Monsoon Mechanism Explained: Jet Streams, ITCZ and Tibetan Plateau for UPSC Prelims
Quick Answer: The Indian monsoon is caused by the seasonal reversal of winds driven by three key factors: the northward shift of the ITCZ over India in summer, the retreat of the Subtropical Westerly Jet Stream (STWJ) north of the Himalayas by mid-June, and intense heating of the Tibetan Plateau, which forms an upper-level anticyclone and generates the Tropical Easterly Jet. Together, these set up the southwest monsoon, normally arriving over Kerala around 1 June.
- Direct Answer: What Causes the Indian Monsoon?
- Monsoon as a Seasonal Reversal of Winds
- The Inter-Tropical Convergence Zone (ITCZ) Shift
- Subtropical Westerly Jet Stream (STWJ) and Onset
- Tropical Easterly Jet (TEJ) and Monsoon Burst
- Tibetan Plateau Heating: The Elevated Heat Source
- Mechanism of Onset of the Southwest Monsoon
- Mechanism of Withdrawal of the Southwest Monsoon
- Somali Jet and Indian Ocean Dipole (Supplementary Factors)
- Northeast (Retreating) Monsoon Mechanics
- Prelims-Style Facts, Dates and Quick Revision Table
- Practice MCQs on Monsoon Mechanism
- Frequently Asked Questions
- Q: What is the normal date of onset of the southwest monsoon over Kerala?
- Q: Where does the ITCZ lie over India during the monsoon season?
- Q: What triggers the formation of the Tropical Easterly Jet?
- Q: From which region does the southwest monsoon withdraw first?
- Q: How do jet streams differ in winter and summer over India?
- Related reading
Direct Answer: What Causes the Indian Monsoon?
The Indian monsoon is not caused by a single factor but by an interplay of planetary-scale and regional mechanisms. For UPSC Prelims, remember the trio: jet streams (STWJ withdrawal and TEJ formation), the ITCZ shift northward over the Indian subcontinent, and Tibetan Plateau heating acting as an elevated heat source. Differential heating of land and sea provides the base energy gradient, but these three mechanisms explain the timing, burst and withdrawal of the monsoon — which is why questions repeatedly probe them.
Monsoon as a Seasonal Reversal of Winds
The word “monsoon” derives from the Arabic mawsim, meaning season. The monsoon is defined as a seasonal reversal of wind direction accompanied by pronounced changes in precipitation:
- Summer (June–September): winds blow from sea to land (southwest), bringing heavy rain.
- Winter (December–February): winds reverse, blowing from land to sea (northeast), keeping most of India dry.
The base mechanism is differential heating: land heats faster than the sea in summer, creating a low-pressure area over the Indian subcontinent that draws in moist oceanic air. In winter, the pattern reverses with a high-pressure cell over the land. India’s monsoon is exceptionally strong because the Himalayas and the Tibetan Plateau amplify this basic land–sea contrast.
The Inter-Tropical Convergence Zone (ITCZ) Shift
The ITCZ is a low-pressure belt near the equator where the trade winds of the two hemispheres converge, forcing moist air to rise and producing thunderstorms. In the Northern Hemisphere summer, following the apparent movement of the sun, the ITCZ migrates northward over the Indian subcontinent, reaching approximately 20–25°N over India in July.
Over the Gangetic plains, this shifted ITCZ is known as the monsoon trough. Its position controls rainfall distribution: when the trough shifts north towards the Himalayan foothills, the “break monsoon” condition occurs with reduced rain over the plains; oscillations southward revive rainfall over central India. The northward ITCZ shift effectively drags the southeast trade winds of the Southern Hemisphere across the equator — where the Coriolis force deflects them to become southwesterlies — delivering the moisture-laden southwest monsoon.
Subtropical Westerly Jet Stream (STWJ) and Onset
The STWJ is a fast-moving belt of westerly winds near the tropopause. In winter, it flows to the south of the Himalayas, roughly along 25–30°N, acting as a barrier and steering western disturbances that bring winter rain to northwest India.
As summer advances, the STWJ shifts north of the Himalayas, typically by mid-June. Its retreat removes the dynamical barrier over north India, allowing the monsoon currents to surge in. The onset over Kerala follows shortly after the jet’s northward migration — one of the most-tested cause-and-effect chains in Prelims.
Tropical Easterly Jet (TEJ) and Monsoon Burst
Strong summer heating of the Tibetan Plateau builds a high-pressure (anticyclonic) system in the upper troposphere. Air flowing out of this upper high generates an easterly jet stream at about 14–15 km altitude, centred near the southern tip of the Indian peninsula — the Tropical Easterly Jet (TEJ).
The TEJ strengthens the monsoon circulation by enhancing upper-level divergence and upper-air outflow over peninsular India, which intensifies surface convergence and convection below. Its establishment is closely associated with the monsoon burst — the sudden spell of heavy rainfall that marks the onset phase. The TEJ exists only during the monsoon season and disappears with the collapse of the Tibetan heat source.
Tibetan Plateau Heating: The Elevated Heat Source
Averaging over 4,500 m in altitude, the Tibetan Plateau heats intensely in summer. Because it is elevated, this heating is applied directly to the middle troposphere — making Tibet a genuine “elevated heat source” rather than a surface-level one.
Consequences for the monsoon:
- It creates a strong upper-level anticyclone over Tibet (~200 hPa level).
- Divergent outflow from this high generates and sustains the Tropical Easterly Jet.
- The rising air over the plateau enhances the meridional (north–south) monsoon circulation, pulling in moist air from the Indian Ocean.
Model studies confirm that without Tibetan heating, the Indian monsoon would be significantly weaker — a favourite statement-based question in Prelims.
Mechanism of Onset of the Southwest Monsoon
The onset sequence, in order:
- The ITCZ begins shifting northward following the sun’s apparent movement.
- The STWJ withdraws north of the Himalayas by mid-June.
- Tibetan Plateau heating peaks, forming the upper-level anticyclone and the TEJ.
- Cross-equatorial moisture-laden winds turn southwesterly and surge toward the coast.
- The monsoon sets in over Kerala around 1 June (IMD revised normal dates, 2015).
- It progresses northward, covering the whole country by around 8 July (over the extreme northwest).
Two branches feed the onset: the Arabian Sea branch striking the Western Ghats and the Bay of Bengal branch moving up the Gangetic plains and into the northeast.
Mechanism of Withdrawal of the Southwest Monsoon
Withdrawal is essentially the reverse of onset, driven by the southward retreat of the sun:
- The ITCZ (monsoon trough) shifts southward back toward the equator.
- The STWJ returns to the south of the Himalayas, restoring westerly control over north India.
- Tibetan heating weakens; the upper anticyclone and the TEJ collapse.
- Withdrawal begins from west Rajasthan and northwest India around 17 September, retreating progressively southward, and is complete over the extreme south peninsula by around 15 December.
As the moist monsoon current retreats, clear skies and high day temperatures produce the characteristic “October heat” of north India — hot, humid, oppressive conditions.
Somali Jet and Indian Ocean Dipole (Supplementary Factors)
Two additional influences frequently appear in Prelims:
- Somali Jet: a strong low-level cross-equatorial jet off the coast of Somalia (Horn of Africa) that accelerates moisture transport from the Southern Indian Ocean into the Arabian Sea, strengthening the southwest monsoon flow toward India.
- Indian Ocean Dipole (IOD): a seesaw in sea-surface temperature anomalies between the western and eastern Indian Ocean. A positive IOD (warmer west) generally boosts monsoon rainfall over India; a negative IOD (warmer east) tends to suppress it.
For authoritative climatology references, see the India Meteorological Department and the NOAA Climate.gov portal.
Northeast (Retreating) Monsoon Mechanics
As winter sets in, the pressure gradient reverses: high pressure over Central Asia and the Tibetan Plateau drives offshore winds from the northeast toward the ocean. These cold, dry winds pick up moisture while crossing the Bay of Bengal, and their deflection by the Eastern Ghats brings significant rainfall to the Tamil Nadu coast from October to December — contributing roughly half of Tamil Nadu’s annual rainfall. Coastal Andhra Pradesh, Rayalaseema and parts of Kerala and Karnataka also receive rain in this season.
Prelims-Style Facts, Dates and Quick Revision Table
| Element | Key Fact for Prelims |
|---|---|
| Normal onset over Kerala | ~1 June (IMD revised normal dates, 2015) |
| Monsoon covers entire country by | ~8 July |
| Withdrawal begins from northwest India | ~17 September |
| Withdrawal completes over south peninsula | ~15 December |
| ITCZ position over India in July | ~20–25°N (monsoon trough over Gangetic plains) |
| STWJ in winter | South of the Himalayas (~25–30°N); brings western disturbances |
| STWJ by mid-June | Shifts north of the Himalayas — precondition for onset |
| Tropical Easterly Jet | ~14–15 km altitude; core near peninsular tip; summer only |
| Tibetan Plateau average elevation | ~4,500 m; acts as elevated heat source |
| Northeast monsoon rainfall season | October–December; Tamil Nadu coast main beneficiary |
Practice MCQs on Monsoon Mechanism
Q1. With reference to the Subtropical Westerly Jet Stream (STWJ), consider the following statements:
- In winter, it flows to the south of the Himalayas.
- Its northward shift beyond the Himalayas by mid-June facilitates monsoon onset.
Which of the statements is/are correct?
(a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2
Q2. The Tropical Easterly Jet forms due to:
(a) Cooling of the Tibetan Plateau in winter
(b) Strong summer heating of the Tibetan Plateau creating an upper-level high
(c) The southward shift of the ITCZ
(d) El Niño conditions in the Pacific
Q3. During the Indian monsoon season, the ITCZ normally lies over India at approximately:
(a) 5–10°N (b) 10–15°N (c) 20–25°N (d) 30–35°N
Q4. The southwest monsoon begins withdrawing first from:
(a) Kerala (b) Northeast India (c) Northwest India (d) Gujarat coast
Q5. Consider the following pairs:
- Somali Jet — cross-equatorial low-level jet strengthening moisture influx
- Positive IOD — generally suppresses Indian monsoon rainfall
Which of the pairs is/are correctly matched?
(a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2
Q6. Rainfall on the Tamil Nadu coast from October to December is primarily caused by:
(a) Southwest monsoon Arabian Sea branch
(b) Western disturbances
(c) Northeast monsoon winds picking up moisture over the Bay of Bengal
(d) Local convection only
Answer Key: 1 – (c); 2 – (b); 3 – (c); 4 – (c); 5 – (a); 6 – (c).
Frequently Asked Questions
Q: What is the normal date of onset of the southwest monsoon over Kerala?
Around 1 June, as per the IMD’s revised normal onset dates issued in 2015. The monsoon then advances northward, covering the entire country by about 8 July.
Q: Where does the ITCZ lie over India during the monsoon season?
It shifts to around 20–25°N, where it is referred to as the monsoon trough — most prominent over the Gangetic plains during July–August.
Q: What triggers the formation of the Tropical Easterly Jet?
Strong summer heating of the Tibetan Plateau creates an upper-level high-pressure system, whose outflow drives easterly winds at roughly 14 km altitude near the peninsular tip — the TEJ, which aids the monsoon burst.
Q: From which region does the southwest monsoon withdraw first?
Northwest India, beginning around 17 September, with the withdrawal proceeding southward and completing over the extreme southern peninsula by mid-December.
Q: How do jet streams differ in winter and summer over India?
In winter, the STWJ flows south of the Himalayas and steers western disturbances bringing rain to northwest India. In summer, it shifts north of the Himalayas, and the TEJ appears over peninsular India — existing only in the monsoon season.
Related reading
- Monsoons Decoded: Jet Streams, ITCZ Shift and Why Monsoon Retreat Gets Delayed — Geography Notes for UPSC & SSC
- Monsoon Mechanism Explained: SW vs NE Monsoon, MJO and Jet Streams for UPSC Geography
Quick revision
- Summer (June–September): winds blow from sea to land (southwest), bringing heavy rain.
- Winter (December–February): winds reverse, blowing from land to sea (northeast), keeping most of India dry.
- It creates a strong upper-level anticyclone over Tibet (~200 hPa level).
- Divergent outflow from this high generates and sustains the Tropical Easterly Jet.
- The rising air over the plateau enhances the meridional (north–south) monsoon circulation, pulling in moist air from the Indian Ocean.
- The ITCZ begins shifting northward following the sun’s apparent movement.
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