Indian Monsoon Mechanism Explained: Southwest vs Northeast Monsoon, ITCZ and Retreating Season
Quick Answer: The Indian monsoon mechanism works through differential heating of land and sea, which shifts the Inter-Tropical Convergence Zone (ITCZ) northward over India around June, reversing winds from dry northeast trades to moisture-laden southwest winds. Jet stream changes — the retreat of the subtropical westerly jet and arrival of the tropical easterly jet — trigger the onset. India’s climate then moves through four seasons: cold weather, hot weather, advancing Southwest monsoon (June–September), and the retreating monsoon (October–November).
- Factors Behind Monsoon Formation: Differential Heating and ITCZ
- Role of Jet Streams: Subtropical Westerly and Tropical Easterly Jets
- Southwest Monsoon (June–September): Onset and Branches
- Break in the Monsoon and Rainfall Patterns
- Retreating Monsoon Season (October–November)
- Northeast Monsoon (December–February): The Tamil Nadu Winter Rains
- El Nino, La Nina and ENSO Impact on Indian Monsoon
- Indian Ocean Dipole (IOD) and Monsoon Variability
- Seasonal Cycle Recap: The Four Seasons of Indian Climate
- Exam-Focused Quick Revision: Key Facts, Dates and Definitions
- Previous Year Questions and Practice MCQs on Indian Monsoon
- Frequently Asked Questions
- Q: When does the Southwest monsoon normally set in over Kerala?
- Q: How does El Nino affect the Indian monsoon?
- Q: What is the role of the Tibetan Plateau in the monsoon mechanism?
- Q: Why does the east coast of Tamil Nadu get rain in winter?
- Q: What is meant by a ‘break’ in the monsoon?
- Related reading
Factors Behind Monsoon Formation: Differential Heating and ITCZ
The Indian monsoon mechanism is fundamentally a thermal response. In summer, the Indian landmass heats faster than the surrounding oceans, creating intense low pressure over the subcontinent — centred over northwestern India. Cooler, moisture-rich oceanic air rushes in to replace rising hot air, establishing the monsoon circulation.
The key driver is the Inter-Tropical Convergence Zone (ITCZ) — the belt of low pressure near the equator where the trade winds of both hemispheres converge. Normally positioned around 5°N, the ITCZ migrates dramatically northward over the Indian subcontinent by late June, sometimes reaching 25°N over the Gangetic plains. This northward shift pulls the southeast trade winds of the Southern Hemisphere across the equator, where the Coriolis force deflects them to the right — turning them into the southwest monsoon winds.
Two additional reinforcing factors:
- Tibetan Plateau heating: The plateau acts as an elevated heat source in summer. Strong heating of its upper atmosphere creates a high-level anticyclone and intensifies upper-air divergence, deepening the monsoon trough over India and strengthening the tropical easterly jet stream.
- Indian Ocean heat capacity: The ocean warms slowly, maintaining the land–sea pressure gradient that sustains the system through the season.
Role of Jet Streams: Subtropical Westerly and Tropical Easterly Jets
Jet streams are high-speed winds near the tropopause (about 9–14 km altitude) that critically govern monsoon onset and withdrawal.
Subtropical Westerly Jet: During winter, this jet flows south of the Himalayas (around 25°N), keeping a westerly regime over northern India and blocking monsoon conditions. As summer advances, it migrates north of the Himalayas — typically by early June. This retreat is a precondition for the monsoon onset.
Tropical Easterly Jet: Once the westerly jet shifts north, an easterly jet stream develops at about 14°N, flowing from east to west over peninsular India. Its formation, linked to Tibetan Plateau heating, marks the burst of the monsoon over Kerala. The sudden onset of heavy rain — the “monsoon burst” — is closely tied to the establishment of this easterly jet. In autumn, the jets reverse positions again, signalling withdrawal.
Southwest Monsoon (June–September): Onset and Branches
The Southwest monsoon is India’s principal rainy season, delivering roughly 75% of the country’s annual rainfall. Per the India Meteorological Department (IMD), the normal onset date over Kerala is around 1 June. The monsoon then splits into two branches:
- Arabian Sea Branch: Strikes the Western Ghats, producing heavy orographic rainfall on the windward (western) slopes; Mumbai receives intense early-season rain from this branch.
- Bay of Bengal Branch: Curves north over the Bay, strikes the Arakan coast and Eastern Himalayas, and brings the first rains to Assam and the northeast — Shillong and the Brahmaputra valley get early June rain — before deflecting westward up the Ganga plain.
The two branches merge over the northwest by around mid-July, covering the entire country.
Break in the Monsoon and Rainfall Patterns
Rainfall distribution is highly uneven. Mawsynram and Cherrapunji in Meghalaya record the world’s highest rainfall — Mawsynram averages over 11,000 mm annually — as the Bay branch is forced up the Khasi Hills. In contrast, the leeward side of the Western Ghats and much of the Deccan interior lie in a rain shadow, receiving far less rain. The Thar Desert and Ladakh remain arid.
A “break” in the monsoon refers to intra-seasonal periods during June–September when the monsoon trough shifts north toward the Himalayan foothills. Rainfall over central and northwest India drops sharply, while the northeast and Himalayan foothills may receive enhanced rain. Breaks lasting over a week can dent seasonal totals.
Retreating Monsoon Season (October–November)
As the sun shifts southward, the ITCZ retreats, low pressure over India weakens, and the monsoon withdraws — from the northwest by mid-September, progressively southward, exiting the peninsula by around mid-December. Characteristic features:
- Winds reverse back to the northeast, but skies remain partly cloudy.
- Temperature and humidity both rise — hot, oppressive, sticky conditions nicknamed “October heat”, common over northwestern India with clear skies and high moisture.
- Tropical cyclones frequently form in the Bay of Bengal (and, less often, the Arabian Sea) during October–November, striking Odisha, Andhra Pradesh and Tamil Nadu coasts.
Northeast Monsoon (December–February): The Tamil Nadu Winter Rains
During winter, a high-pressure system dominates northwestern India and the ITCZ lies south of the equator. Winds blow from land to sea as dry, cold northeast trades. However, while crossing the Bay of Bengal, these winds pick up moisture and deposit it on the Tamil Nadu coast — giving Chennai and coastal Tamil Nadu their main rainy season (October–December). Tamil Nadu receives little benefit from the Southwest monsoon because it lies in the rain shadow of both the Western Ghats and, for coastal areas, the parallel wind flow along the Coromandel coast.
El Nino, La Nina and ENSO Impact on Indian Monsoon
ENSO (El Nino–Southern Oscillation) is an irregular Pacific Ocean–atmosphere phenomenon:
- El Nino: Abnormal warming of the eastern-central equatorial Pacific. It weakens the Walker circulation and tends to suppress Indian monsoon rainfall — many drought years (e.g., 2002, 2009, 2015) were El Nino years.
- La Nina: Abnormal cooling of the same region. It generally strengthens the monsoon and is associated with above-normal rainfall and stronger Bay of Bengal cyclone activity.
The correlation is strong but not absolute — not every El Nino year is a drought, and IOD conditions can offset ENSO’s influence. For authoritative monitoring, see the IMD Pune and NOAA Climate.gov ENSO pages.
Indian Ocean Dipole (IOD) and Monsoon Variability
The IOD is an irregular oscillation of sea-surface temperatures in the Indian Ocean — western pole (Arabian Sea) versus eastern pole (near Sumatra):
- Positive IOD: Warmer western Indian Ocean, cooler east — favours a stronger monsoon with above-normal rainfall.
- Negative IOD: Cooler west, warmer east — tends to suppress monsoon rainfall.
IOD and ENSO interact: a strong positive IOD can neutralise an El Nino’s drying effect, as seen in some recent seasons.
Seasonal Cycle Recap: The Four Seasons of Indian Climate
Per NCERT and IMD classification, India’s climate year has four seasons:
- Cold weather season: December–February; northeast trade winds; dry, except Tamil Nadu rains.
- Hot weather season: March–May; rising temperature, low pressure, local storms like kaal baisakhi and mango showers.
- Advancing Southwest monsoon: June–September; main rainy season.
- Retreating monsoon: October–November; transition, October heat, cyclones, Tamil Nadu rains.
Exam-Focused Quick Revision: Key Facts, Dates and Definitions
| Term / Fact | Key Point |
|---|---|
| ITCZ | Equatorial low-pressure convergence belt; shifts to ~25°N over India in monsoon |
| Onset over Kerala (MOK) | Normal date: ~1 June |
| Country-wide cover | By ~15 July |
| SW monsoon branches | Arabian Sea branch; Bay of Bengal branch |
| Subtropical westerly jet | Retreats north of Himalayas before onset |
| Tropical easterly jet | Forms with Tibetan heating; signals monsoon burst |
| Monsoon trough | Low-pressure trough over Gangetic plains; north shift = “break” |
| El Nino | Weakens monsoon |
| La Nina | Strengthens monsoon |
| Positive IOD | Favours good rainfall |
Previous Year Questions and Practice MCQs on Indian Monsoon
- UPSC Prelims style: The burst of the monsoon over Kerala is closely associated with the — (a) subtropical westerly jet (b) tropical easterly jet (c) polar jet (d) Somali jet only. Answer: (b)
- SSC CGL style: Which state receives most of its rainfall from the Northeast monsoon? (a) Kerala (b) Tamil Nadu (c) Goa (d) Assam. Answer: (b)
- UPSC style: The ITCZ during the Indian summer monsoon normally shifts to about — (a) 5°N (b) 10°N (c) 25°N (d) equator. Answer: (c)
- SSC style: “October heat” is caused by — (a) high temperature with high humidity during retreating monsoon (b) heat waves in May (c) La Nina (d) Western disturbances. Answer: (a)
- UPSC style: A positive Indian Ocean Dipole generally — (a) weakens the monsoon (b) strengthens the monsoon (c) has no effect (d) causes cyclones only. Answer: (b)
Frequently Asked Questions
Q: When does the Southwest monsoon normally set in over Kerala?
Around 1 June per IMD’s normal onset date, advancing to cover the entire country by mid-July.
Q: How does El Nino affect the Indian monsoon?
El Nino (warm eastern Pacific) generally weakens the monsoon and reduces rainfall; La Nina tends to strengthen it — though the correlation is not absolute.
Q: What is the role of the Tibetan Plateau in the monsoon mechanism?
Summer heating of the plateau creates an upper-level low-pressure/anticyclonic outflow that strengthens monsoon circulation and drives the tropical easterly jet.
Q: Why does the east coast of Tamil Nadu get rain in winter?
Northeast monsoon winds blow from land to sea but pick up moisture while crossing the Bay of Bengal and strike the Tamil Nadu coast as rain-bearing winds.
Q: What is meant by a ‘break’ in the monsoon?
Intra-seasonal spells during June–September when the monsoon trough shifts north and rainfall over central India reduces sharply, while the northeast may get more rain.
Related reading
- Rivers of Peninsular India: East-Flowing vs West-Flowing Drainage — Mapped Notes for UPSC & SSC
- Ocean Currents Explained: Warm vs Cold Currents, El Niño and Monsoon Linkage for UPSC
Quick revision
- Tibetan Plateau heating: The plateau acts as an elevated heat source in summer.
- Indian Ocean heat capacity: The ocean warms slowly, maintaining the land–sea pressure gradient that sustains the system through the season.
- Arabian Sea Branch: Strikes the Western Ghats, producing heavy orographic rainfall on the windward (western) slopes; Mumbai receives intense early-season rain from this…
- Bay of Bengal Branch: Curves north over the Bay, strikes the Arakan coast and Eastern Himalayas, and brings the first rains to Assam and the northeast — Shillong and the…
- Winds reverse back to the northeast, but skies remain partly cloudy.
- Temperature and humidity both rise — hot, oppressive, sticky conditions nicknamed “October heat”, common over northwestern India with…
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