Cyclone Season in India: Why Cyclones Peak in May and November

Overview

The cyclone season in India is the part of the year when tropical cyclones form over the Bay of Bengal and the Arabian Sea and strike the Indian coast. It comes in two parts, set by the monsoon. The pre-monsoon season runs from March to May and the post-monsoon season from October to December, and the monthly number of cyclonic storms shows two maxima, in May and in November. November is the primary peak and May the secondary one.

Cyclone Season in India and the Monsoon Calendar

Two Peaks in the Monthly Frequency of Cyclones

The months of the southwest monsoon, June to September, lie between the two cyclone seasons. June, when the monsoon sets in, can still bring severe storms, but from July to September the North Indian Ocean produces many depressions and few cyclones, and those few are generally not intense. The cause of this pattern is the monsoon's own wind structure, which shears young storms apart in July and August and releases its hold once the monsoon withdraws.

The North Indian Ocean forms about five cyclones a year, four in the Bay of Bengal and one in the Arabian Sea. In the long record from 1891 to 2019 the basin averaged about 12.4 cyclonic disturbances (depressions and stronger systems) and 5.2 cyclonic storms a year. More than 80 per cent of all cyclonic disturbances occur from May to November, and the largest number of them forms in August. Cyclonic storms follow a different curve, with two maxima, in May and November. The August maximum is therefore made up mostly of depressions, while the May and November maxima are of systems that reach cyclone strength.

The strongest storms come around the two peaks. May and June, and October and November, produce cyclones of severe intensity, while cyclones that develop in July to September are generally not so intense.

Table 1. The four seasons of the cyclone year in the North Indian Ocean
Season Months Main weather systems Cyclone activity
Winter season January and February Northeasterly winds over the Bay of Bengal; western disturbances over north-west India Few cyclones form
Pre-monsoon season March to May Rising heat before the monsoon Depressions most likely to intensify in April; secondary peak and severe storms in May
Southwest monsoon season June to September Monsoon onset in June; monsoon trough, monsoon lows and monsoon depressions Severe storms possible in June as the monsoon sets in; many depressions and few cyclones in July to September, which are generally not intense
Post-monsoon season October to December Retreating southwest monsoon and the northeast monsoon over the south Peninsula Primary peak in November; most landfalls on the east coast
Timeline of the twelve months. IMD seasons: Winter in January and February, pre-monsoon season March to May, southwest monsoon season June to September, post-monsoon season October to December. Normal monsoon dates: Kerala onset 1 June, covers India 8 July, withdrawal begins 17 September, withdrawn from most of India by 15 October. The northeast monsoon spans October to December. Secondary peak of cyclones in May, primary peak in November; in June, the onset phase, severe storms are possible; July to September has high wind shear and few cyclones.

Vertical Wind Shear and the Quiet Monsoon Months

Vertical wind shear is the change in wind speed and direction with height. A tropical cyclone can form only where this shear is low, less than about 10 metres per second (20 knots) between about 1.5 km and 12 km above the surface, along with a warm sea of at least 26.5 °C, moist middle layers, enough Coriolis force and a starting disturbance. Large shear disturbs the vertical transport of latent heat and the organisation of thunderstorms around the centre, so it prevents genesis; it can also weaken or destroy a cyclone that has already formed.

The southwest monsoon creates exactly the high shear that cyclones cannot survive. In July and August the surface winds south of the monsoon trough are westerly or south-westerly, and they reach their greatest speed, 20 to 25 knots, at about 900 to 800 hPa. Higher up, easterly winds strengthen with height above 200 hPa and are strongest at about 150 to 100 hPa, blowing at 60 to 80 knots over the Peninsula. The lower troposphere thus blows from the west and the upper troposphere from the east, and the difference between them is far larger than a young cyclone can withstand.

Two cross-sections on a pressure axis from 1000 to 100 hPa. Left, July, the monsoon: Westerlies of 20 to 25 knots near the surface and the Tropical Easterly Jet of 60 to 80 knots near 150 to 100 hPa blow in opposite directions; the large shear tears a cloud column apart. Right, May or November, the transition: Light winds at all levels and shear under 10 metres per second; the column stays upright around an eye.

Jet streams, fast-flowing, narrow air currents near the altitude of the tropopause, blow in both hemispheres; each hemisphere has a polar jet, and the southern one circles Antarctica. The upper easterlies of the summer monsoon, strongest at about 150 to 100 hPa, are concentrated into one such jet, the Tropical Easterly Jet, which lies near the latitude of Chennai in July.

It runs from the east coast of Vietnam to the west coast of Africa and is fed by the outflow of the Tibetan High, a warm anticyclone over the Tibetan Plateau in the middle and upper troposphere. The jet sets in along 15° N only after the westerly jet stream has withdrawn from its position over the north Indian plain, south of the Himalayas, and it is held responsible for the burst of the monsoon.

The jet that comes with the monsoon rains therefore also keeps cyclones away. It weakens to less than 50 knots over India in September, and as it weakens and the monsoon withdraws, the shear over the seas falls.

Pre-Monsoon Cyclone Season from March to May

Why the May Peak Comes Before Monsoon Onset

The pre-monsoon season, from March to May, is the part of the year when a depression in the North Indian Ocean is most likely to grow into a cyclone. The probability that a depression becomes a cyclone is highest in April, followed by March, May and November, and the probability that it becomes a severe cyclonic storm is highest in May, followed by April and November. More than half of the cyclonic disturbances that form in March, April, May, November and December intensify into storms.

Warm water and low shear lie behind this. Cyclone Remal showed the combination when high sea surface temperatures over the northern Bay of Bengal and low wind shear let it intensify on 26 May 2024, before it made landfall over Bangladesh and adjoining West Bengal that night.

May is the busiest pre-monsoon month. From 1961 to 2024 the North Indian Ocean formed 73 cyclonic disturbances in May, 52 in the Bay of Bengal and 21 in the Arabian Sea. Of these, 55 became cyclonic storms (39 in the Bay and 16 in the Arabian Sea), and 43 crossed a coast as cyclones.

Most of the Bay's May cyclones crossed the coasts of Bangladesh (18) and Myanmar (seven), against four for Andhra Pradesh and three each for West Bengal and Odisha. Pre-monsoon storms in the Bay are more likely to recurve towards the north-east, which carries many of them towards Bangladesh and Myanmar. Cyclone Mocha, for example, made landfall just north of Sittwe in Myanmar in May 2023.

Grouped bar chart of the May climatology from 1961 to 2024. Cyclonic disturbances formed: Bay of Bengal 52, Arabian Sea 21. Cyclonic storms formed: Bay of Bengal 39, Arabian Sea 16. Crossed the coast as cyclones: Bay of Bengal 36, Arabian Sea 7.

Pre-monsoon storms are also smaller. The average diameter of a pre-monsoon storm is about 800 km, against about 1,200 km for a post-monsoon storm, although intensity does not depend on size.

The pre-monsoon season ends when the monsoon sets in over Kerala, normally around 1 June, and the shear builds again. Severe storms can still form in June while the Tropical Easterly Jet is setting in.

Southwest Monsoon Season: Onset Cyclones, Lows and Depressions

Onset Vortex, Somali Jet and June Cyclones in the Arabian Sea

The first weeks of the southwest monsoon season have their own pattern over the Arabian Sea, tied to the arrival of the monsoon over Kerala. The Somali jet is a low-level jet, about 1 to 1.5 km above sea level, that carries air across the equator along the east coast of Africa. It originates near Mauritius and northern Madagascar, moves in May into eastern Africa and then over the Arabian Sea, and reaches the west coast of India in June, attaining its maximum strength in July.

As this monsoon current arrives, a cyclonic vortex can develop at its leading edge over the south-east or east-central Arabian Sea. This onset vortex sometimes grows into a cyclone, and other storms form over the Arabian Sea in the same weeks.

  • Ashobaa, 2015: The monsoon set in over Kerala on 5 June 2015, and the onset vortex over the east-central Arabian Sea intensified into Cyclonic Storm Ashobaa, which lasted from 7 to 12 June.
  • Vayu, 2019: The monsoon set in over Kerala on 8 June 2019, about seven days late. The onset vortex off the west coast then developed into Very Severe Cyclonic Storm Vayu (10 to 17 June), and the monsoon's further advance remained sluggish until Vayu dissipated.
  • Biparjoy, 2023: This extremely severe cyclonic storm formed over the Arabian Sea on 6 June 2023, two days before the monsoon set in over Kerala on 8 June. It crossed the Saurashtra and Kutch coast close to Jakhau Port between 2230 and 2330 hours IST on 15 June as a very severe cyclonic storm, and it brought extremely heavy rainfall to Rajasthan in June.

Vayu showed how such a storm can hold back the monsoon. It drew clouds and moisture over the Arabian Sea away from the developing monsoon trough, and this delayed the monsoon's progress towards northern India by a further week.

Monsoon Trough and Monsoon Depressions in July and August

The monsoon trough is an elongated low-pressure area that extends from the heat low over Pakistan to the head of the Bay of Bengal. It is the summer position of the Inter Tropical Convergence Zone (ITCZ), which lies at about 20° N to 25° N over the Gangetic plain in July. An area of low pressure that forms within the monsoon circulation is a monsoon low, and one that intensifies, with winds of 17 to 33 knots, is a monsoon depression. Monsoon lows and depressions are the principal rain-bearing systems of the southwest monsoon, and the westward passage of depressions forming in the Bay of Bengal generates substantial amounts of rainfall.

The normal frequency is about six monsoon depressions a season, although the number varies widely from year to year. In July and August they usually form north of 18° N in the north-west Bay, and in September the site of genesis shifts southward into the central Bay. Those that form in early June help the monsoon advance and are not strictly monsoon depressions. The 2026 southwest monsoon season (June to September) had 14 low-pressure systems, six of them monsoon depressions. Monsoon depressions over the east coast of India have shown a significant decreasing trend in recent decades, while monsoon lows appear to have become more frequent and longer-lasting.

Two features keep these systems below cyclone strength. The high shear of the monsoon caps their growth, and their place of birth gives them little time over water. A depression that forms in the north Bay crosses the Odisha-West Bengal coast in a day or two, so it spends little time over the sea.

The position of the trough also governs the rain. When the trough moves south of its normal position, the monsoon is active over most of India; when it shifts north towards the Himalayan foothills, a break in the monsoon follows, with heavy rain along the foothills.

Monsoon Depression and Cyclone Compared

A monsoon depression and a tropical cyclone are both low-pressure systems with anticlockwise winds in the Northern Hemisphere, but they are built differently. A monsoon depression has a cold core near the surface and in the lower levels and a warm core aloft. It is roughly elliptical and 6 to 9 km deep, and its strongest winds blow at 0.9 to 1.5 km. It tilts southward with height, and when it moves westward its heaviest rain falls in its south-west quadrant. Light and variable winds cover a large area near its centre.

A cyclonic storm is a warm-core system, with the greatest warming at about 300 hPa. A cyclone that reaches very severe cyclonic storm strength (64 knots) usually shows an eye, and the eye can be 10 °C or more warmer than its surroundings at about 12 km, because the air in it sinks slowly and warms by compression.

Table 2. Monsoon depression and tropical cyclone compared
Feature Monsoon depression Tropical cyclone (cyclonic storm and above)
Season June to September, along the monsoon trough Mainly May and October to December, with severe storms also in June
Place of genesis North-west Bay of Bengal, north of 18° N, in July and August Lower latitudes with the ITCZ; south-east Bay of Bengal, Andaman Sea and south-east Arabian Sea
Wind speed 17 to 33 knots 34 knots and above
Thermal structure Cold core in the lower levels, warm core aloft Warm core, greatest warming at about 300 hPa
Centre Light and variable winds over a large area An eye, usually seen once the storm reaches very severe cyclonic storm strength
Main effect Most of the monsoon rain over India Destructive winds, storm surge and very heavy coastal rain

Post-Monsoon Cyclone Season: October to December

How Post-Monsoon Cyclones Originate in the Bay of Bengal

Post-monsoon cyclones originate as the monsoon withdraws and the shear that suppressed them in July and August disappears. The southwest monsoon normally begins to withdraw from north-west India on 17 September and withdraws from most parts of the country, except the south Peninsula and some parts of neighbouring central India, by 15 October; the northeast monsoon then gets established over the south Peninsula.

By the beginning of October the monsoon trough of low pressure, which lay over the Gangetic plain in July, has shifted to the northern Bay of Bengal, and by early November it lies over Karnataka and Tamil Nadu. As it retreats south, the zone of genesis returns to lower latitudes, where cyclones form along the ITCZ, as they do in the pre-monsoon season.

Bay of Bengal cyclones, in any season, have two kinds of origin. Many develop in situ over the south-east Bay and the adjoining Andaman Sea, often from an easterly wave, a westward-moving trough in the trade winds that supplies the seedling disturbance. In October and November the cyclonic depressions that originate over the Andaman Sea cross the east coast of the southern Peninsula. Other Bay cyclones are the remnants of typhoons that move across the South China Sea from the north-west Pacific. That basin is the most active on the planet, with about one third of all tropical cyclone activity, so the Bay receives a steady supply of such remnants.

The post-monsoon season is the main cyclone season of the Bay. From 1965 to 2020 the frequency of genesis and of landfall of every category of cyclone over the Bay was about three to four times higher after the monsoon than before it. Landfall moves south as the season advances. Bay of Bengal cyclones most often strike the Odisha-West Bengal coast in October, the Andhra Pradesh coast in November and the Tamil Nadu coast in December. The most intense cyclone in the recorded history of the basin, the Odisha super cyclone, crossed the coast near Paradip on 29 October 1999, and Cyclone Montha made landfall near Narasapuram in Andhra Pradesh in late October 2025.

Cyclones and Northeast Monsoon Rainfall in Tamil Nadu

The post-monsoon season is also the rain season of south-east India. The Coromandel coast of Tamil Nadu stays largely dry during the southwest monsoon, because it lies parallel to the Bay of Bengal branch of the monsoon and in the rain-shadow of the Arabian Sea branch. Its rain comes later. During October and November the northeast monsoon picks up moisture while crossing the Bay of Bengal and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, south-east Karnataka and south-east Kerala. In 2026 the southwest monsoon withdrew from the entire country on 15 October, and northeast monsoon rainfall commenced the same day over Tamil Nadu, Puducherry, coastal Andhra Pradesh, Rayalaseema, south interior Karnataka and Kerala.

Much of this rain arrives with the Andaman Sea depressions as they cross the coast, and the bulk of the rainfall of the Coromandel coast comes from depressions and cyclones. Tamil Nadu receives about 48 per cent of its annual rainfall in the October to December season, and the south Peninsula as a whole about 30 per cent. The same systems are very destructive, and the thickly populated deltas of the Godavari, Krishna and Kaveri are their preferred targets.

  • Gaja, 2018: It made landfall between Nagapattinam and Vedaranyam in Tamil Nadu late on 15 November 2018.
  • Michaung, 2023: It brought heavy rain to north-eastern Tamil Nadu, including Chennai, before making landfall near Bapatla in Andhra Pradesh on 5 December 2023.
  • Fengal, 2024: It made landfall near Puducherry on the evening of 30 November 2024.

Bay of Bengal and Arabian Sea Cyclone Seasons Compared

The Bay of Bengal and the Arabian Sea have the same two cyclone seasons but differ in how many cyclones each forms and when. The Bay of Bengal forms about four cyclones for every one in the Arabian Sea. The Arabian Sea is relatively colder than the Bay, which inhibits the formation and intensification of storms, and most Bay cyclones weaken over land after landfall, so few cross the Peninsula into the Arabian Sea.

The Bay is busiest after the monsoon, while the Arabian Sea's two seasons are more evenly matched; from 1965 to 2020 its landfall frequency was almost the same before and after the monsoon. Arabian Sea storms that move north of about 15° N tend to recurve, which leads to more landfalls on the Gujarat coast, the Indian coast most prone to Arabian Sea cyclones. In both seasons the Arabian Sea's landfalls are most frequent on the combined coasts of Iran, Arabia and Africa, followed by Saurashtra and Kutch in Gujarat.

The Bay of Bengal shares its cyclone-prone character with two other warm seas of the tropics, the South China Sea and the Gulf of Mexico, and the same conditions explain all three.

  • Warm water: Their surface water stays at 26.5 °C or more through a sufficient depth, and enclosed seas in the low latitudes record higher temperatures than the open seas.
  • Coriolis force: They lie more than about 500 km from the equator, where the Coriolis force is strong enough to keep a low-pressure system spinning.
  • Seedling disturbances: They receive a regular supply of disturbances, such as easterly waves, from which cyclones can grow.
  • Moist air and low shear: A disturbance deepens where the middle troposphere is moist, the air diverges aloft and the vertical wind shear is low, as it is over the Bay in May and in October and November.

Two of the three seas lie in large, active ocean basins. The South China Sea is part of the north-west Pacific, the most active basin on the planet, and the Gulf of Mexico is part of the North Atlantic basin, where most tropical storms and hurricanes form between 1 June and 30 November.

The balance between the seas changes from year to year. In 2019 the Arabian Sea formed five cyclones and the Bay of Bengal three. Over the long term the annual frequency of tropical cyclones over the North Indian Ocean has fallen significantly since the middle of the twentieth century (1951 to 2018), while the number of very severe cyclonic storms in the post-monsoon season rose significantly between 2000 and 2018. Over the Arabian Sea the frequency of extremely severe cyclonic storms shows an increasing trend.

Table 3. Bay of Bengal and Arabian Sea cyclone seasons compared
Feature Bay of Bengal Arabian Sea
Share of North Indian Ocean cyclones About four in five About one in five
Main cyclone season Post-monsoon, October to December Both seasons, with landfalls almost equally frequent
Systems in the monsoon months Monsoon depressions in the north-west Bay Fewer systems; onset-phase cyclones in June
Usual landfall in the pre-monsoon season Bangladesh and Myanmar Iran, Arabia and Africa, then Saurashtra and Kutch
Usual landfall in the post-monsoon season Odisha-West Bengal in October, Andhra Pradesh in November, Tamil Nadu in December Iran, Arabia and Africa, then Saurashtra and Kutch
Cyclones crossing a coast in May, 1961‑2024 36 7
Map of the North Indian Ocean with India, Sri Lanka, Bangladesh and Myanmar, the Arabian Sea, Bay of Bengal and Andaman Sea. Dashed zones of genesis in both cyclone seasons over the south-east Bay of Bengal and Andaman Sea and the south-east Arabian Sea; a zone of monsoon depressions over the north-west Bay. An arrow shows Arabian Sea storms recurving north of 15 degrees north to the Gujarat coast. Numbered landfalls: 1 Biparjoy 2023 at Jakhau Port, an onset-phase cyclone of June; 2 Mocha 2023 at Sittwe and 3 Remal 2024 on the Bangladesh coast, pre-monsoon cyclones; 4 Gaja 2018 at Vedaranyam, 5 Michaung 2023 at Bapatla, 6 Fengal 2024 at Puducherry and 7 Montha 2025 at Narasapuram, post-monsoon cyclones. East coast landfall by month: October Odisha-West Bengal, November Andhra Pradesh, December Tamil Nadu.

Previous Year UPSC‑CSE Questions

Each Prelims question is explained option by option, and each Mains question comes with an answer outline.

  1. UPSC Prelims 2020Consider the following statements:
    1. Jet streams occur in the Northern Hemisphere only.
    2. Only some cyclones develop an eye.
    3. The temperature inside the eye of a cyclone is nearly 10°C lesser than that of the surroundings.

    Which of the statements given above is/are correct?

    1. a 1 only
    2. b 2 and 3 only
    3. c 2 only
    4. d 1 and 3 only
    How to approach this Prelims question

    Question type: Multiple statements

    Approach: Test each statement against how jet streams and the eye of a cyclone work, then match the true statements to the codes.

    • Statement 1: Jet streams blow in both hemispheres; each hemisphere has a polar jet, and the southern one circles Antarctica, so the statement is wrong.
    • Statement 2: An eye usually appears only once a storm reaches very severe cyclonic storm strength (64 knots); before that, the dense cloud over its centre usually shows no eye, so the statement is correct.
    • Statement 3: The eye has the warmest temperatures aloft and can be 10 °C or more warmer than its surroundings at about 12 km, because its slowly sinking air warms by compression; the statement reverses this and is wrong.

    Only statement 2 is correct, which is code (c).

    Trap to watch: Students who remember the figure of 10 °C pick a code containing statement 3, missing that the eye can be 10 °C or more warmer, not cooler, than its surroundings.

    Why each option is right or wrong:

    • (a) 1 only: Wrong. Jet streams are not confined to the Northern Hemisphere; the Southern Hemisphere has its own polar jet around Antarctica.
    • (b) 2 and 3 only: Wrong. Statement 2 is correct, but the eye is warmer than its surroundings, so statement 3 is wrong.
    • (c) 2 only: Correct. Only some cyclones develop an eye, and the other two statements are false.
    • (d) 1 and 3 only: Wrong. Both statements are false, because jet streams occur in both hemispheres and the eye is warmer than its surroundings.

    Answer: (c) 2 only. An eye usually forms only in a cyclone of very severe strength, and that eye is warmer, not cooler, than the air around it.

  2. UPSC Mains 2014 GS‑ITropical cyclones are largely confined to South China Sea, Bay of Bengal and Gulf of Mexico. Why?
    Answer outline

    The outline sets out what the question asks, the main arguments, relevant examples and the direction of the conclusion; it is not a full model answer.

    Directive verb: None (a "why" question asking for causes)

    Approach: Name the conditions a tropical cyclone needs, then show how each of the three seas meets them.

    Introduction: Tropical cyclones form only over warm tropical seas where several conditions meet at once, and the South China Sea, the Bay of Bengal and the Gulf of Mexico are among the seas where these conditions are met most often.

    Body (sub-themes to develop):

    • Warm water: The sea surface stays at 26.5 °C or more through a sufficient depth, and enclosed seas in the low latitudes are warmer than the open seas.
    • Coriolis force: All three seas lie more than about 500 km from the equator, where the Coriolis force can keep a low-pressure system spinning.
    • Seedling disturbances: Easterly waves supply the starting lows, and remnants of typhoons from the north-west Pacific, the most active basin, cross into the Bay of Bengal.
    • Favourable air above: A moist middle troposphere and divergence aloft let a disturbance deepen into a cyclone.
    • Low shear and season: Over the Bay most cyclones form in May and in October and November, when the shear is low; most Gulf of Mexico storms form between 1 June and 30 November.

    Conclusion: These seas combine warm water, adequate Coriolis force, frequent disturbances and seasons of low shear, which is why cyclones are far more frequent over them than over colder or equatorial waters.

Sources

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This article draws on the India Meteorological Department's own publications, the current NCERT textbooks and the other sources listed in the Sources section.

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