Indian Monsoons

Introduction to Monsoons

The word monsoon derives from the Arabic word mausin (or Malay monsin), meaning season. It describes a phenomenon where winds completely reverse direction between summer and winter.

• Monsoons are periodic or secondary winds that reverse direction with seasonal change. They can be understood as land and sea breezes on a continental scale, or convection cells operating at a planetary level.
• They form a double system of seasonal winds:
  • Summer (SW Monsoon): Sea → Land | Brings heavy rainfall (June–September)
  • Winter (NE Monsoon): Land → Sea | Relatively dry (October–December)
• Monsoons are most pronounced over the Indian Subcontinent compared to any other region on Earth. The unique combination of the Himalayas, the Tibetan Plateau, the Indian Ocean, and the ITCZ makes India's monsoon the world's strongest.
• Other affected regions: Southeast Asia, parts of Central-West Africa, northern Australia.
• India, Bangladesh, Myanmar, and Indonesia receive most annual rainfall from the SW Monsoon; South-East China and Japan receive it from the NE Monsoon.

UPSC Prelims 2014: The seasonal reversal of winds is the typical characteristic of: (c) Monsoon climate ✓

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Factors Responsible for Monsoon Formation

Factors Responsible for SW Monsoon Formation

1. Intense heating of the Tibetan Plateau (elevation ~4,500 m) during April–May — acts as a heat engine, creating an intense low-pressure cell.
2. Permanent high-pressure cell (Mascarene High) in the South Indian Ocean (east-northeast of Madagascar in summer) — acts as the source of moisture-laden winds.
3. Differential heating between the Indian landmass and the surrounding Indian Ocean (land heats faster than water).

Factors Influencing the Onset of SW Monsoons

(In addition to the above factors)

• Subtropical Jet Stream (STJ) — its northward shift across the Himalayas triggers monsoon onset.
• Tropical Easterly Jet (African Easterly Jet / Somali Jet) — a low-level jet stream that feeds moisture into the monsoon cell.
• Inter-Tropical Convergence Zone (ITCZ) — its northward migration to ~20°–25°N is essential for onset.

Factors Influencing the Intensity of SW Monsoons

Factors Responsible for NE Monsoon Formation

• Formation and strengthening of high-pressure cells over Tibetan and Siberian Plateaus in winter.
• Westward migration and weakening of Mascarene High in the Southern Indian Ocean.
• Southward migration of ITCZ to south of India (October–November).

UPSC Prelims 1996: High temperature and low pressure over the Indian subcontinent during summer draws air from the Indian Ocean leading to: (b) South-west monsoon ✓

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Theories of Monsoon Mechanism

1. Classical Theory — Sir Edmund Halley (17th Century)

Edmund Halley (1686) proposed that the monsoon is essentially a large-scale land and sea breeze driven by differential heating between land and sea.

• Summer: Land heats up faster → low pressure over land → ocean winds blow inland (SW Monsoon).
• Winter: Land cools faster → high pressure over land → winds blow seaward (NE Monsoon).

Limitations:

• Cannot explain the sudden burst of monsoons.
• Does not explain variability and breaks in monsoon.
• Cannot account for the role of jet streams, ITCZ, or upper-tropospheric dynamics.
• Monsoons do not develop uniformly everywhere on Earth despite similar differential heating.

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2. Air Mass Theory — Role of ITCZ

This theory explains monsoon as a modification of planetary trade winds through the seasonal migration of the ITCZ.

Mechanism:

• The Inter-Tropical Convergence Zone (ITCZ) is where SE Trade Winds (Southern Hemisphere) and NE Trade Winds (Northern Hemisphere) converge.
• ITCZ is characterised by ascending air, maximum cloud cover, and heavy rainfall.
• Its location shifts north and south of the equator with the seasons.

Summer (SW Monsoon) process:

1. Sun moves over Tropic of Cancer → ITCZ shifts northward to ~20°–25°N over the Indo-Gangetic Plain.
2. SE Trade Winds of Southern Hemisphere cross the equator.
3. Under Coriolis force, they deflect to blow southwest to northeast → become SW Monsoon Winds.
4. The Monsoon Front / Monsoon Trough (ITCZ over India) is a zone of heavy rainfall.

Seasonal Labels:

• NITCZ (Northern ITCZ) — July, over Indo-Gangetic Plain → brings heavy monsoon rainfall.
• SITCZ (Southern ITCZ) — January, south of India → associated with dry winter.

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3. Modern Theory — Jet Stream Theory (Most Accepted)

The most scientifically accepted theory. Explains monsoon through upper tropospheric dynamics — primarily the Subtropical Jet Stream (STJ).

Pre-Monsoon Phase (March–May):

• STJ blows south of the Himalayas, maintaining a subtropical high-pressure belt over NW India.
• This high pressure blocks the development of low pressure needed for monsoon onset.

Onset Trigger (Late May – Early June):

• With the sun moving toward Tropic of Cancer, the STJ shifts northward across the Himalayas to over the Tibetan Plateau (approximately June 1–20).
• This removes the blocking high pressure from India.
• A Tropical Easterly Jet (TEJ) forms in the upper troposphere over Indian plains (first week of June) — it creates:
  • Anticyclonic conditions in upper troposphere
  • Cyclonic (low pressure) conditions at surface
• Combined with the deep Tibetan Low, this rapidly develops the subcontinental monsoon cell.
• The cell operates between:
  • High Pressure: Mascarene High (South Indian Ocean, ~40°S–60°E)
  • Low Pressure: Tibetan Plateau (~30°N)

Rainy Season (June–September):

• The Somali Jet (low-level jet stream over Arabian Sea) feeds warm, moist air into the monsoon cell.
• The TEJ fluctuates, causing spatial variation in rainfall intensity.

End of Monsoon (August–October):

• Tibetan Plateau begins cooling → STJ transitions back south of Himalayas.
• Subtropical high-pressure belt returns to Indian plains.
• Sinking air masses replace the convective cells → monsoon withdraws.

How do Jet Streams affect surface weather? Jet streams develop ridges (warm air pushes into cold) and troughs (cold air drops into warm). Surface below a trough = low pressure (cyclonic). Surface below a ridge = high pressure (anticyclonic).

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Seasonal Rhythm of Indian Monsoons

March–May: Pre-Monsoon / Hot Weather Season

• Solar heating of the Indian subcontinent and Tibetan Plateau intensifies.
• Subtropical High-Pressure Belt occupies NW India, blocking monsoon development.
• STJ still south of Himalayas — monsoon onset inhibited.
• Hot, dry conditions with dust storms (andhi) in NW India.
• Pre-monsoon showers (mango showers) along the Western Ghats and Kerala coast.
• Nor'westers (Kalbaisakhi) — violent thunderstorms in West Bengal and Assam.
• Loo — hot, dry westerly winds in Punjab, Haryana, UP.

June: Onset of SW Monsoon

• STJ shifts north of Tibetan Plateau.
• TEJ establishes over Indian Plains.
• Monsoon cell develops rapidly (within days).
• Normal onset over Kerala: ~June 1 (IMD revised normal date).

Recent Kerala Onset Dates:

Important: There is no direct relationship between the onset date of monsoon over Kerala and the total seasonal rainfall over India. A late onset does not necessarily mean a deficient monsoon.

June–September: SW Monsoon Season (Advancing)

• SW Monsoon advances over the entire country in two branches:
  • Arabian Sea Branch (western/northern India)
  • Bay of Bengal Branch (northeastern/central India)
• Progression takes 6–8 weeks to cover all of India.
• Normal date of coverage: Entire India by mid-July.

October–December: NE Monsoon / Retreating Monsoon

• SW Monsoon withdraws from NW India first (September 1 — normal).
• Withdraws from the entire country by late November/early December.
• NE Monsoon establishes — brings rainfall to Tamil Nadu, coastal Andhra Pradesh.
• Tamil Nadu receives 48–50% of its annual rainfall from the NE Monsoon.

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SW Monsoon: Two Branches

Arabian Sea Branch

• Strongest and moisture-laden of the two branches.
• Splits into three sub-branches:
  1. Western Ghats branch — strikes the windward western slopes; heavy rainfall (>250 cm); Cherrapunji, Mawsynram, and Agumbe type; rain shadow effect on the eastern side.
  2. Gujarat branch — moves along the Narmada and Tapi valleys into the interior; turns NE into Rajasthan/Punjab.
  3. Oman Sea branch — crosses the Arabian Sea toward Pakistan/Rajasthan; weakest sub-branch.
• The Somali Jet (Findlater Jet) energises the Arabian Sea Branch.

Bay of Bengal Branch

• Strikes the Myanmar and Assam coast first; deflected westward by the Himalayan arc.
• Moves up the Ganga valley toward the northwest.
• Orographic rain on the Meghalaya plateau (Mawsynram — world's highest rainfall).
• Meets the Arabian Sea branch over the Punjab plains.

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Break in Monsoon

• Periods of weak or no rainfall over plains during the rainy season, even though the monsoon has onset.
• Caused by:
  1. Northward shift of the ITCZ / Monsoon Trough toward the Himalayan foothills.
  2. Weakening of the Somali Jet.
  3. Strengthening of the High Pressure over the Bay of Bengal.
• During a break, heavy rainfall occurs in the Himalayan foothills and northeastern India (trough moves north).
• Prolonged breaks → drought conditions in peninsular and central India.

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Variability of Monsoon — El Niño, IOD, and La Niña

El Niño–Southern Oscillation (ENSO)

• El Niño: Anomalous warming of the central/eastern Pacific Ocean.
  • Causes eastward shift of Walker Circulation → sinking air over India → below-normal monsoon.
  • ~60% of El Niño years see deficient Indian monsoon rainfall.
• La Niña: Anomalous cooling of central/eastern Pacific.
  • Strengthens Walker Circulation → enhanced convection over India → above-normal or normal monsoon.
• Neutral ENSO: No significant influence; IOD and other factors dominate.

2023-24 El Niño (One of Top-5 Strongest on Record):

• Peaked in winter 2023-24; weakened by early 2024.
• August 2023 rainfall: 36% deficit — driest single month since 2018.
• Led to: rice export restrictions, 40% duty on onion exports, duty-free pulse imports.

2024 SW Monsoon (Post El Niño):

• El Niño dissipated → monsoon recovered strongly.
• Overall: 108% of LPA (934.8 mm against LPA of 868.6 mm, 1971-2020 base).
• Annual rainfall 2024: 104% of normal (1,206.6 mm vs normal 1,160 mm).
• July 2024: 109% | August 2024: 116% | September 2024: 111% of LPA.
• 78% of districts received normal/excess/large excess — best performance in over a decade.
• Only 3 meteorological sub-divisions had deficient rainfall (lowest in years).
• Extreme events: 2,632 instances of very-heavy rainfall + 473 extremely-heavy rainfall events — highest in 5 years (source: IMD Monsoon Report 2024).
• Regional disparity: Central India: +19% departure; East & NE India: −13% (shortfall).

2025 SW Monsoon (confirmed, IMD):

• IMD April 2025 forecast: 105% of LPA (above normal).
• Actual outcome: 107.9% of LPA (937.2 mm vs LPA 868.6 mm) — 5th highest since 2001, 38th highest since 1901.
• Regional variation: NW India 127% | Central India 115% | South Peninsula 110% | NE India 80% (shortfall).
• Monsoon core zone (rain-fed agriculture belt): 22% above LPA — excellent for kharif crops.
• Monthly: June 109% | July 105% | August 105% | September 115% of LPA.
• 14 of 18 monsoon weeks had excess or large excess rainfall.
• Second consecutive year of above-normal monsoon (after 2024's 108%).
• Neutral ENSO conditions prevailed during the season.

2026 Context (as of April 2026):

• Neutral ENSO expected for 2026 SW Monsoon season.
• IMD monitoring La Niña decay and possible ENSO transition.

Indian Ocean Dipole (IOD)

• Positive IOD: Warmer Sea Surface Temperature (SST) in western Indian Ocean, cooler in eastern Indian Ocean.
  • Weakens Indian monsoon — reduces moisture flow.
  • Associated with droughts in Indonesia and Australia.
• Negative IOD: Cooler western, warmer eastern Indian Ocean.
  • Strengthens Indian monsoon — associated with flood years in India.

2023: ~80% probability of positive IOD predicted (June–August 2023) — contributed alongside El Niño to deficit rainfall. 2024: Positive IOD re-emerged in latter half of 2024 — back-to-back positive IOD, first occurrence since 1960; partial offset by post-El Niño recovery.

IOD vs ENSO: IOD is an independent variable but often acts synergistically with ENSO. A combination of El Niño + positive IOD = most severe monsoon deficits.

La Niña 2024-25

• Expected onset: Late 2024 / early 2025 — observed delayed onset (October–December 2024).
• No significant impact on 2025 SW Monsoon — La Niña weakened/dissipated before the season.
• If a full La Niña establishes before monsoon season: above-normal rainfall likely; extended winters in Punjab, Haryana, Rajasthan.

Walker Circulation and Climate Change (Updated 2026)

• A warmer climate is causing the Walker Circulation to bring more persistent high pressure over India even during non-El Niño years.
• ENSO–Monsoon teleconnection is weakening with rising CO2 concentrations — making monsoon prediction harder.
• Risk of unprecedented monsoon failures (40–70% rainfall reduction) may increase in future decades.
• Key concern: Climate change may increase year-to-year variability even as the mean monsoon stays stable.

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North-East Monsoon (Retreating Monsoon)

• After SW Monsoon withdraws, the ITCZ shifts south of India and high pressure develops over the Tibetan and Siberian Plateaus.
• NE Trade Winds blow from land to sea (NE → SW).
• These winds pick up moisture while crossing the Bay of Bengal → bring rainfall to Tamil Nadu, Sri Lanka, and coastal Andhra Pradesh.
• Season: October–December.

Rainfall by NE Monsoon:

• Tamil Nadu: ~48–50% of annual rainfall from NE Monsoon.
• Coastal Andhra Pradesh (south): 30–35%.
• Rest of India: Very little to negligible.

Why NE Monsoon brings rain only to Southeast India? NE Monsoon winds blow from land → they are mostly dry. Only when they travel over the Bay of Bengal (sufficient fetch) do they pick up moisture. Tamil Nadu coast is at a right angle to these winds, allowing orographic and cyclonic rainfall.

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Annual Rainfall Distribution in India

Rainfall Zones

Wettest places:

• Mawsynram, Meghalaya — officially world's wettest place (~11,871 mm annual average). [Replaced Cherrapunji as record holder]
• Cherrapunji (Sohra): ~11,430 mm average; holds world record for most rainfall in a single month (9,296 mm in July 1861).

Driest places:

• Leh, Ladakh: ~8–10 mm annual rainfall (cold desert, rain shadow of Himalayas + Karakoram).
• Jaisalmer, Rajasthan: ~100 mm (hot desert).

Spatial Variability

• Rainfall generally decreases from the coasts toward the interior.
• Western coast (windward side of Western Ghats): Very high rainfall.
• Eastern coast (leeward side): Low rainfall during SW Monsoon; compensated by NE Monsoon.
• Rain shadow zone: Deccan Plateau, areas east of Western Ghats receive significantly lower rainfall.

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UPSC Corner

High-Frequency Prelims Topics

• Mausin (Arabic) / Monsin (Malay) = origin of word 'Monsoon'
• Monsoon trough = ITCZ over India during July
• SW Monsoon onset over Kerala = ~June 1 (revised normal)
• NE Monsoon = also called Retreating Monsoon
• Findlater Jet = Somali Jet = same thing (UPSC uses both)
• Mascarene High = source high-pressure cell for SW Monsoon
• Mawsynram = wettest place (not Cherrapunji for average; Cherrapunji for records)
• Purvaiya (Bhojpuri) = local name for SW Monsoon in eastern UP/Bihar region

UPSC Mains GS1 Angles

1. "Critically analyse the role of the Tibetan Plateau in the Indian Monsoon mechanism."
2. "How does El Niño affect the Indian Monsoon? Discuss with recent examples." [Use 2023 data]
3. "What is the Indian Ocean Dipole? How does it interact with ENSO to affect Indian rainfall?"
4. "Why is the South-West Monsoon called 'Purvaiya' in the Bhojpur region? How has it influenced the cultural ethos?" [UPSC 2023]
5. "Climate change is altering Indian monsoon patterns. Examine." [Use Walker Circulation weakening data]

UPSC Prelims PYQ Answers

MCQ Trap Awareness

• Trap: "Cherrapunji is the wettest place in India" → Incorrect for average rainfall (Mawsynram holds that title). Cherrapunji holds single-month and single-year records.
• Trap: "El Niño always causes drought in India" → Incorrect (~60% of El Niño years are deficit; not all).
• Trap: "NE Monsoon affects all of peninsular India" → Incorrect — primarily Tamil Nadu and coastal Andhra Pradesh.
• Trap: "The monsoon trough and ITCZ are different things" → They are the same phenomenon — ITCZ when over India in July is called the monsoon trough.
• Trap: "Late Kerala onset means deficient total rainfall" → No direct relationship (IMD confirmed).

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Current Affairs 2023–2026

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Summary: Key Facts at a Glance

• Monsoon = seasonal reversal of winds; word from Arabic mausin = season
• SW Monsoon: June–September | NE Monsoon: October–December
• Normal onset over Kerala: ~June 1
• India LPA (1971–2020): 868.6 mm
• SW Monsoon 2024: 108% LPA (934.8 mm)
• SW Monsoon 2025: 108% LPA
• Wettest place (average): Mawsynram, Meghalaya (~11,871 mm)
• Driest mainland area: Jaisalmer, Rajasthan (~100 mm)
• Tamil Nadu NE Monsoon share: ~48–50% of annual rainfall
• El Niño → deficit (60% cases) | La Niña → surplus | Positive IOD → deficit | Negative IOD → surplus
• Monsoon trough = ITCZ over India in July (~20°–25°N)