind shear seasons**: Weak upper winds in the pre-monsoon and October to December windows produce Bay storms like Phailin (2013), while the Gulf peaks in August to October, as with Katrina (2005).

**Why Other Waters See Fewer Cyclones**

1. **Cold eastern margins**: The Canary, Benguela, Humboldt and California currents cool the eastern sides of oceans and stabilise the air.
2. **Bay versus Arabian Sea**: Upwelling off Somalia and Oman and dry Arabian air leave the Arabian Sea with about a quarter of the Bay's cyclones.
3. **South Atlantic and equator**: Shear and cool water keep the South Atlantic almost barren, Catarina (2004) excepted, and within about 5° of the equator the Coriolis force is too weak.

**Conclusion**

Warmth, rotation, a trigger and calm upper air meet in these seas, making them the world's cyclone nurseries. For India, IMD's accurate warning before Phailin, which kept deaths to a few dozen against about 10,000 in 1999, shows that preparedness can blunt this geography.

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

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