India Monsoons: Origin, Mawsynram Rainfall and Retreating Season Explained
Quick Answer: The monsoons of India originate from differential heating of land and sea, the northward shift of the ITCZ, and jet stream reversal, driving moisture-laden southwest winds from June to September. Mawsynram in Meghalaya is the wettest place on Earth (~11,871 mm annually) because Khasi hills funnel and lift Bay of Bengal moisture. The retreating monsoon (mid-September to November) brings the northeast monsoon, Tamil Nadu rainfall, October heat, and Bay of Bengal cyclones.
## What Is Differential Heating and Why It Starts the Monsoon
The fundamental trigger of the Indian monsoon is differential heating — the fact that land heats and cools faster than the sea. From March onwards, the Indian subcontinent heats up rapidly under the overhead sun. By June, intense heating creates a strong low-pressure area over the northwest Indian plains and the Tibetan Plateau, while the surrounding oceans remain comparatively cooler with higher pressure.
Air always flows from high pressure to low pressure. So moisture-laden winds from the Indian Ocean rush toward the low-pressure zone over the land. Because these winds blow from the southwest (deflected right by the Coriolis force in the Northern Hemisphere), we call them the southwest monsoon.
In winter, the pattern reverses: the land cools faster, a high-pressure area forms over Central Asia and northern India, and cold, dry winds blow from the northeast — the northeast monsoon. This seasonal reversal of wind direction is the defining feature of the monsoon, a word derived from the Arabic mausim, meaning “season.”
## Role of the ITCZ (Inter-Tropical Convergence Zone)
The Inter-Tropical Convergence Zone is the equatorial belt where trade winds from both hemispheres converge, forcing air to rise and producing a belt of low pressure and heavy rainfall. In the Northern Hemisphere summer, the ITCZ shifts northward following the sun.
Key facts for exams:
– In July, the ITCZ shifts to roughly 25°N over the Indian subcontinent — an unusually high latitude compared with other regions.
– Over India, this seasonal ITCZ is called the monsoon trough, a low-pressure trough running roughly from Sri Ganganagar (Rajasthan) to the Bay of Bengal.
– The position of the monsoon trough decides where rain falls: when it lies over the plains, central India gets good rainfall; when it shifts north to the Himalayan foothills, a monsoon break occurs.
– The Tibetan Plateau’s high altitude acts as an elevated heat source, intensifying the low pressure and helping pull the ITCZ dramatically northward.
## Jet Streams: Tropical Easterly Jet and Subtropical Westerly Jet
Jet streams — narrow bands of fast-moving winds in the upper troposphere — act as the “switch” for the monsoon:
– Subtropical Westerly Jet (STWJ): In winter, this jet flows south of the Himalayas, keeping a westerly regime over north India. Its northward retreat to the north of the Himalayas (Tibet) by mid-June removes the barrier to monsoon winds and permits the onset of the southwest monsoon.
– Tropical Easterly Jet (TEJ): The intense heating of Tibet in summer creates a strong north-south pressure gradient in the upper atmosphere, producing an easterly jet stream at about 14–15 km altitude, blowing from east to west across peninsular India. The establishment of the TEJ is closely linked with the onset and strength of the southwest monsoon; when the TEJ weakens in September, the monsoon begins to retreat.
– In October, the subtropical westerly jet stream returns south of the Himalayas, heralding the establishment of the northeast monsoon and the transition to winter conditions.
Remember for prelims: TEJ onset → SW monsoon onset; STWJ’s return → SW monsoon withdrawal.
## Onset of the Southwest Monsoon: Kerala and Branches
The normal date of onset over Kerala, as per the India Meteorological Department (IMD), is 1 June. The onset is dramatic — a sudden rise in humidity, a sharp increase in rainfall, and gusty winds — traditionally called the burst of the monsoon.
The monsoon reaches India through two main branches:
– Arabian Sea Branch: Strikes the Western Ghats first, giving heavy rainfall on the windward side (Mumbai, Konkan coast) and rain-shadow dryness on the leeward Deccan (Pune side). It then curves to bring rain to Gujarat, Rajasthan and the Indo-Gangetic plain.
– Bay of Bengal Branch: Moves north in the Bay, strikes the Arakan coast and Meghalaya hills (causing Mawsynram’s record rainfall), then deflects westward over the Ganga plain, covering Delhi and Punjab by early July.
By mid-July, the monsoon covers the entire country. Roughly 75% of India’s annual rainfall comes from the southwest monsoon.
## The Mawsynram Factor: Why It Is the Wettest Place on Earth
Mawsynram, in the East Khasi Hills of Meghalaya, records an average annual rainfall of about 11,871 mm, the highest in the world. The reasons are a classic orographic story:
– The Bay of Bengal branch of the monsoon sweeps north through the Bangladesh plains, carrying enormous moisture.
– The Khasi Hills rise abruptly as a wall across the path of these south-to-north winds.
– The deep valleys and the funnel-shaped topography of the hills channel and compress the moist winds, intensifying their ascent.
– The forced ascent (orographic lift) cools the air, condenses the moisture, and produces incessant rain on the windward slopes.
Cherrapunji (Sohra), barely 15–20 km away in the same hills, records slightly less (~11,000+ mm annually) though it holds records for the highest rainfall in single months and years. The difference is frequently asked in SSC exams: both lie in the Khasi Hills, but Mawsynram’s average annual rainfall exceeds Cherrapunji’s.
## Breaks and Variability in the Monsoon
The monsoon is not uniformly wet. Important variability concepts:
– Monsoon break: During July–August, the monsoon trough sometimes shifts north to the Himalayan foothills, causing rain to concentrate there while most of India stays dry for days or weeks.
– El Niño / Southern Oscillation (ENSO): Warming of the eastern and central Pacific weakens the Walker circulation; El Niño years are historically associated with deficient Indian monsoon rainfall (e.g., 2002, 2009, 2015 drought years).
– Indian Ocean Dipole (IOD): A positive IOD (warmer western Indian Ocean) generally supports a good monsoon and can offset El Niño; a negative IOD weakens it.
For verified, current monsoon data, always refer to the India Meteorological Department and PIB releases.
## Retreating Monsoon Season: Withdrawal Pattern
The southwest monsoon begins withdrawing from extreme northwest India (west Rajasthan) around 17 September. Key features:
– Withdrawal proceeds southward and from northwest to southeast, completing from most of the country by around 15 October, and from the southern peninsula by late October/early November.
– Unlike onset, withdrawal is gradual, not a “burst.”
– During retreat, winds reverse direction, and the land is still moist and warm.
– The retreat period coincides with the northeast monsoon picking up moisture over the Bay of Bengal, bringing rain to coastal Tamil Nadu and parts of Andhra Pradesh.
## Northeast Monsoon and Tamil Nadu Winter Rainfall
From October to December, winds blow from land to sea (northeast direction). Crossing the Bay of Bengal, they pick up moisture and deposit it on the Tamil Nadu coast, coastal Andhra Pradesh and parts of Sri Lanka.
Why Tamil Nadu gets winter rain while the rest of India dries:
– Tamil Nadu lies leeward of the Western Ghats during the SW monsoon, so it receives little June–September rain.
– In Oct–Dec, the northeast monsoon winds are deflected by the Coriolis force onto the Tamil Nadu coast, bringing most of the state’s annual rainfall.
– This season also spawns tropical cyclones in the Bay of Bengal (e.g., the historic 1977 Diviseema cyclone), which strike the eastern coast.
## October Heat and the Transition Season
October–November is the season of October heat across north India:
– The retreating monsoon leaves the ground wet and skies partly cloudy.
– Day temperatures rise again (the sun’s rays become more vertical over the tropics), while high humidity prevents evaporation of sweat — making the weather hot, damp and oppressive.
– This muggy, uncomfortable weather — not extreme heat — defines “October heat” in geography textbooks.
– The transition also favours Bay of Bengal cyclones, which frequently make landfall in October–November, especially on the Odisha–Andhra–Tamil Nadu coasts.
## Previous Year Questions: How UPSC & SSC Frame Monsoon Topics
UPSC Prelims typically asks mechanism-based questions (jet streams, ITCZ, ENSO/IOD), while SSC focuses on factual recall. One-line points to memorise:
– Monsoon word origin: Arabic mausim.
– Normal onset over Kerala: 1 June; withdrawal from NW India: 17 September.
– ITCZ position in July: ~25°N over India.
– Tropical Easterly Jet forms due to heating of the Tibetan Plateau.
– Mawsynram average annual rainfall: ~11,871 mm — wettest place on Earth.
– SW monsoon contributes ~75% of India’s annual rainfall.
– Tamil Nadu’s rainy season: October–December (northeast monsoon).
– La Niña generally favours above-normal monsoon rainfall; El Niño the opposite.
## Revision Table: Key Facts, Dates and Figures
| Item | Key Fact / Figure |
|---|---|
| Onset over Kerala | 1 June (normal date, IMD) |
| Complete coverage of India | Mid-July (~15 July) |
| Withdrawal begins (NW India) | ~17 September |
| Withdrawal completes (peninsula) | By late October / early November |
| ITCZ position in July | ~25°N (monsoon trough) |
| TEJ trigger | Summer heating of Tibetan Plateau |
| STWJ role | Northward shift (June) permits onset; return (October) ends monsoon |
| Mawsynram rainfall | ~11,871 mm average annual (wettest on Earth) |
| Cherrapunji (Sohra) | Same Khasi Hills; slightly lower annual average |
| SW monsoon share of annual rain | ~75% |
| NE monsoon rainy belt | Tamil Nadu coast, Oct–Dec |
| October heat | High temperature + high humidity, Oct–Nov |
## Frequently Asked Questions
### When does the southwest monsoon normally onset over Kerala and withdraw from India?
Normal onset over Kerala is around 1 June. Withdrawal begins from northwest India around 17 September and completes by around mid-October for most of the country, per the IMD’s revised normals.
### Which branch of the monsoon brings rain to Tamil Nadu?
The retreating (northeast) monsoon from October to December. These dry continental winds pick up moisture while crossing the Bay of Bengal and strike the Tamil Nadu coast.
### What causes October heat?
The retreating monsoon leaves the ground moist and humidity high while temperatures rise again, so the plains feel hot and oppressive during October–November.
### How is Mawsynram different from Cherrapunji?
Both lie in Meghalaya’s Khasi Hills, but Mawsynram records a slightly higher average annual rainfall (~11,871 mm) than Cherrapunji (Sohra), making it the wettest place on Earth.
### What is a monsoon “break”?
A period during July–August when the monsoon trough shifts north to the Himalayan foothills, weakening rainfall over most of India while rain continues in the hills and parts of the northeast.
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