Cyclones in India: where and when

Detail

Why India gets cyclones from two seas

• India is a peninsula. A peninsula is land with water on three sides. India's southern part narrows into the Indian Ocean.
• The Bay of Bengal lies to the east and the Arabian Sea to the west.

• So cyclones can strike both the east coast and the west coast.

• Tropical cyclone: a large, low-pressure storm that forms over warm tropical sea. Winds spiral inwards around a calm centre called the eye. In the Northern Hemisphere they spiral anticlockwise.

• Landfall: the moment the centre of a cyclone crosses the coast. After landfall the storm loses its supply of warm, moist sea air and weakens.

Bay of Bengal vs Arabian Sea: which sea has more?

• Cyclones in the Bay of Bengal and Arabian Sea: over the long-term record, the Bay has had about four times as many cyclones as the Arabian Sea.
• The Bay usually has 2–4 cyclones a year [5].

Why the Bay makes more cyclones

• Warmer water. Cyclones draw their energy from warm sea water. A warmer sea gives more heat and moisture.
• Fresher, layered (stratified) surface water. Stratification means the sea is set in layers that do not mix easily.
• Big rivers (Ganga–Brahmaputra, Mahanadi, Godavari, Krishna) pour fresh water into the Bay.
• Fresh water is lighter than salt water, so it stays on top as a thin, warm layer.

• A cyclone's winds stir the sea, but they cannot easily bring cold water up from below → the surface stays warm → the storm keeps its fuel.

• Leftover Pacific typhoons. A typhoon is the name for a tropical cyclone in the western Pacific.

• Weak remains of these storms drift west across Myanmar and Thailand.

• Over the warm Bay they can grow again into fresh depressions or cyclones.

• Shallow, funnel-shaped head. The northern Bay (coasts of West Bengal and Bangladesh) is shallow and narrows like a funnel.

• A storm surge is a wall of seawater pushed onto land by a cyclone's winds and low pressure.
• In a shallow, narrowing bay the water has nowhere to spread → it piles up higher → surges are much worse.

Why the Arabian Sea has fewer cyclones (historically)

• Upwelling: cold water from deep below rises to the surface. Monsoon winds push surface water away from the Somalia and Oman coasts, and cold water comes up in its place. This cools the Arabian Sea.
• Dry air from Arabia: dry desert air blows over the sea. It holds little moisture, so storms struggle to grow.

Feature	Bay of Bengal	Arabian Sea
Share of cyclones (long-term)	About 4 times more	Fewer
Sea surface	Warmer; fresh, layered top water	Cooler; upwelling off Somalia and Oman
Extra source of storms	Remains of western Pacific typhoons	None
Coast shape	Shallow, funnel-shaped head → large surges	Open coast
Most hit (India)	Odisha, Andhra Pradesh, Tamil Nadu, West Bengal	Gujarat; less often Maharashtra, Kerala
Recent trend	Still the main cyclone basin	Rising number and strength

The Arabian Sea is changing

• 2019: the Arabian Sea had 5 of the 8 cyclones in the north Indian Ocean (62.5%). It was the first year with more Arabian Sea cyclones than Bay cyclones [5].
• 2020: 2 of the 5 cyclones were in the Arabian Sea [5].
• Stronger storms. Before 1995, Category 2–3 cyclones were rare in the Arabian Sea and Category 4 storms did not occur. After 1995, Category 2–3 cyclones became much more common each year [5].

• Example: Tauktae (May 2021) reached peak winds of about 220 km/h [5].

• The sea is warming faster:

• Average warming was 0.0101 °C a year from 1960 to 2011 [5].

• The rate went from 7.7 to 13.4 millidegrees a year after the mid-1990s, about 74% faster [5].

• Causes put forward [5]:

• more heat stored in the ocean;
• weaker natural cooling (less upwelling and winter mixing);
• the Indian Ocean Dipole (IOD: a see-saw in sea temperature between the western and eastern Indian Ocean);

• aerosols (tiny particles of pollution in the air) released by people.

• Biparjoy (June 2023) is one of the longest-lived cyclones ever seen in the north Indian Ocean. It lasted 13 days 3 hours, about twice the regional average of 6 days 3 hours [3].

When cyclones come: a two-peak (bimodal) year

• Bimodal season: a season with two peaks.
• Pre-monsoon: April–May.

• Post-monsoon: October–December. October–November is the main peak for the Bay of Bengal.

• Why so few cyclones in the monsoon months (June–September)?

• Vertical wind shear means the wind changes a lot in speed or direction between low and high levels of the air.
• In the monsoon, strong westerly winds blow near the surface and the strong easterly jet blows high above.
• This shear tilts the storm and tears apart its tall column of clouds, so it cannot grow into a full cyclone.
• Weaker monsoon depressions (low-pressure systems weaker than cyclones) still form in the Bay in these months. They bring much of the monsoon rain to central India.

Season	Months	Main basin	Notes
Pre-monsoon	April–May	Both seas	Fani 2019 and Amphan 2020 (Bay)
Monsoon	June–September	Few cyclones	Strong shear; monsoon depressions form instead
Post-monsoon	October–December	Bay (peak October–November)	Andaman Sea and southern Bay; hits the east coast

Where Bay cyclones form (genesis zones)

• Genesis: the place where a cyclone is first born.
• In October–November, Bay storms mostly form between about 16° and 21° N, west of 92° E. (NCERT's printed figure, "16°–2°N", is a misprint.)
• By July, the birthplace moves north to about 18° N, west of 90° E, near the Sundarbans delta (the Ganga–Brahmaputra delta shared by West Bengal and Bangladesh).

Post-monsoon cyclones and the Coromandel coast

• Post-monsoon cyclones are cyclones that form after the south-west monsoon pulls back (October–December). They form over the Andaman Sea and the southern Bay of Bengal and move west to cross the east coast.
• They bring rain, and they cause damage:
• Rain: with the north-east monsoon (winds blowing from the north-east in winter), they give much of the Coromandel coast's rain. The Coromandel coast is the Tamil Nadu coast. Tamil Nadu gets most of its rain in October–December, not in summer.
• Damage: they often wreck the low, crowded, farm-rich deltas of the Godavari, Krishna (Andhra Pradesh) and Kaveri (Tamil Nadu). There, high winds, surges and floods ruin crops, homes and fishing boats.

Recent cyclones: three case studies

Cyclone	Year, season	Sea	Coast hit	Key facts
Fani	May 2019, pre-monsoon	Bay of Bengal	Odisha	Over the Bay on 1 May 2019 with steady winds of about 190 km/h (Category 3 level), heading for India's east coast [4]
Amphan	May 2020, pre-monsoon	Bay of Bengal	West Bengal (Sundarbans) and Bangladesh	Reached Super Cyclonic Storm level. Landfall on 20 May 2020 as a very severe cyclonic storm, between Digha (West Bengal) and Hatiya Islands (Bangladesh) [2]
Biparjoy	June 2023, early monsoon	Arabian Sea	Gujarat (Kachchh)	Landfall on 15 June 2023. Lasted 13 days 3 hours. Flooded Gujarat, Rajasthan and Madhya Pradesh as it moved slowly inland to the north-east [3]

Amphan in detail: the funnel-shaped Bay at work

• Winds: IMD forecast steady winds of 155–165 km/h, gusting to 185 km/h, at landfall [2].
• Storm surge: forecast at 4–5 m above the normal tide in West Bengal and 2.5–3.5 m in Bangladesh [2].
• Evacuation:
• India moved about 1 million people to safety.
• Bangladesh moved nearly 2 million people and 500,000 livestock.

• More than 12,000 cyclone shelters were used [2].

• Loss: at least 72 deaths in India and 25 in Bangladesh were reported at the time. About 500,000 families lost their homes [2].

• Lesson: early warnings for several hazards together (wind, surge and rain) kept deaths far below the thousands killed by cyclones in this region in the past [2].

Most exposed coasts

• East coast (most exposed):
• Odisha
• Andhra Pradesh (Godavari and Krishna deltas)
• Tamil Nadu (Kaveri delta, Coromandel coast)

• West Bengal (Sundarbans, the funnel-shaped head of the Bay)

• West coast:

• Gujarat (Kachchh and Saurashtra) is the most exposed.

• Maharashtra and Kerala are hit less often.

• NCERT's map of wind and cyclone hazard zones (Class 11, Fig. 6.4) shows the highest risk along the east coast and the Gujarat coast.

• The east coast is hit by several cyclones every year.

Warning and management

• IMD (India Meteorological Department) tracks each cyclone from birth to landfall: where it forms, how it grows and where it will cross the coast.
• NDRF (National Disaster Response Force) carries out rescue and relief. (NCERT: 12 battalions.)
• Cyclones are easier to predict than earthquakes. A cyclone can be tracked for days before landfall, so people have time to prepare. Key steps:
• Cyclone shelters: strong buildings on high ground near the coast.
• Embankments: raised earth walls that hold back surge water.
• Mangroves: salt-tolerant coastal forests, such as the Sundarbans. Their dense roots slow the wind and break the surge.
• Early warning and evacuation: this is how Amphan (2020) was handled [2].

Prelims Hooks

• The Bay of Bengal has historically had about four times as many cyclones as the Arabian Sea.
• 2019: Arabian Sea 5, Bay 3 of the 8 north Indian Ocean cyclones. It was the first year the Arabian Sea had more [5].
• Seasons are bimodal: April–May and October–December. The Bay's peak is October–November. Cyclones are rare in June–September because of vertical wind shear.
• Trap: the Arabian Sea is cooler because of upwelling off Somalia and Oman and dry Arabian air. It is not cooled by fresh river water. Fresh water layering is what keeps the Bay warm.
• Bay genesis zone in October–November: about 16–21° N, west of 92° E. In July it moves to about 18° N, west of 90° E, near the Sundarbans.
• Post-monsoon cyclones form over the Andaman Sea and southern Bay and add to Tamil Nadu's north-east monsoon rain.
• Delta–state pairs: Godavari and Krishna → Andhra Pradesh; Kaveri → Tamil Nadu; Sundarbans → West Bengal (and Bangladesh).
• Amphan (2020): reached Super Cyclonic Storm level. Landfall between Digha (West Bengal) and Hatiya Islands (Bangladesh) [2].
• Biparjoy (2023): an Arabian Sea storm that hit Gujarat. It lasted 13 days 3 hours, among the longest-lived in the north Indian Ocean [3].
• Most cyclone-prone west coast state: Gujarat (Kachchh, Saurashtra).

Mains Points

• Spatial pattern (GS-I): the Bay of Bengal leads in cyclones for four reasons. Its water is warm, its surface is fresh and layered by river inflow, it receives leftover Pacific typhoons, and its shallow, funnel-shaped head drives high storm surges. This explains why Odisha, Andhra Pradesh, Tamil Nadu and West Bengal are the most exposed states, and why the Sundarbans and the Godavari, Krishna and Kaveri deltas suffer the most.
• Changing pattern (GS-I/III): the Arabian Sea is warming faster. It warmed 74% faster after the mid-1990s [5]. Strong, long-lived storms such as Tauktae (2021) and Biparjoy (2023) now hit Gujarat and the west coast [3][5]. Plans for Gujarat, Maharashtra and Kerala must now assume more cyclone risk than the historical record shows.
• Seasonal pattern (GS-I): cyclones come in two peaks, April–May and October–December, and are held back in the monsoon by wind shear. Post-monsoon cyclones are both a hazard and a resource. They bring most of Tamil Nadu's rain but damage delta farming.
• Disaster management (GS-III): because cyclones can be tracked days ahead, India's approach combines several parts:
• IMD forecasts;
• mass evacuation, as for Amphan: about 1 million people moved and more than 12,000 shelters used [2];
• NDRF rescue;
• embankments, and mangroves as a natural barrier.

Together these cut cyclone deaths from thousands in the past to tens or low hundreds today.