Indian Monsoon: Mechanism of Monsoon, ITCZ, Jet Streams and Drivers

Indian Monsoon: Meaning, Types and Monsoon Climate

What Is the Indian Monsoon?

The word monsoon comes from the Arabic mawsim, meaning season, and a monsoon climate is one associated with the seasonal reversal in the direction of winds. India has a hot monsoonal climate, the prevalent climate of South and South-east Asia, and this shared regime ties India to the rest of the region.

India has two monsoons, the south-west monsoon of summer and the north-east monsoon of winter, joined by a season of retreat:

• South-west monsoon: June to September; moist winds from the Indian Ocean bring the main rainy season to most of the country, reaching India in two branches, the Arabian Sea branch and the Bay of Bengal branch.
• Retreating monsoon: October and November; as the trough over the Ganga plain moves south, the south-west monsoon withdraws and the north-east winds set in, skies clear in the north, and the east of the Peninsula gets its rainiest months.
• North-east monsoon: The winter winds that blow from the land; in October and November, as the south-west monsoon retreats, they cross the Bay of Bengal, pick up moisture and cause torrential rain on the Tamil Nadu coast, southern Andhra Pradesh, south-east Karnataka and south-east Kerala.

The average annual rainfall of India is about 125 cm, but it varies greatly from place to place, from more than 1,000 cm in parts of the Khasi and Jaintia hills to below 50 cm in most of western Rajasthan. This regional variation of the Indian monsoon gives rise to the country's Köppen climatic regions.

Why India Has a Monsoon Type of Climate

India has a monsoon type of climate because three features work together. A large landmass heats quickly in summer; the warm Indian Ocean surrounds the peninsula on three sides and supplies moisture; and the Himalaya to the north shields the subcontinent from cold northern winds and traps the monsoon winds, forcing them to shed their moisture within the subcontinent.

The rain that results has a recognisable character:

• Seasonal: It falls between June and September, so the year has a wet summer and a long dry spell.
• Governed by relief: The windward side of the Western Ghats receives over 250 cm, and the north-eastern hills are among the wettest places in the country.
• Declining away from the sea: Along the northern plains, rainfall drops from 100 to 200 cm in Bihar to 50 to 100 cm in western Uttar Pradesh, Delhi, Haryana and Punjab, as the Bay of Bengal branch loses moisture on its way west.
• In spells: Rain comes in wet spells of a few days, separated by rainless breaks.
• Heavy downpours: Summer rain falls heavily, causing considerable runoff and soil erosion.
• Uncertain timing: The rains can begin late or end early, damaging standing crops and making the sowing of winter crops difficult.

Mechanism of Monsoon: Heating, ITCZ and Coriolis Force

Thermal Theory: Differential Heating of Land and Sea

The oldest explanation treats the monsoon as a giant sea breeze. Towards the end of the nineteenth century, the differential heating of land and sea in summer was believed to be the mechanism that sets the stage for the monsoon winds. In April and May the sun shines vertically over the Tropic of Cancer and the large landmass north of the Indian Ocean heats intensely, forming an intense low pressure over the north-western part of the subcontinent. Water heats up slowly, so pressure over the Indian Ocean stays high, and the low draws the south-east trades across the equator.

The low over the arid north-west is called the heat low. In June the ground there passes about 160 watts per square metre of heat into the air, and the heat low that forms is very shallow, reaching only up to the 850 hPa level, about 1.5 km.

• Strong heat low: When pressure in the heat low is below normal, it acts as a suction device for moist air along the monsoon trough, which goes with a good monsoon.
• Weak heat low: When pressure is above normal, rain is deficient or scanty over a vast area; in 1987 the central pressure of the heat low stayed mostly above normal, and the year was a drought year.

Dynamic Theory: ITCZ Shift and the Monsoon Trough

The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone near the equator where the trade winds of the two hemispheres converge, so it is a zone where air tends to rise. The dynamic theory explains the monsoon as the result of this belt moving north and south with the vertical sun. In July the ITCZ lies around 20° to 25° N over the Gangetic plain, where it is called the monsoon trough, and this trough encourages the thermal low over north and north-west India.

The monsoon trough is an elongated low running from the heat low over Pakistan to the head of the Bay of Bengal, over the Thar desert, Patna and the Chotanagpur plateau. It is a semi-permanent feature of the monsoon circulation, and its eastern end swings north and south. When it moves south of its normal position, the monsoon is active over most of India; when it moves north to the foothills of the Himalaya, most of India gets a break while the foothills get heavy rain and the Brahmaputra may flood. The trough can shift 5° to the north or south within 24 hours.

Coriolis Force, Cross-Equatorial Flow, Mascarene High and Somali Jet

A low pressure zone draws winds in from every side. When the ITCZ moves north, the south-east trade winds of the southern hemisphere are drawn across the equator between 40° and 60° E. Once across, the Coriolis force, the deflection caused by the Earth's rotation, turns them to the right, so they blow from the south-west to the north-east. The south-west monsoon is therefore a continuation of the south-east trades, deflected towards the subcontinent after crossing the equator, and passing over warm equatorial waters it brings moisture in abundance.

The Coriolis force has two properties that explain this turn:

• Latitude: The force increases with latitude, in proportion to the sine of the latitude; it is maximum at the poles and absent at the equator. It deflects winds to the right in the northern hemisphere and to the left in the southern hemisphere.
• Speed: The deflection is greater when the wind velocity is high.

Two features of the southern Indian Ocean drive this flow. The Mascarene High is a high pressure area around the Mascarene Islands during the monsoon season; it drives the cross-equatorial flow through the south Arabian Sea, and surges in its strength bring heavy rain to the west coast. The Somali jet is a low-level jet, 1 to 1.5 km above sea level, that originates near Mauritius and northern Madagascar, runs along the East African coast, reaches the west coast of India in June and attains its maximum strength in July. First documented by Findlater in 1969, it gives strong monsoons over the Peninsula when it strengthens.

Jet Streams, Tibetan Plateau and the Himalaya in the Monsoon Mechanism

Subtropical Westerly Jet in Winter and Its Northward Shift

In winter a subtropical westerly jet stream blows high in the upper troposphere. It flows as a single stream as far as Afghanistan, where the Himalayan massif splits it into two branches, one north and one south of the Himalaya, which rejoin over China. The southern branch lies over the North Indian Plain and steers the western disturbances from the Mediterranean into north-west India, where they bring the light winter rain that helps the rabi crops.

The shift of this jet in early summer is linked closely with the monsoon. Meteorologists have found an interrelationship between the northward shift of the ITCZ and the withdrawal of the westerly jet from over the North Indian Plain, and it is generally believed that the two are cause and effect. A timely northward shift of the subtropical westerly jet is critical to the onset; if the shift is delayed, so is the monsoon, and an early shift brings an early monsoon. The shift is not a slow, gradual change, which helps explain why the rains arrive suddenly; this account of the onset is known as the jet stream theory.

Tropical Easterly Jet and the Tibetan High

The tropical easterly jet sets in along 15° N only after the westerly jet has withdrawn from the region, and it is held responsible for the burst of the monsoon in India. Its source is the heated plateau. In summer a warm high-pressure system, the Tibetan High, forms over the Tibetan Plateau in the middle and upper troposphere, centred near 28° N, 98° E. The air flowing out of it concentrates into an easterly jet centred near the latitude of Chennai at the 150 hPa level in July, running from the east coast of Vietnam to the west coast of Africa.

• Steering the rain: The easterly jet steers tropical depressions into India, and the tracks of these depressions are the areas of highest rainfall.
• Strength of the season: A strong easterly jet over central India goes with a strong monsoon, and a weak jet with a weak one.
• Breaks and the end of the season: During break conditions the jet moves north to about 20° N, and it weakens to less than 50 knots over India in September.

Himalaya and Tibetan Plateau as Barrier and Heat Source

The Himalaya does more than make rain fall on its slopes. It confines the monsoon to the subcontinent. Without it, the south-west winds would blow right over India into Tibet, Afghanistan and Russia without causing much rain. The range also acts as an effective climatic divide, shielding the subcontinent from cold northern winds that originate near the Arctic Circle and blow across central and eastern Asia. Its rivers, the Indus, the Ganga and the Brahmaputra, brought down the alluvial deposits that form the northern plains.

The plateau behind the range is a heat engine. In summer it heats the air high in the atmosphere, creating the Tibetan High and with it the easterly jet. Over a much longer span, the strengthening of the Asian monsoon has been linked to the uplift of the Tibetan Plateau after India collided with Asia around 50 million years ago.

Onset, Burst, Branches, Breaks and Withdrawal of the Monsoon

Onset over Kerala: IMD Criteria and the Burst of the Monsoon

The monsoon reaches the Indian region well before it reaches Kerala. Initial monsoon rains fall in May over the south Andaman Sea, and the monsoon enters the subcontinent through the Andaman and Nicobar Islands around 20 May before setting in over the Kerala coast on 1 June, its normal date.

By early June the low over the north-western plains has grown powerful enough to pull in the southern trades, and the rains begin abruptly. This sudden onset of moisture-laden winds with violent thunder and lightning is the burst of the monsoon. It may come in the first week of June on the coasts of Kerala, Karnataka, Goa and Maharashtra, and in the first week of July in the interior; the day temperature falls by 5 °C to 8 °C between mid-June and mid-July.

The India Meteorological Department declares onset over Kerala by criteria adopted in 2016, designed to rule out a false, pre-monsoon onset:

1. Rainfall: After 10 May, if 60 per cent of 14 listed stations, from Minicoy and Thiruvananthapuram to Mangalore, report 2.5 mm or more for two consecutive days, onset is declared on the second day, provided the next two tests are met.
2. Wind field: The westerlies extend up to 600 hPa in the box from the equator to 10° N and 55° E to 80° E.
3. Clouds: Satellite-measured outgoing longwave radiation is below 200 watts per square metre over the south-east Arabian Sea, showing deep cloud.

Before the burst, the hot season produces its own local storms where moist air meets dry air at the edge of the trough:

• Mango showers: Pre-monsoon showers in Kerala and coastal Karnataka that help mangoes ripen early.
• Blossom showers: Showers that make coffee flowers blossom in Kerala and nearby areas.
• Nor'westers: Dreaded evening thunderstorms of Bengal and Assam. Bengal calls them Kalbaisakhi, "a calamity of the month of Baisakh", and Assam calls them Bardoisila; for all their damage, their rain helps the tea, jute and rice crops.
• Loo: The hot, dry wind of the northern plains from Punjab to Bihar, strongest between Delhi and Patna.

Arabian Sea Branch and Bay of Bengal Branch of the Monsoon

As the winds approach the land, relief and the thermal low over north-west India modify their direction, and the monsoon reaches the landmass in two branches.

The Bay of Bengal branch strikes the coast of Myanmar and south-east Bangladesh, but the Arakan Hills deflect a big portion of it towards the subcontinent. It therefore enters West Bengal and Bangladesh from the south and south-east instead of the south-west. The surface winds over north Bengal and Bihar are easterly or south-easterly, so in the Bhojpur region the rain-bearing wind arrives from the east and is called Purvaiya, the easterly; its arrival opens the kharif season. Because the farm calendar follows the rain, many of the region's festivals are tied to the seasons and the farming year.

The Tamil Nadu coast stays dry in this season for two reasons. It lies parallel to the Bay of Bengal branch, and it lies in the rain-shadow area of the Arabian Sea branch.

Monsoon Depressions, Active Spells and Breaks in the Monsoon

The rain of the season comes mainly from travelling low pressure systems. Monsoon lows and depressions, which form mostly in the Bay of Bengal and move westward across the country, are the principal rain-bearing systems of the south-west monsoon. On average two depressions form in each monsoon month; in July and August they usually form north of 18° N in the north-west Bay. Strong vertical wind shear in the monsoon months keeps them from intensifying into cyclonic storms, which is why cyclones are rare in July and August.

If rain fails for one or more weeks after a few rainy days, it is called a break in the monsoon. Breaks have different causes in different regions:

• Northern India: Rain fails when the rain-bearing storms along the monsoon trough over the region are not frequent.
• West coast: Dry spells come on days when the winds blow parallel to the coast instead of onto it.
• The trough shift: When the trough moves north to the Himalayan foothills, most of India gets a break while the foothills get heavy rain.

The opposite of a break is an active spell. A meteorological subdivision is declared active when its rainfall is one and a half to four times the normal. On the west coast, an offshore trough of low pressure that develops anywhere from north Kerala to south Gujarat strengthens the rain, and offshore vortices, eddies formed when the monsoon winds are deflected round the Western Ghats, bring heavy to very heavy rain to the coast.

Withdrawal of the Monsoon and New Normal Dates

The monsoon withdraws as the trough over the Ganga plain moves south in response to the southward march of the sun. The India Meteorological Department declares withdrawal from the extreme north-west by criteria adopted in 2006, considered only after 1 September. Rainfall must cease over the area for 5 continuous days, an anticyclone must be established in the lower troposphere at 850 hPa and below, and satellite water-vapour imagery must show a considerable fall in moisture. The monsoon withdraws from the southern Peninsula, and so from the whole country, around 15 October, when the winds change over from the south-westerly regime.

In 2020 the India Meteorological Department adopted new normal dates. The old ones came from 149 stations for 1901 to 1940; the new onset dates use 1961 to 2019 data and the new withdrawal dates 1971 to 2019. The monsoon now covers the entire country by 8 July, nearly a week earlier than the old 15 July, and starts withdrawing from the north-west around 17 September, more than two weeks later than the old 1 September.

Kochi has the monsoon from 1 June until the north-east monsoon takes over, while Bikaner has it only from early July to mid-September, so the season grows shorter from southern to northern India.

Monsoon Variability Drivers: ENSO, IOD, EQUINOO and MJO

El Niño, La Niña and the Southern Oscillation

From year to year, the variation of the south-west monsoon is governed mainly by the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). El Niño is a complex weather system that appears once every three to seven years, when a warm current appears off the coast of Peru in the eastern Pacific and raises the temperature of the water there by 10 °C. It distorts the equatorial atmospheric circulation, and it is used in India for forecasting long-range monsoon rainfall. In 1990‑91 an El Niño delayed the onset of the south-west monsoon over most of the country by five to twelve days.

The atmospheric half of ENSO was found in India. Sir Gilbert Walker, Director General of Observatories in India, noticed the see-saw of pressure between Tahiti and Darwin, the Southern Oscillation, and saw that the monsoon rain was often negligible in years of high pressure over Darwin. The monsoon tends to be weak, with droughts, in El Niño years, while La Niña years, the reverse condition, bring particularly strong monsoons. Of the 16 El Niño years since 1960, nine depressed India's monsoon rainfall, so El Niño raises the risk of a weak season without deciding it.

Indian Ocean Dipole and the Equatorial Indian Ocean Oscillation

ENSO does not explain every season. The strong El Niño of 1997 did not cause a drought in India. The reason, identified in 1999 by a team of Indian and Japanese scientists that included researchers at the Indian Institute of Science, is the Indian Ocean Dipole, an irregular oscillation in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part. The IOD develops in the equatorial Indian Ocean from April to May and peaks in October. A positive IOD often negates the effect of El Niño, giving more monsoon rain in years such as 1983, 1994 and 1997.

The atmospheric part of the dipole was found later, and the combined system was named the Equatorial Indian Ocean Oscillation (EQUINOO). Taking EQUINOO into account explains some failed forecasts, such as the acute drought of 2002. Other slower patterns, the North Atlantic Oscillation and the Pacific Decadal Oscillation, also contribute to the year-to-year swings.

Madden-Julian Oscillation and Intraseasonal Oscillations

Within a single season, the active and break spells follow oscillations of a few days to a few weeks. The most important is the Madden-Julian Oscillation (MJO), discovered by Madden and Julian in 1971. It is a planetary-scale pulse of deep convection and rain that moves slowly eastward over the Indian and Pacific Oceans at about 4 to 8 metres per second, with each cycle lasting about 30 to 60 days and divided into eight phases. When its wet phase lies over the Indian Ocean, the monsoon tends to be active; when its dry phase arrives, rain is suppressed.

Monsoon Forecasting: Long Period Average and the 2026 Season

Long Period Average and Monsoon Rainfall Categories

A forecast needs a yardstick, and India's is the Long Period Average (LPA), the average rainfall of a region for a month or season over a long period such as 30 or 50 years. The all-India normal for the south-west monsoon is now 868.6 mm, replacing the earlier 880.6 mm based on 1961 to 2010. Because the standard deviation is about 10 per cent of the mean, season rainfall for the country as a whole within 90 to 110 per cent of the LPA is called normal. Rain below 90 per cent of the LPA is called deficient; the India Meteorological Department does not use the word drought.

The India Meteorological Department has forecast the monsoon for more than a century. Its long-range forecast for all-India season rainfall was reintroduced in 1988 with a 16-parameter model, and new models followed in 2003 and 2007. Today it combines a Statistical Ensemble Forecasting System, built on eight predictors with strong physical links to the monsoon, with the Monsoon Mission Climate Forecast System, a coupled dynamical model adapted for India by the Indian Institute of Tropical Meteorology, Pune.

Monsoon of 2026: Late Onset, El Niño and an 87 Per Cent Season

The 2026 season showed how El Niño can weaken the monsoon. The monsoon set in over Kerala on 4 June, three days after its normal date, and covered the entire country on 9 July, a day later than its normal date. A weak El Niño formed in June and strengthened to moderate in July, and the season ended with 759.4 mm, 12.6 per cent below the 868.6 mm normal, or 87 per cent of the LPA, the lowest monsoon rainfall since 2015.

Sources

• NCERT: India: Physical Environment (Class XI), Climate (current edition)
• NCERT: India: Physical Environment (Class XI), Structure and Physiography
• NCERT: Fundamentals of Physical Geography (Class XI), Atmospheric Circulation and Weather Systems
• NCERT: Exploring Society: India and Beyond (Class VII), Climates of India
• India Meteorological Department (Pune): Frequently Asked Questions on Monsoon
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• Wikipedia: Somali Jet
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• Wikipedia: Coriolis force
• UPSC: Civil Services Examination question papers