Tropical Cyclones in the Indian Ocean Part 3: Bay of Bengal versus Arabian Sea

Why Basin Geography Determines Cyclone Behaviour

Definition: Seven Global Cyclone Basins

A tropical cyclone basin is an ocean region with sustained warm sea-surface temperatures, organised atmospheric circulation, and a sufficient Coriolis force to support cyclonic genesis. Seven such basins exist worldwide: the North Atlantic, the Northeast Pacific, the Northwest Pacific, the North Indian Ocean, the South-West Indian Ocean, and the combined Australian plus South Pacific region. Each basin is governed by a designated Regional Specialised Meteorological Centre that issues warnings and assigns names from a basin-specific list.

Basin geography determines who gets hit, when, and how often. The Bay of Bengal is the world's most cyclone-vulnerable shallow shelf because its shape funnels storms into a narrowing coastline and amplifies storm-surge heights to dangerous levels.

The Arabian Sea has historically produced one-fifth of North Indian Ocean cyclones because of cooler sea surface temperatures and stronger vertical wind shear, though a warming signal is now reversing this. The Indian east coast carries the majority of Indian cyclone landfalls because it directly faces the Bay of Bengal's storm-track climatology.

Why Hurricane, Typhoon, and Cyclone Are the Same Storm

What is the significance of treating the seven basins as a single global system. A tropical cyclone is the same physical phenomenon everywhere; only the regional name changes. A storm at 150 km/h sustained wind is a hurricane in the Atlantic and the East Pacific, a typhoon in the Northwest Pacific, and a cyclone in the Indian Ocean and the Southern Hemisphere.

The underlying physics, the formation conditions covered in Part 1, and the classification scales covered in Part 2 apply uniformly across all seven basins. The regional nomenclature is institutional, not physical: the storm does not change behaviour when it crosses a basin boundary, only the name and the responsible warning centre change.

The Seven Global Cyclone Basins

From the North Atlantic Hurricane Belt to the South Pacific

The seven basins fall into three activity tiers, ordered by annual storm count and impact profile.

• Most prolific (Northwest Pacific and North Atlantic): The Northwest Pacific produces about twenty-five point seven cyclones per year of which sixteen reach typhoon strength; Japan, the Philippines, Vietnam, China, and Taiwan are the dominant landfall countries. The North Atlantic averages fourteen named storms per year, seven hurricanes, and three major hurricanes (Category 3 or higher on the Saffir-Simpson scale covered in Part 2); the Gulf of Mexico, the Caribbean, and the United States east coast are the primary impact zones.
• High-volume open-ocean (Northeast Pacific and South-West Indian): The Northeast Pacific produces fifteen to sixteen storms annually, nine of which reach hurricane strength; most head west into open ocean rather than making landfall, which keeps the basin’s damage footprint smaller than the Atlantic’s despite a comparable count. The South-West Indian basin averages nine tropical storms, five of which intensify to cyclonic-storm strength; Madagascar, Mozambique, and Reunion are the principal landfall zones.
• Smaller count, disproportionate impact (Australian and South Pacific, and North Indian Ocean): The Australian and South Pacific regions each produce about seven cyclones per year, with year-on-year variability tied to the El Nino-Southern Oscillation phase. The North Indian Ocean basin is the smallest in count terms with only four to six storms per year, yet carries disproportionate impact because Bay of Bengal storms hit the densely populated and topographically flat coastlines of Bangladesh and eastern India.

Bay of Bengal: The World's Most Cyclone-Vulnerable Shelf

Why the Bay of Bengal Carries Four Out of Five North Indian Ocean Storms

Three converging factors compound the Bay's cyclone-favourable conditions. The Bay is the more cyclone-active of the two North Indian Ocean sub-basins because of three converging factors that compound favourable formation conditions.

• Higher sea-surface temperatures: Sea-surface temperatures in the Bay are routinely above twenty-eight degrees Celsius during the cyclone seasons; the six conditions for formation covered in Part 1 are met more frequently than in the Arabian Sea.
• Accumulated cyclone energy from the Northwest Pacific: The Bay enjoys spillover from the world’s most-active basin via the South China Sea; typhoons that weaken across the Indochinese peninsula sometimes regenerate over the Bay’s warm waters.
• Funnel-shaped geometry: The Bay narrows toward the head-of-bay coastline, which amplifies storm-surge heights against Bangladesh, West Bengal, and Odisha during landfall events.

The Bay's coastline exposes five Indian states and Bangladesh to direct cyclone risk, with the exposure profile varying by sub-region.

• West Bengal and Odisha: Face the head-of-bay funnel and absorb the largest single-event landfalls, including the 1999 Odisha Super Cyclone covered in Part 10 of this series.
• Andhra Pradesh: Faces frequent low-to-moderate-intensity storms with periodic high-intensity exceptions including Phailin (October 2013) and Hudhud (October 2014).
• Tamil Nadu: Faces the southeast-trajectory storms that emerge from the southern Bay during the November peak, with Chennai metropolitan flooding the dominant impact mode.
• Bangladesh: Lying on the head of the funnel, has historically carried the world’s highest cyclone fatality rate before its cyclone-shelter network expanded after 1991.

Arabian Sea: Historical Quiet, Rising Signal

Why the Arabian Sea Is Both the Quieter and the Faster-Changing Sub-Basin

The Arabian Sea historically produces approximately one in five North Indian Ocean cyclones, far fewer than the Bay of Bengal's roughly four in five. The historical quiet has two physical drivers.

First, Arabian Sea surface temperatures average two to three degrees cooler than Bay of Bengal SSTs during the same months, especially before the warming signal of the last two decades. Second, the Arabian Sea sits under stronger upper-tropospheric wind-shear regimes during much of the year, which tears apart the vertical cyclone column at the formation stage (the low-shear condition from Part 1 fails more often).

The recent trend reverses both drivers. Arabian Sea warming over the last two decades has pushed sea surface temperatures above the formation threshold for longer windows. Cyclone Tauktae in May 2021 (Extremely Severe Cyclonic Storm; Gujarat landfall) and Cyclone Biparjoy in June 2023 (Very Severe Cyclonic Storm; Gujarat landfall) are the canonical reference cases for the shift.

The Gujarat and Maharashtra coastlines that historically were rarely tested are now the emerging high-priority zone for Arabian Sea cyclone preparedness. Part 12 of this series develops the climate-change linkage that drives this trend toward parity between the two sub-basins.

Seasonal Occurrence: May and November Peaks, Monsoon Suppression

Two Peaks per Year, Four Quiet Months

The North Indian Ocean cyclone calendar has two distinct peaks per year and a deep quiet period. The pre-monsoon peak runs from March through June with the modal month being May; storms in this window form over the warming Bay of Bengal as Indian Ocean surface temperatures rise toward the summer maximum.

The post-monsoon peak runs from October through December with the modal month being November; storms in this window form over the post-monsoon residual heat retained in the upper ocean after the southwest monsoon withdraws.

The monsoon period from June through September is the quiet window. Very few cyclones form during these four months despite favourable sea surface temperatures, because the monsoon circulation produces high vertical wind shear across the basin that prevents organised cyclone column development (the low-shear condition from Part 1 fails throughout the southwest monsoon active phase).

The monsoon also keeps the upper troposphere cooled by latent-heat extraction over the Indian land mass, reducing the temperature gradient that cyclones require. This combination of basin geography and seasonal suppression is precisely why tropical cyclones are confined to specific ocean regions and specific months.

East Coast Vulnerability and Coastal Disaster Preparedness

Why the East Coast Carries Most Indian Landfalls

The Indian East Coast from West Bengal through Odisha, Andhra Pradesh, and Tamil Nadu carries the bulk of Indian cyclone landfalls because the Bay of Bengal basin is more cyclone-active than the Arabian Sea, and Bay of Bengal storm tracks recurve into the east coast more often than they recurve into Bangladesh.

Three contemporary signals shape the next decade of Indian cyclone exposure. First, the Arabian Sea warming trend is shifting cyclone risk westward, threatening Gujarat, Maharashtra, Goa, and Kerala coasts that have historically been low-priority for cyclone preparedness.

Second, the track-recurvature patterns of Bay of Bengal storms are showing decadal shifts under warming sea-surface temperatures, with more landfalls north of the historical 18-degree-latitude centroid. Third, the rapid intensification events that Part 12 of this series develops are now occurring closer to coastlines, compressing the warning-and-evacuation timeline.

Part 4 of this series develops the cyclogenesis mechanism deep dive that explains why these basin-geography factors translate into the specific intensity outcomes observed in landfall events. Part 10 covers the major Indian cyclone case studies from the 1999 Odisha Super Cyclone to 2024 Dana.

Sources

• Fundamentals of Physical Geography, Class 11, Chapter 11: World Climate and Climate Change
• Tropical cyclone basins
• North Indian Ocean tropical cyclone
• Atlantic hurricane
• Pacific typhoon
• RSMC New Delhi Best Track Archive and Annual Cyclone Reports
• Cyclone Bulletins for North Indian Ocean
• Tropical Cyclone Programme and Six Regional Specialised Meteorological Centres
• Atlantic Hurricane Best Track Database and Climatology
• Bay of Bengal Storm Surge and Sea-State Bulletins
• Best Track Archive for North Indian Ocean and Western Pacific
• Assessment Report 6 Working Group 1 Chapter 11: Weather and Climate Extreme Events