Mechanism of the Indian Monsoon
How the Indian monsoon works for UPSC — differential heating, ITCZ, jet streams, El Niño and IOD, onset and withdrawal, the four seasons and local winds.
SyllabusGS I · Salient features of world's physical geography; physical geography of India — climate, monsoon and important geophysical phenomena
Prerequisites: Climatology · Physiography of India
The Hook: The Driest August in a Century
In 2023 an El Niño developed in the Pacific. India’s south-west monsoon (June–September) ended at about 94% of the long period average, in the “below normal” band, and August 2023 was the driest August since records began in 1901 (India Meteorological Department). Sowing was delayed in many districts and food prices came under pressure.
Why should warm water off the coast of Peru starve the fields of Maharashtra of rain? Answering that requires understanding how the monsoon actually works.
The Core Concept
The word monsoon comes from the Arabic mausim — season. A monsoon is a seasonal reversal of winds: in summer, moist winds blow from the sea to the land (south-west in India); in winter, dry winds blow from the land to the sea (north-east). India has a “tropical monsoon” type of climate even though the Tropic of Cancer runs through its middle, because the Himalayas shut out cold Central Asian air and make the whole subcontinent behave as one tropical unit.
Factors that shape India’s climate
| Factor | Effect |
|---|---|
| Latitude | Tropic of Cancer divides India into a tropical south and a subtropical north |
| The Himalayas | Block cold winds; force monsoon winds to rise and shed rain |
| Land–sea distribution | Land heats and cools faster than the sea — the root of the monsoon |
| Distance from the sea | Coasts have equable climates; the interior has extremes (continentality) |
| Relief | Windward slopes get heavy rain; leeward sides lie in rain shadow |
| Upper-air circulation | Jet streams and pressure cells decide onset, strength and breaks |
How the south-west monsoon is set up
graph TD
A[Summer sun heats the Indian landmass and the Tibetan plateau] --> B[Intense low pressure over north-west India]
A --> C[Tibetan plateau heats the upper air]
C --> D[Sub-tropical westerly jet shifts north of the Himalayas]
C --> E[Tropical Easterly Jet develops over the Peninsula]
B --> F[ITCZ shifts north over the Ganga plain as the monsoon trough]
G[Mascarene High east of Madagascar] --> H[South-east trades cross the equator]
H --> I[Coriolis force turns them into the south-west monsoon]
F --> I
D --> I
E --> I
I --> J[Arabian Sea branch and Bay of Bengal branch]NCERT lists the main elements of the mechanism:
- Differential heating of land and sea creates a low over the Indian landmass and highs over the surrounding seas.
- Shift of the ITCZ: in summer the Inter-Tropical Convergence Zone moves north and settles over the Ganga plain as the monsoon trough.
- The Mascarene High: a high-pressure area east of Madagascar, around 20°S in the Indian Ocean, whose intensity and position affect the monsoon.
- Heating of the Tibetan plateau in summer produces strong rising air currents over the plateau, which help organise the upper-air circulation of the monsoon.
- Jet streams: the sub-tropical westerly jet moves north of the Himalayas, and the Tropical Easterly Jet develops over the Peninsula in summer.
- ENSO and the Indian Ocean Dipole modulate the strength of the season from year to year.
The south-east trade winds of the southern hemisphere cross the equator, are deflected to the right by the Coriolis force, and arrive as the south-west monsoon.
Onset, advance and withdrawal (IMD normal dates, revised in 2020)
| Event | Normal date |
|---|---|
| Onset over Kerala | 1 June |
| Covers the whole country | 8 July |
| Withdrawal begins from north-west India | 17 September |
| Withdrawal from the whole country | 15 October |
The long period average (LPA) of all-India June–September rainfall, based on 1971–2020, is 87 cm (IMD). IMD calls a season normal when rainfall is 96–104% of the LPA, below normal at 90–95%, deficient below 90%, above normal at 105–110% and excess above 110%.
The two branches
| Branch | Path and effects |
|---|---|
| Arabian Sea branch | Strikes the Western Ghats — heavy orographic rain on the windward western slopes, rain shadow on the Deccan to the east; moves north along the west coast and into the interior |
| Bay of Bengal branch | Moves towards Myanmar and the north-east; is deflected west along the Himalayas into the Ganga plain; the funnel-shaped Khasi hills give Mawsynram and Cherrapunji some of the highest rainfall in the world |
Two famous consequences:
- The Tamil Nadu coast stays relatively dry in the south-west monsoon — it lies parallel to the Bay of Bengal branch and in the rain shadow of the Arabian Sea branch. It gets most of its rain in October–December from the north-east monsoon and retreating-monsoon cyclones.
- Rainfall declines westward across the northern plains (from the Bengal delta towards Punjab and Rajasthan), and eastward across the Peninsula from the west coast.
Breaks in the monsoon: wet spells alternate with dry spells. A break occurs when the monsoon trough shifts north to the Himalayan foothills — the plains go dry while the foothills and the north-east receive heavy rain.
ENSO and the Indian Ocean Dipole
| Phenomenon | What happens | Usual effect on the Indian monsoon |
|---|---|---|
| El Niño | Abnormal warming of the central and eastern equatorial Pacific; the Walker circulation weakens and shifts | Tends to weaken the monsoon (2023 is a recent example) |
| La Niña | Abnormal cooling of the same waters | Tends to favour a good monsoon |
| Southern Oscillation | See-saw of air pressure between the eastern Pacific (Tahiti) and the western Pacific / Indian Ocean (Darwin) | A negative Southern Oscillation Index accompanies El Niño |
| Positive IOD | Western Indian Ocean warmer than the eastern Indian Ocean | Tends to support the monsoon and can offset El Niño |
| Negative IOD | Eastern Indian Ocean warmer than the western | Tends to weaken the monsoon |
These are tendencies, not rules — India has had good monsoons in El Niño years and weak ones in La Niña years.
The four seasons (IMD)
| Season | Months | Highlights |
|---|---|---|
| Cold weather | December–February | Clear skies, low temperatures in the north; western disturbances from the Mediterranean bring rain and snow to the north-west (valuable for rabi wheat); north-east monsoon rain in Tamil Nadu |
| Hot weather | March–May | Heat waves; the loo in the northern plains; pre-monsoon storms (below) |
| South-west monsoon | June–September | Bulk of India’s annual rainfall |
| Retreating monsoon | October–November | Clear skies and high humidity — “October heat”; cyclones from the Bay of Bengal hit the east coast |
Pre-monsoon local storms and winds (hot weather season):
| Name | Region | Significance |
|---|---|---|
| Mango showers | Kerala and coastal Karnataka | Help mangoes ripen early |
| Blossom showers | Kerala and nearby areas | Make coffee flowers blossom |
| Kalbaisakhi (Nor’westers) | West Bengal, Assam | Violent evening thunderstorms; useful for tea, jute and rice; called “Bardoli Chheerha” in Assam |
| Loo | Northern plains (Punjab to Bihar) | Hot, dry, gusty afternoon winds |
Climatic regions (Köppen, as applied to India by NCERT) — selected
| Code | Type | Where |
|---|---|---|
| Amw | Monsoon with short dry season | West coast south of Goa |
| As | Monsoon with dry summer | Coromandel coast of Tamil Nadu |
| Aw | Tropical savanna | Most of the Peninsular plateau |
| BWhw | Hot desert | Extreme western Rajasthan |
| Cwg | Monsoon with dry winter | Ganga plain |
Memory Anchors
NCERT’s six monsoon drivers: “Don’t Ignore My Tired Jet Engine” → Differential heating · ITCZ shift · Mascarene High · Tibetan heating · Jet streams · ENSO/IOD.
IMD normal dates: “1-8, 17-15” → onset Kerala 1 June; all-India 8 July; withdrawal starts 17 September; complete 15 October.
El Niño vs positive IOD: El Niño is a Pacific “thief” of Indian rain; a positive IOD is an Indian Ocean “friend”.
One-line Mains hook: “The Indian budget is a gamble on the monsoon” — the old saying shows why rainfall variability is an economic, not merely a meteorological, question.
For Prelims
- In summer, the sub-tropical westerly jet moves north of the Himalayas and the Tropical Easterly Jet forms over the Peninsula.
- The ITCZ in July lies over the Ganga plain as the monsoon trough.
- The Mascarene High lies east of Madagascar in the southern Indian Ocean.
- Tamil Nadu gets most of its rainfall from October to December.
- Western disturbances originate over the Mediterranean region and bring winter rain to north-west India.
- Kalbaisakhi — Bengal and Assam; mango showers — Kerala and coastal Karnataka; blossom showers — coffee.
- IMD’s LPA for 1971–2020 is 87 cm; normal monsoon = 96–104% of LPA.
For Mains
- Dimensions: physical (mechanism and theories); economic (kharif output, rural demand, food inflation, hydropower, reservoir storage); social (drought, distress migration); disaster (floods, cloudbursts, heat waves, cyclones); climate change (more extreme rain events, longer dry spells).
- Keywords: differential heating, ITCZ, monsoon trough, Tropical Easterly Jet, teleconnection, ENSO, IOD, MJO, break in monsoon, rainfall variability, rain shadow.
- Value-addition: IMD’s revised normal onset and withdrawal dates (2020); the 2023 El Niño season (94% of LPA; driest August since 1901); the classical thermal theory (Halley) versus the dynamic (ITCZ / jet stream) view.
Common Traps
In the News
- 2023 — El Niño year; IMD records June–September rainfall at about 94% of LPA and the driest August since 1901.
- May 2025 — the monsoon set in over Kerala on 24 May, about a week ahead of the normal date of 1 June.
- See monthly updates under Current Affairs and the Climate Change chapter.
Practice
Prelims
Consider the following statements regarding the south-west monsoon:
- In summer, the sub-tropical westerly jet stream shifts to the north of the Himalayas.
- The Tropical Easterly Jet develops over peninsular India during the summer months.
- The intensification of the high-pressure area east of Madagascar is irrelevant to the Indian monsoon.
Which of the statements given above is/are correct?
Solution
Statements 1 and 2 are correct — the westerly jet’s northward shift and the Tropical Easterly Jet over the Peninsula are both part of the summer set-up. Statement 3 is wrong — NCERT lists the high-pressure area east of Madagascar (the Mascarene High) among the factors that affect the monsoon.
Answer: C
The Tamil Nadu coast receives relatively little rainfall during the south-west monsoon season. Which one of the following best explains this?
Solution
The Coromandel coast runs parallel to the Bay of Bengal branch of the monsoon, so the winds blow along it rather than onto it, and it lies in the rain shadow of the Arabian Sea branch, which sheds its moisture on the Western Ghats. Western disturbances affect north-west India in winter; there is no cold current or jet blocking along this coast.
Answer: A
Consider the following pairs:
| Local weather phenomenon | Region / effect |
|---|---|
| 1. Mango showers | Kerala and coastal Karnataka |
| 2. Kalbaisakhi | West Bengal |
| 3. Loo | Northern plains |
| 4. Blossom showers | Flowering of coffee |
How many of the pairs given above are correctly matched?
Solution
All four are correct. Mango showers are pre-monsoon showers on the Kerala and Karnataka coasts; Kalbaisakhi are violent evening thunderstorms of Bengal (and Assam); the loo is the hot, dry wind of the northern plains in May–June; blossom showers help coffee plants flower.
Answer: D
Mains
Explain the mechanism of the Indian south-west monsoon. How do El Niño and the Indian Ocean Dipole influence its year-to-year variability?
Solution
Introduction: The south-west monsoon is a seasonal reversal of winds that brings the bulk of India’s annual rainfall between June and September. Its onset, strength and breaks are governed by surface and upper-air circulation.
Body — mechanism:
- Differential heating: intense summer heating creates a low over north-west India; the seas remain relatively high-pressure areas.
- ITCZ shift: the convergence zone moves north over the Ganga plain as the monsoon trough, pulling in the south-east trades, which cross the equator and turn south-west.
- Tibetan plateau and jet streams: heating of the plateau helps push the westerly jet north of the Himalayas and creates the Tropical Easterly Jet over the Peninsula — both aid onset.
- Mascarene High east of Madagascar drives cross-equatorial flow.
- Relief: the Western Ghats and the north-eastern hills produce orographic rain; the Deccan and Tamil Nadu coast lie in rain shadow.
Body — variability:
- El Niño: warming of the eastern Pacific weakens the Walker circulation and tends to suppress Indian rainfall (2023: about 94% of LPA).
- La Niña: tends to favour a good monsoon.
- IOD: a positive IOD tends to support the monsoon and can offset El Niño; a negative IOD tends to weaken it.
- Intra-seasonal factors such as the MJO cause active and break spells.
Conclusion: The monsoon is a coupled land–ocean–atmosphere system. Better forecasting of ENSO and IOD, crop contingency plans and water storage are essential to reduce the economy’s exposure to its variability.
Quick Revision
| Point | One-liner |
|---|---|
| Monsoon | Seasonal reversal of winds (Arabic mausim) |
| Mechanism | Differential heating, ITCZ shift, Mascarene High, Tibetan heating, jet streams, ENSO/IOD |
| IMD normal dates | Kerala 1 June; all-India 8 July; withdrawal 17 Sept – 15 Oct |
| LPA | 87 cm (1971–2020); normal = 96–104% |
| Tamil Nadu | Rain mainly October–December |
| Western disturbances | Mediterranean origin; winter rain in north-west India |
| El Niño / positive IOD | Weakens / supports the monsoon (tendencies) |
| Local storms | Mango showers (Kerala), Kalbaisakhi (Bengal), loo (north) |
Key Takeaways
- The monsoon results from differential heating plus the northward shift of the ITCZ and the jet streams — not from heating alone.
- ENSO and the IOD drive year-to-year variability; their effects are probabilistic, not certain.
- Regional exceptions (Tamil Nadu’s winter rain, rain shadows, western disturbances) are favourite Prelims material.
Related Topics
- Indian Climate and the Monsoon — quick revision
- Indian Climate and the Monsoon — practice questions
- Climatology
- Agricultural Geography
- Climate Change