GS Paper 1 10 marks · 150w 9 min Easy
Tropical cyclones are largely confined to South China Sea, Bay of Bengal and Gulf of Mexico. Why?
Subtopic: Geography · tropical cyclone genesis and distribution
How to structure your answer
Introduction: the three seas as warm western arms of tropical oceans → SST above 26–27°C sustained by warm currents and trade-wind piling → Latitude 5°–20° supplies Coriolis force; low wind shear preserves structure → ITCZ and easterly waves seed disturbances; semi-enclosed shallow basins heat fast → Contrast with cool eastern margins and the South Atlantic → Conclusion
Detailed model answer
207 words · target 150 words · 9 min
Tropical cyclones concentrate in the South China Sea, the Bay of Bengal and the Gulf of Mexico because these warm, semi-enclosed western arms of tropical oceans combine every condition of cyclogenesis.
Conditions these seas satisfy
- Sea-surface temperatures above 26–27°C through a deep surface layer: trade winds pile warm water on the western sides of oceans, and warm currents — the Kuroshio near the South China Sea, the Loop Current–Gulf Stream system in the Gulf of Mexico — sustain the heat and moisture supply.
- Location between roughly 5° and 20° latitude, providing enough Coriolis force to impart rotation, unlike the equatorial belt where cyclones cannot spin up.
- Low vertical wind shear, which lets the warm core stay vertically organized.
- Abundant seed disturbances — the migrating ITCZ and easterly waves repeatedly generate low-pressure cells over these waters.
- Semi-enclosed, relatively shallow basins heat rapidly; the Bay of Bengal’s freshwater stratification from the Ganga–Brahmaputra–Irrawaddy discharge keeps its surface layer especially warm, explaining its high share of the deadliest cyclones.
Why other areas lag
- Eastern ocean margins are chilled by cold currents and upwelling (Peru, Canary, California currents), while the South Atlantic lacks warm enough water and has high shear — so cyclones rarely form there.
Hence cyclone frequency peaks in these three warm western basins.
What an examiner expects to see
- All three are western, semi-enclosed arms of tropical oceans where trade winds pile up warm water
- SST above 26–27°C through a deep mixed layer — maintained by the Kuroshio and Loop Current–Gulf Stream warm currents
- 5°–20° latitude band supplies adequate Coriolis force; the equatorial belt itself is cyclone-free
- Low vertical wind shear and high mid-troposphere humidity preserve the storm's warm core
- ITCZ migration and easterly waves provide repeated seed disturbances
- Bay of Bengal specifics: freshwater stratification keeps surface water warm; funnel shape worsens surge impacts
- Contrast: cold currents and upwelling on eastern ocean margins suppress cyclogenesis
Concrete cases, schemes and judgments
- Bay of Bengal storms — Cyclones Amphan (2020) and Mocha (2023); historically the 1970 Bhola cyclone, the deadliest on record
- Gulf of Mexico hurricanes — Katrina (2005) intensifying over the warm Loop Current
- South China Sea typhoons — Haiyan (2013) approaching through the warm western Pacific
- Near-absence of South Atlantic cyclones (rare exception: Catarina, 2004) due to cool SSTs and shear
Terminology to weave into the answer
cyclogenesissea-surface temperatureCoriolis forcevertical wind sheareasterly wavesITCZ