Monsoons of India Explained: SW and NE Monsoon Mechanism, MJO and Retreat for UPSC
Quick Answer: The Indian monsoon is a seasonal reversal of winds caused by differential heating of land and sea and the northward shift of the Inter-Tropical Convergence Zone (ITCZ). The Southwest monsoon (June–September) brings about 75% of India’s annual rainfall; the Northeast or retreating monsoon (October–December) brings rain mainly to Tamil Nadu and the east coast.
- What Is the Indian Monsoon? — Direct Answer
- Mechanism of the Indian Monsoon: The Basic Drivers
- Inter-Tropical Convergence Zone (ITCZ) and Its Seasonal Shift
- Role of Jet Streams: Subtropical Westerly and Tropical Easterly Jets
- Onset of the Southwest Monsoon
- Southwest Monsoon Rainfall: Breaks and Inter-Tropical Variability
- El Niño, La Niña and ENSO-Monsoon Link
- Retreat and Withdrawal of the Southwest Monsoon
- Northeast (Retreating) Monsoon Mechanism
- Classification and Seasons: India’s Four-Monsoon Cycle
- Prelims and Mains Practice: Key Facts, PYQ Patterns, Diagrams
- Frequently Asked Questions
- Q: When does the Southwest monsoon normally arrive over Kerala and withdraw from India?
- Q: What is the difference between Southwest and Northeast monsoon?
- Q: How does El Niño affect the Indian monsoon?
- Q: What are ‘breaks’ in the monsoon?
- Q: Which states receive rainfall from the Northeast monsoon?
- Related reading
What Is the Indian Monsoon? — Direct Answer
The word monsoon derives from the Arabic mausim, meaning season. For Indian monsoon mechanism UPSC preparation, the standard definition is: a complete reversal of wind direction between summer and winter, driven by the different rates at which land and sea heat up, and by the seasonal migration of the ITCZ. In summer, winds blow from sea to land (Southwest monsoon); in winter, they reverse and blow from land to sea (Northeast monsoon). The India Meteorological Department (IMD) is the official agency that declares onset and withdrawal dates each year.
Mechanism of the Indian Monsoon: The Basic Drivers
Three basic drivers explain the monsoon in plain English:
- Differential heating: Land heats faster than ocean. In summer, the Indian subcontinent and especially the Tibetan Plateau heat up intensely, creating a low-pressure zone over north India, while seas remain relatively cool and high pressure persists over the oceans.
- Pressure gradients: Two pressure systems dominate — the Tibetan (seasonal) low over the Tibetan Plateau and north India, and the Mascarene High, a semi-permanent high-pressure cell near the Mascarene Islands in the southern Indian Ocean. The pressure difference between these two drives moisture-laden winds from the Indian Ocean towards India — the “engine” of the Southwest monsoon. The strength of the Mascarene High is a key determinant of monsoon strength.
- Coriolis effect: As winds move from the southern to the northern hemisphere, the Coriolis force deflects them — south-easterlies curve to become south-westerlies, giving the Southwest monsoon its name.
Inter-Tropical Convergence Zone (ITCZ) and Its Seasonal Shift
The ITCZ is a low-pressure belt near the equator where the trade winds of both hemispheres converge, forcing air to rise and producing heavy convectional rainfall. Normally located around 5°N, the ITCZ shifts north with the apparent movement of the sun. During the Indian summer, it migrates unusually far north — to about 20–25°N over the Indian subcontinent — because the Himalayan wall blocks its further movement and the Tibetan Plateau intensifies heating.
This northward shift of the ITCZ over India is effectively the monsoon trough, and its arrival is what triggers the burst of the Southwest monsoon. In winter, the ITCZ shifts south of the equator, and winds over India reverse to blow from the northeast.
Role of Jet Streams: Subtropical Westerly and Tropical Easterly Jets
- Subtropical Westerly Jet (STWJ): In winter, this jet flows south of the Himalayas, keeping the ITCZ and monsoon systems away from India and aiding western disturbances. In early summer, as the Tibetan Plateau heats up, the STWJ retreats north of the Tibetan Plateau — a key precondition for monsoon onset. Its northward jump typically occurs in stages during May–June.
- Tropical Easterly Jet (TEJ): Formed in summer due to heating of the Tibetan Plateau (upper-level high pressure aloft), it flows from east to west at about 14–15°N over peninsular India, at around 14 km altitude. Its establishment is closely linked with the burst of the monsoon over Kerala and the strengthening of monsoon circulation.
Onset of the Southwest Monsoon
The normal date of onset over Kerala is around 1 June. The onset then progresses in two branches — the Arabian Sea branch moving up the west coast, and the Bay of Bengal branch entering the Northeast and moving across the Gangetic plains — covering the whole country by around mid-July (normal date of full coverage: ~15 July).
A critical component is the Somali jet (or Findlater jet): a strong low-level cross-equatorial jet off the Somali coast (Kenya–Somalia) that funnels moisture from the southern Indian Ocean into the Arabian Sea. The accelerated evaporation over the Arabian Sea supplies the moisture for the heavy “burst” of rainfall along the Western Ghats at onset.
Southwest Monsoon Rainfall: Breaks and Inter-Tropical Variability
Monsoon rainfall is not continuous. An “active” spell brings heavy rain over the monsoon trough zone, while a “break” monsoon means rainfall weakens or stops over central India for days to weeks — because the monsoon trough shifts north to the Himalayan foothills, and heavy rain occurs there instead.
The Madden-Julian Oscillation (MJO) is the dominant driver of this intra-seasonal variability. The MJO is an eastward-moving pulse of cloud and rainfall along the tropics, circling the globe in roughly 30–60 days. When the active (wet) phase of the MJO lies over the Indian longitudes, monsoon rainfall strengthens; when the suppressed (dry) phase is over India, breaks occur. MJO phase tracking is used by IMD and global agencies for extended-range monsoon forecasts.
El Niño, La Niña and ENSO-Monsoon Link
- El Niño: Abnormal warming of the central and eastern equatorial Pacific. It shifts convection eastward, weakens the Walker circulation, and weakens the Indian monsoon — historically linked with drought years (e.g., 2002, 2009, 2015). Roughly 60% of El Niño years have seen deficient Indian monsoon rainfall.
- La Niña: Abnormal cooling of the same Pacific region — strengthens the Walker circulation and typically strengthens the Indian monsoon, often bringing above-normal rainfall (e.g., strong monsoon years such as 2020 and 2021 occurred during La Niña).
- Indian Ocean Dipole (IOD): A modifier. A positive IOD (warmer western Indian Ocean, cooler east) enhances monsoon rainfall and can offset El Niño (as in 1997 and 2019); a negative IOD suppresses it.
Authoritative data on ENSO and IOD can be checked at the NOAA Climate.gov portal and IMD (mausam.imd.gov.in).
Retreat and Withdrawal of the Southwest Monsoon
Withdrawal begins from extreme northwestern India around 17 September on the normal date schedule, progressing southwestward — it withdraws from most of northwest India by end-September, from the Gangetic plains and peninsula through October, and from the extreme southern peninsula by around mid-December. The retreat occurs as the ITCZ shifts southwards and the subtropical westerly jet re-establishes itself south of the Himalayas, reversing the pressure pattern. Retreating monsoon winds picking up moisture over the Bay of Bengal cause the rains of October–December.
Northeast (Retreating) Monsoon Mechanism
In winter, the pressure belts reverse: high pressure builds over cold land, low pressure over warmer seas. Winds blow from land to sea as north-easterlies. They are generally dry over most of India — but while crossing the Bay of Bengal, they pick up moisture and strike the Coromandel Coast (Tamil Nadu), causing rainfall from October to December.
This answers the classic exam question — why does Tamil Nadu get winter rain? Two reasons: (i) the retreating NE winds gain moisture over the Bay of Bengal; and (ii) Tamil Nadu’s eastern coastal location lies in their path, whereas the Western Ghats block the Southwest monsoon’s Arabian Sea branch from reaching interior Tamil Nadu during June–September. Hence Tamil Nadu receives most of its rainfall (about 48%) in the NE monsoon season.
Classification and Seasons: India’s Four-Monsoon Cycle
The IMD classifies the Indian year into four seasons:
| Season | Period | Key Feature |
|---|---|---|
| Pre-monsoon / Summer | March–May | “Mango showers” in Kerala/Karnataka; “Kaal Baisakhi” in Bengal; “Loo” in north India |
| Southwest Monsoon | June–September | ~75% of annual rainfall; onset over Kerala ~1 June |
| Post-monsoon / Retreating | October–December | NE monsoon; Tamil Nadu rains; tropical cyclones in Bay of Bengal |
| Winter / Cold season | January–February (Dec–Feb) | Western disturbances; NE trade winds |
Prelims and Mains Practice: Key Facts, PYQ Patterns, Diagrams
- Prelims trends: PYQs repeatedly test ITCZ position, jet stream behaviour, El Niño effects, and NE monsoon rainfall states. Expect statement-based questions on the Somali jet and MJO.
- Mains (GS-I): “Explain the mechanism of the Indian monsoon” style questions need structured answers — differential heating → ITCZ shift → jet streams → onset → withdrawal.
- Essential diagrams: (i) seasonal shift of the ITCZ over India, (ii) Somali jet and cross-equatorial flow, (iii) El Niño vs normal Walker circulation, (iv) monsoon withdrawal isochrone map.
- High-yield one-liners: Kerala onset ~1 June; withdrawal from NW India ~17 September; ITCZ reaches 20–25°N in July; TEJ forms due to Tibetan heating; positive IOD favours good monsoon.
Frequently Asked Questions
Q: When does the Southwest monsoon normally arrive over Kerala and withdraw from India?
The normal onset date over Kerala is around 1 June (with an IMD-declared model error of ±4 days). Withdrawal begins from extreme northwestern India around 17 September and completes over the southernmost peninsula by about mid-December.
Q: What is the difference between Southwest and Northeast monsoon?
The Southwest monsoon (June–September) brings most of India’s rainfall via moisture-laden winds from the Arabian Sea and Bay of Bengal, crossing the country from sea to land. The Northeast monsoon (October–December) is the retreating phase — dry continental north-easterlies that pick up moisture over the Bay of Bengal and bring rain mainly to Tamil Nadu and the east coast.
Q: How does El Niño affect the Indian monsoon?
El Niño warms the central-eastern Pacific, shifting convection eastward and weakening the Walker circulation. This weakens the Indian monsoon circulation and typically reduces monsoon rainfall, with El Niño years historically associated with droughts.
Q: What are ‘breaks’ in the monsoon?
Breaks are periods of markedly weak or absent rainfall over central India when the monsoon trough shifts north to the Himalayan foothills. They are often linked to the suppressed phase of the MJO and can last several days to weeks.
Q: Which states receive rainfall from the Northeast monsoon?
Tamil Nadu, coastal Andhra Pradesh, Rayalaseema, Kerala, south interior Karnataka, and Puducherry receive October–December rainfall from the Northeast monsoon — with Tamil Nadu receiving the largest share of its annual rainfall in this season.
Related reading
- Monsoons of India: Origin, Mawsynram Factor and Retreating Season for SSC & UPSC
- Thursday Teaser Quiz: 12 Geography and Environment MCQs from This Week's Posts
Quick revision
- Differential heating: Land heats faster than ocean.
- Pressure gradients: Two pressure systems dominate — the Tibetan (seasonal) low over the Tibetan Plateau and north India, and the Mascarene High, a semi-permanent…
- Coriolis effect: As winds move from the southern to the northern hemisphere, the Coriolis force deflects them — south-easterlies curve to become south-westerlies,…
- Subtropical Westerly Jet (STWJ): In winter, this jet flows south of the Himalayas, keeping the ITCZ and monsoon systems away from India and aiding western disturbances.
- Tropical Easterly Jet (TEJ): Formed in summer due to heating of the Tibetan Plateau (upper-level high pressure aloft), it flows from east to west at about 14–15°N over…
- El Niño: Abnormal warming of the central and eastern equatorial Pacific.
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