Monsoon in India: Mechanism, Types, Onset and Impact

The monsoon is India. Everything — crops, rivers, festivals, famines, floods, droughts, the entire agricultural economy — revolves around this seasonal reversal of winds. A good monsoon year means prosperity; a failed one means distress for millions.

For UPSC, the monsoon is one of the most frequently tested topics across Prelims, Mains, and Essay. You need to understand not just facts but the mechanism that drives it.

What Is a Monsoon?

"Monsoon" comes from the Arabic word mawsim meaning season. Technically, a monsoon is a seasonal reversal of wind direction — winds that blow from ocean to land in summer (bringing rain) and from land to ocean in winter (dry).

But India’s monsoon is more than just wind reversal. It’s a massive atmospheric circulation system driven by differential heating between the Asian landmass and the Indian Ocean, amplified by the Tibetan Plateau, and modulated by global phenomena like El Niño and the Indian Ocean Dipole.

Types of Monsoon in India

India experiences two monsoon systems:

The Southwest Monsoon: Complete Mechanism

The Southwest Monsoon is the dominant system. Understanding its mechanism is crucial for UPSC Mains.

Step-by-Step Mechanism

Step 1: Summer Heating of the Asian Landmass From April onwards, the Asian landmass — especially the Thar Desert and the Indian subcontinent — heats up rapidly under intense solar radiation. Land heats faster than the sea (lower specific heat capacity). By May-June, the Thar Desert region has temperatures exceeding 45–48°C.

Step 2: Development of Low Pressure over Northwest India This intense heating creates a thermal low-pressure system over the Thar Desert (Rajasthan, Sind) and extends across Pakistan. The low pressure is relatively dry — it's not a true cyclonic depression but a heat-induced trough.

Step 3: High Pressure over the Indian Ocean Meanwhile, the southern Indian Ocean (Mascarene High) maintains a subtropical high-pressure system. The temperature gradient between the hot land and cooler ocean creates a pressure gradient — air flows from high to low pressure, i.e., from the ocean toward India.

Step 4: ITCZ Shifts Northward The Inter-Tropical Convergence Zone (ITCZ) is the belt near the equator where the northeast and southeast trade winds converge. In winter it sits near the equator. In summer, as the Asian landmass heats up, the ITCZ shifts northward — sometimes to 20–25°N over India. This shift is critical: it brings the southern hemisphere's moisture-laden southeast trade winds across the equator.

Step 5: Cross-Equatorial Flow and Coriolis Deflection The southeastern trade winds from the South Indian Ocean cross the equator and enter the northern hemisphere. In the northern hemisphere, the Coriolis force deflects them to the right — converting them from southeast to southwest direction. This is now the Southwest Monsoon.

Step 6: Role of the Tibetan Plateau The Tibetan Plateau (average 4,500 m elevation) heats up in summer. This creates an upper-level anticyclone (high pressure) over Tibet around June-July. This high pressure helps pull the low-level monsoon flow northward into India, deepening the monsoon's penetration. The Tibetan Plateau is often called the "driving engine" of the Asian monsoon.

Step 7: Jet Streams

• The subtropical westerly jet stream flows over northern India in winter. In summer, as Tibet heats up, it shifts north of the Tibetan Plateau (~45°N). Its withdrawal from over India removes a blocking mechanism, allowing the monsoon to advance.
• The tropical easterly jet stream develops over peninsular India (~15°N) around late June-July, flowing from east to west at about 14 km altitude. This upper-level easterly helps maintain the monsoon circulation. Its strength correlates with monsoon intensity.
• The Somali Jet (or Findlater Jet) — a low-level jet stream over the Somali coast of East Africa — intensifies in summer, channelling moisture from the Arabian Sea toward India at high speed. It's one of the strongest low-level jets on Earth.

The Two Branches of the SW Monsoon

The Southwest Monsoon splits into two branches after entering India:

Arabian Sea Branch:

• Picks up moisture over the Arabian Sea
• First hits the Western Ghats (windward/western slope) — causes very heavy rainfall (2,000–4,000 mm)
• Leeward side (Deccan Plateau) is the rain shadow — much less rain (400–700 mm)
• Advances northward along the west coast
• Reaches Mumbai (~June 10), then Gujarat, Rajasthan
• Another arm crosses the Western Ghats and moves into central India

Bay of Bengal Branch:

• Picks up moisture over the Bay of Bengal
• Moves along the Myanmar-Bangladesh coast and penetrates northeastern India
• Gets deflected westward by the Himalayas (which act as a barrier)
• Moves westward across the Indo-Gangetic Plain
• Responsible for most rainfall in eastern and central India, UP, Bihar

The two branches merge over the central Indo-Gangetic Plain around late June.

Related: Seasons in India: Climate Patterns

Monsoon Onset: State-by-State

The onset (arrival) of the SW monsoon follows a predictable progression:

The meteorological agency (IMD) declares the onset when: (1) sustained rainfall of ≥2.5 mm/day for 2 consecutive days occurs; (2) specific OLR (outgoing longwave radiation) thresholds are met; (3) winds shift to southwest; (4) specific atmospheric conditions are satisfied.

2023 Monsoon Onset

In 2023, the monsoon arrived in Kerala on June 8 — a week late. By the time it covered all of India, it was running below normal. The 2023 season ended as below-normal overall — partly linked to the El Niño developing in the Pacific.

Monsoon Withdrawal

The retreat of the SW monsoon is slower than the onset.

The withdrawal leaves behind the post-monsoon season and ushers in the Northeast Monsoon.

The Northeast Monsoon

When the SW Monsoon retreats from the Indian subcontinent, the ITCZ shifts southward and high pressure re-establishes over the Asian landmass. Now winds flow from the land toward the sea — northeast trade winds.

These northeast winds blow from the Tibetan Plateau and Central Asia southward and southwestward toward the Bay of Bengal. As they cross the Bay of Bengal, they pick up moisture and dump it on the southeastern coast of India — particularly Tamil Nadu.

Tamil Nadu's monsoon season: October–December, with peak rainfall in November.

Key facts about the NE Monsoon:

• Covers Tamil Nadu, Puducherry, Andhra Pradesh coast, Kerala (partly)
• Chennai receives ~55% of its annual rainfall from the NE Monsoon
• 2015 Chennai floods — catastrophic flooding during NE Monsoon, among the worst in Indian history
• The Bay of Bengal is warmer in October–November, enhancing moisture pick-up by NE winds

Related: Longest Rivers in India

Rainfall Distribution During the SW Monsoon

India's monsoon rainfall is highly uneven — both spatially and temporally.

Highest Rainfall Areas

• Mawsynram (Meghalaya): ~11,871 mm/year — world's highest
• Cherrapunji (Meghalaya): ~11,777 mm/year — second highest
• Why: Bay of Bengal branch funnelled into the Khasi Hills valley, forcing orographic lifting
• Western coast: Mumbai (~2,200 mm), coastal Karnataka (~3,000–5,000 mm), Kerala (~3,000 mm)
• Northeastern states: High rainfall from both Bay of Bengal branch and local topography

Lowest Rainfall Areas

• Jaisalmer (Rajasthan): ~160 mm/year
• Leh (Ladakh): ~100 mm/year
• Western Rajasthan, interior Gujarat, rain shadow of Western Ghats (Pune gets ~700 mm vs. Mumbai's 2,200 mm, though they're ~150 km apart)

Rainfall Map Pattern

Isohyets (lines of equal rainfall) in India broadly show:

• Very high rainfall in northeast and west coast
• Decreasing rainfall toward the northwest and interior Deccan
• The rain shadow of the Western Ghats creates a marked gradient

Breaks in the Monsoon

The monsoon doesn't rain uniformly throughout June–September. There are "breaks" — 7–15 day periods when rainfall pauses or becomes deficient.

During a Break

• The ITCZ shifts northward to the Himalayan foothills
• Rainfall concentrates along the Himalayan foothills, northeast, and extreme south
• Central and peninsular India experience dry spells
• A break in July or August can be devastating for standing kharif crops

Active Spells vs. Break Spells

Extended breaks lasting more than 2 weeks in July are strongly correlated with below-normal monsoon years and agricultural distress.

El Niño, La Niña and the Indian Monsoon

These Pacific Ocean phenomena have profound impacts on Indian rainfall.

El Niño

El Niño = Anomalous warming of the central and eastern Pacific Ocean near the equator.

Mechanism of impact on India:

• Warmer eastern Pacific weakens the east-west temperature gradient (Walker Circulation)
• The Walker Circulation normally drives moisture westward toward India; El Niño weakens it
• The atmospheric pressure gradient between India and the Pacific weakens
• This results in weakened SW Monsoon — less moisture transport to India

Historical El Niño years with deficit monsoon:

• 1987: -19% deficit
• 2002: -19% deficit
• 2009: -23% deficit (worst in decades)
• 2014: -12% deficit
• 2023: Below-normal monsoon

Note: Not every El Niño year means drought. About 60–65% of strong El Niño events coincide with below-normal Indian monsoon.

La Niña

La Niña = Anomalous cooling of the central/eastern Pacific.

Impact on India:

• Strengthens the Walker Circulation
• Enhances moisture transport toward India
• Generally associated with above-normal monsoon rainfall
• Can cause flooding in India, Bangladesh, Nepal
• 2010: Extreme La Niña → severe flooding in Pakistan and India
• 2020: La Niña → above-normal monsoon

Indian Ocean Dipole (IOD)

The Indian Ocean Dipole is the sea surface temperature difference between the western Indian Ocean (Arabian Sea) and the eastern Indian Ocean (near Indonesia).

2019 case study: El Niño was active (usually negative for India), but a strong Positive IOD counteracted it. India received 110% of normal rainfall in 2019 — above normal despite El Niño.

Madden-Julian Oscillation (MJO)

The MJO is an intra-seasonal oscillation (30–90 day cycle) that moves eastward around the tropics. When the active phase of MJO is over the Indian Ocean (phases 2–3), it enhances Indian monsoon rainfall. When in the "break" phase, it suppresses rainfall.

IMD now uses MJO tracking for extended-range forecasts (2–3 weeks ahead).

Monsoon Forecasting

The India Meteorological Department (IMD) issues:

• Long-range forecast (April): Forecast for the full season rainfall as % of Long Period Average (LPA)
• Update (June): Refined forecast
• Monthly forecasts: Each month of the monsoon season
• Extended Range Forecasts: 2-week outlooks

Long Period Average (LPA): 87 cm (870 mm) — the average seasonal rainfall for 1971–2020 period.

Normal year definition: 96–104% of LPA Below normal: <96% of LPA Excess: >104% of LPA Deficient: <90% of LPA Large excess: >110% of LPA

Impact of Monsoon on Agriculture

The link between monsoon and agriculture is direct and enormous.

Kharif Crops (Monsoon Crops)

Planted at monsoon onset (June), harvested at monsoon end (September–November):

• Rice — India's primary food crop; requires 1,200–2,000 mm
• Cotton — rain-fed in black soil belt
• Sugarcane — planted in spring, irrigated with monsoon water
• Bajra (Pearl millet) — drier areas
• Jowar (Sorghum) — Deccan plateau
• Maize — Bihar, UP, Karnataka
• Groundnut — Gujarat, AP, Tamil Nadu
• Soybean — Madhya Pradesh, Maharashtra

A 10% deficit in seasonal rainfall can reduce kharif output by 5–8% and trigger food price inflation.

Rabi Crops (Winter Crops)

Planted November–December, harvested March–April:

• Wheat — requires Western Disturbances for moisture; benefits from residual soil moisture
• Mustard, Chickpea (chana) — drier areas
• Barley

Rabi crops depend less directly on the SW Monsoon but benefit from adequate soil moisture retained after the monsoon season.

Monsoon Deficit and the Economy

A severe monsoon deficit year (like 2002 or 2009) causes:

• Agricultural GDP decline — farm sector contracts
• Food inflation — prices of vegetables, cereals spike
• Rural distress — farm income falls; migration increases
• Power shortage — hydroelectric generation falls
• Water scarcity — reservoirs underfilled, affecting subsequent Rabi irrigation

India's overall GDP growth has a 0.3–0.5% correlation with monsoon performance — lower now than in the 1980s–90s due to higher irrigation coverage and services sector share.

Monsoon and Climate Change

Climate change is altering India’s monsoon in complex ways:

• Increased variability: More extreme events — intense bursts followed by prolonged dry spells
• Delayed onset: Monsoon onset has been irregular; significant early or late arrival years more common
• Shift toward extremes: More "extremely heavy rainfall" events (>100 mm/day) even as total seasonal rainfall remains similar or below normal
• Dry day frequency: The number of dry days during the monsoon season has increased in many regions, even when total rainfall is normal
• Arabian Sea warming: The Arabian Sea is warming faster than the Bay of Bengal — affecting the moisture content of the Arabian Sea branch of the monsoon
• Western Disturbances: Are now producing more extreme precipitation in winter over the Himalayas

The 2013 Uttarakhand cloudburst, 2018 Kerala floods, 2019 Maharashtra floods, 2020 Assam-Bihar floods — all point to more extreme monsoon behaviour.

Key Monsoon Facts Table

Related: Atmosphere: Layers and Composition

Frequently Asked Questions