Monsoons Decoded: Jet Streams, ITCZ Shift and Why Monsoon Retreat Gets Delayed — Geography Notes for UPSC & SSC
Quick Answer: How Jet Streams and ITCZ Drive the Indian Monsoon
Quick answer: The Indian monsoon is a seasonal reversal of winds driven by differential heating between land and sea. In summer, the ITCZ shifts north to about 25°N over India, the subtropical westerly jet retreats north of the Himalayas, Tibetan Plateau heating sets up an upper-level high-pressure pump, and the Tropical Easterly Jet and Somali Jet form — together delivering the southwest monsoon that onsets over Kerala around 1 June. Retreat begins from west Rajasthan around 17 September and gets delayed when the monsoon trough persists, the ITCZ returns south slowly, or El Niño/IOD conditions alter circulation.
- Quick Answer: How Jet Streams and ITCZ Drive the Indian Monsoon
- What Is the Indian Monsoon? Seasonal Reversal of Winds Explained
- ITCZ Shift: The Tropical Convergence Zone and Its Northward Migration
- Role of Tibetan Plateau Heating in Monsoon Onset
- Jet Streams: Subtropical Westerly Jet and Tropical Easterly Jet
- Somali Jet (Findlater Jet): The Low-Level Cross-Equatorial Flow
- Mechanism of Monsoon Onset: Kerala Onset and Burst
- Monsoon Withdrawal (Retreat): Dates and Phases
- Why Monsoon Retreat Gets Delayed: Key Reasons
- El Niño, La Niña and Indian Ocean Dipole: Oceanic Influences
- Mrunal-style Exam Framing: PYQs and How UPSC/SSC Ask This Topic
- Revision Table: Monsoon Mechanism at a Glance
- Frequently Asked Questions
- What causes the onset of the southwest monsoon over Kerala?
- Why does the monsoon retreat from northwest India first?
- How does El Niño affect the Indian monsoon?
- What is the difference between monsoon ‘burst’ and ‘break’?
- Is the Somali Jet important for UPSC prelims?
- Related reading
What Is the Indian Monsoon? Seasonal Reversal of Winds Explained
The monsoon is not just “rain” — it is a wind system that reverses direction between summer and winter. In summer (June–September), winds blow from the southwest Indian Ocean onto the subcontinent, loaded with moisture. In winter (October–December), the pattern flips: dry, cool northeasterly winds flow from land to sea.
The fundamental cause is differential heating: land heats faster than ocean. By June, a intense low pressure forms over northwestern India (the seasonal low), pulling in moisture-laden oceanic air. The India Meteorological Department (IMD) tracks and forecasts this system operationally — see IMD’s monsoon portal.
ITCZ Shift: The Tropical Convergence Zone and Its Northward Migration
The Inter-Tropical Convergence Zone (ITCZ) is the belt near the equator where northeast and southeast trade winds converge, forcing air to rise and producing heavy convective rainfall.
- In summer, the ITCZ migrates northward and lies over the Ganga plain at roughly 20°N–25°N — unusually far north compared with other longitudes, because the Himalayan landmass heats strongly.
- Over India, this shifted ITCZ appears as the monsoon trough, an elongated low-pressure zone from Sri Ganganagar (Rajasthan) to the Bay of Bengal.
- Depressions forming in the Bay of Bengal travel along this trough, distributing rainfall across the plains.
- When the trough shifts to the Himalayan foothills, plains get a dry spell — the monsoon “break” condition.
Role of Tibetan Plateau Heating in Monsoon Onset
The Tibetan Plateau, averaging over 4,500 m, acts as an elevated heat source in summer. Strong insolation heats the plateau, and this warmth is transferred to the upper troposphere (around 200 hPa), creating a high-pressure anticyclone aloft over Tibet — sometimes called a “heat pump.”
This upper-level high drives air outward at high altitude (toward the east and south), which in turn intensifies surface inflow from the ocean. The result: the monsoon circulation is strengthened and sustained. Without Tibetan heating, the Indian monsoon would be far weaker — a point UPSC loves to frame as a statement-based prelims question.
Jet Streams: Subtropical Westerly Jet and Tropical Easterly Jet
Two jet streams matter most for the monsoon mechanism:
- Subtropical Westerly Jet (STWJ): In winter it flows south of the Himalayas over north India, bringing western disturbances and blocking monsoonal flow. Its northward withdrawal over Tibet (around mid-June) is a precondition for monsoon onset over northern India.
- Tropical Easterly Jet (TEJ): Forms in June due to Tibetan upper-level heating, flowing from east to west at ~14°N and around 200 hPa, with core speeds of 140–150 km/h. The TEJ strengthens the monsoon by enhancing upper-level divergence (rising air) over the Indian peninsula.
Somali Jet (Findlater Jet): The Low-Level Cross-Equatorial Flow
The Somali or Findlater Jet is a low-level, fast cross-equatorial current that blows from the southern Indian Ocean, crossing the equator off the East African (Somali) coast, then curving east to strike India’s west coast. Channelled and accelerated by the East African highlands, it transports enormous moisture and momentum into the Arabian Sea branch of the monsoon — the reason the Western Ghats coast receives intense rains. It typically strengthens as the monsoon sets in over Kerala.
Mechanism of Monsoon Onset: Kerala Onset and Burst
The IMD declares monsoon onset over Kerala when sustained rainfall, wind field and outgoing longwave radiation criteria are met — the normal date is 1 June. The sequence:
- ITCZ shifts north over the Indian landmass.
- STWJ withdraws north of the Himalayas.
- Tibetan heating builds the upper high; TEJ forms.
- Somali Jet surges, feeding moisture to the west coast.
- Onset over Kerala — followed by the “burst”: an abrupt, sharp rise in rainfall within days.
The monsoon then advances northwest, covering the whole country by around 8 July (normal dates range from late May in the south to mid-July in Rajasthan).
Monsoon Withdrawal (Retreat): Dates and Phases
Withdrawal begins from west Rajasthan around 17 September, as the monsoon trough/ITCZ starts shifting south and the subtropical westerly jet re-establishes south of the Himalayas. Typical phases:
- By 15 October: withdrawn from most of northwest India.
- By 15 December: withdrawn from the entire country, with the northeast (winter) monsoon rains beginning over Tamil Nadu.
During retreat, the skies turn clear and humid heat builds — the “October heat” of north India.
Why Monsoon Retreat Gets Delayed: Key Reasons
- Persistence of the monsoon trough: lingering low-pressure systems and Bay depressions keep the trough over the plains.
- Slow southward return of the ITCZ: warmer-than-normal land or sea-surface conditions delay the seasonal migration.
- Jet stream behaviour: delayed re-establishment of the subtropical westerly jet south of the Himalayas prolongs monsoon circulation.
- El Niño/La Niña and IOD: ocean-atmosphere coupling can extend or truncate the season.
- Climate change: IMD studies show withdrawal dates shifting later over recent decades — a favourite current-affairs hook for mains answers.
El Niño, La Niña and Indian Ocean Dipole: Oceanic Influences
El Niño (warm central-east Pacific) weakens the Walker circulation and typically reduces Indian monsoon rainfall, sometimes delaying onset and altering retreat. La Niña generally favours a stronger monsoon. The Indian Ocean Dipole (IOD) — a positive IOD (warm west Indian Ocean) boosts rainfall and can offset El Niño; a negative IOD suppresses it. Track ENSO status via NOAA Climate.gov and Copernicus Climate Change Service.
Mrunal-style Exam Framing: PYQs and How UPSC/SSC Ask This Topic
UPSC prelims asks this as statement-matching: “The Somali Jet is a low-level cross-equatorial jet (true)”; “The TEJ forms due to Tibetan heating (true)”; “The ITCZ shifts to 25°N over India in July (true).” SSC CGL asks date-based one-liners: Kerala onset — 1 June; withdrawal starts — west Rajasthan, mid-September. For mains (GS1), frame answers around factors → mechanism → outcome: differential heating → ITCZ shift + jet reversal → onset; and add delayed-withdrawal causes with IMD data for the climate-change angle.
Revision Table: Monsoon Mechanism at a Glance
| Factor | Effect on Monsoon |
|---|---|
| Differential land–sea heating | Seasonal wind reversal; low pressure over NW India |
| ITCZ shift to ~25°N | Monsoon trough over Ganga plain; depressions and rain |
| Tibetan Plateau heating | Upper-level high-pressure pump; strengthens circulation |
| STWJ withdrawal north of Himalayas | Precondition for onset over north India |
| Tropical Easterly Jet | Upper-level divergence; stronger updrafts and rain |
| Somali (Findlater) Jet | Cross-equatorial moisture supply to west coast |
| El Niño / negative IOD | Weaker monsoon; altered onset/retreat timing |
| La Niña / positive IOD | Stronger monsoon; may extend the season |
Frequently Asked Questions
What causes the onset of the southwest monsoon over Kerala?
Northward ITCZ shift, withdrawal of the subtropical westerly jet north of the Himalayas, Tibetan heating, and the development of the Tropical Easterly and Somali Jets — leading to onset around 1 June.
Why does the monsoon retreat from northwest India first?
As the ITCZ/monsoon trough shifts south, high pressure builds over northwest India first, and the westerly jet re-establishes over north India from the north — so withdrawal starts from west Rajasthan.
How does El Niño affect the Indian monsoon?
El Niño weakens the Walker circulation, generally reducing monsoon rainfall and often altering onset and retreat timing, though a strong positive IOD can offset its impact.
What is the difference between monsoon ‘burst’ and ‘break’?
Burst = sudden, sharp rise in rainfall at monsoon onset. Break = a dry spell during the monsoon caused by the monsoon trough shifting to the Himalayan foothills.
Is the Somali Jet important for UPSC prelims?
Yes — it is frequently asked as a cross-equatorial, low-level jet that supplies moisture to India’s west coast and drives the Arabian Sea branch of the monsoon.
Related reading
- Soil Types of India: ICAR Classification, Crop Fit and Erosion Hotspots for Prelims
- Geography Rapid Quiz: 15 PYQ-Style MCQs on Drainage, Climate and Soils with Explanations
Quick revision
- In summer, the ITCZ migrates northward and lies over the Ganga plain at roughly 20°N–25°N — unusually far north compared with other longitudes,…
- Over India, this shifted ITCZ appears as the monsoon trough, an elongated low-pressure zone from Sri Ganganagar (Rajasthan) to the Bay of Bengal.
- Depressions forming in the Bay of Bengal travel along this trough, distributing rainfall across the plains.
- When the trough shifts to the Himalayan foothills, plains get a dry spell — the monsoon “break” condition.
- Subtropical Westerly Jet (STWJ): In winter it flows south of the Himalayas over north India, bringing western disturbances and blocking monsoonal flow.
- Tropical Easterly Jet (TEJ): Forms in June due to Tibetan upper-level heating, flowing from east to west at ~14°N and around 200 hPa, with core speeds of 140–150 km/h.
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