The Indian Monsoon: Mechanism, Seasons and Variability

Why the monsoon is central to India

The word monsoon comes from the Arabic *mausim*, meaning season. It refers to a seasonal reversal of winds that brings most of India's rain in about four months. Roughly 70–75 per cent of India's annual rainfall arrives during the south-west monsoon (June to September). About half of India's net sown area is still rain-fed, so the monsoon affects crops, food prices, rural incomes, hydropower, drinking water and even the inflation targets of the Reserve Bank. UPSC asks about its mechanism, the roles of El Niño, the Indian Ocean Dipole and jet streams, the seasons, and the patterns of rainfall.

Factors that shape India's climate

• Latitude: the Tropic of Cancer passes through the middle of India, so the south is tropical and the north sub-tropical.
• The Himalayas block cold Siberian winds in winter and trap the monsoon winds, forcing them to rise and shed rain over the subcontinent.
• Distribution of land and water: the land heats up faster than the Indian Ocean in summer, creating a pressure contrast.
• Distance from the sea: coastal areas have an equable (maritime) climate, while interior places like Delhi have an extreme (continental) climate.
• Altitude and relief: hill stations are cooler; windward slopes of the Western Ghats get heavy rain while the leeward side lies in a rain-shadow.

Mechanism of the monsoon: the classic and modern views

Classical thermal view (Halley, 1686): in summer the land heats up, forming a low-pressure area over north-west India; winds blow from the high-pressure ocean towards the land, picking up moisture. In winter the pattern reverses. This explains the basic reversal but not the sudden "burst", the breaks, or year-to-year variation.

Modern view – the ITCZ and jet streams:

• Shift of the ITCZ: the Inter-Tropical Convergence Zone, a belt of low pressure where trade winds meet, moves north in summer to lie over the Indo-Gangetic plain (as the monsoon trough). The south-east trade winds of the southern hemisphere cross the equator, are deflected by the Coriolis force, and turn into the south-west monsoon.
• Tibetan Plateau heating: the plateau, about 4,000 m high, heats the middle troposphere and creates an upper-level high, which helps set up an easterly flow aloft.
• Jet streams: in winter the sub-tropical westerly jet stream lies south of the Himalaya and brings western disturbances to north-west India. In early June it shifts north of the Tibetan Plateau, and the tropical easterly jet develops over peninsular India – this shift coincides with the sudden onset of the monsoon.
• Somali jet: a strong low-level cross-equatorial current off East Africa that carries moisture towards India.

Ocean–atmosphere links

The four seasons (IMD classification)

1. Cold weather season (December–February): clear skies, low temperatures, north-east trade winds over land. Western disturbances from the Mediterranean bring winter rain and snow to the north-west, vital for rabi wheat. Tamil Nadu receives rain from the north-east monsoon.
2. Hot weather season (March–May): temperatures rise; the loo blows over the northern plains. Local storms: mango showers in Kerala and Karnataka (help ripen mangoes), cherry blossoms / coffee showers in Karnataka, Kalbaisakhi (Nor'westers) in Bengal and Assam, useful for tea, jute and rice.
3. South-west monsoon season (June–September): the monsoon reaches Kerala around 1 June and covers the whole country by about 8 July (IMD's revised normal dates). It has two branches:

• Arabian Sea branch – strikes the Western Ghats, giving very heavy rain on the windward side (Mumbai, Konkan) and a rain-shadow in the interior Deccan; another stream moves along the Narmada–Tapi and up to Gujarat and Rajasthan, where it gives little rain because the Aravallis lie parallel to it.
• Bay of Bengal branch – moves north-east to the Meghalaya hills, where the funnel-shaped Khasi hills produce the world's heaviest rainfall at Mawsynram and Cherrapunji (Sohra), and then turns west along the Himalaya into the Ganga plain, with rainfall decreasing from east to west.
• Tamil Nadu coast stays relatively dry in this season because it lies parallel to the Bay branch and in the rain-shadow of the Arabian Sea branch.

1. Retreating (post-) monsoon season (October–November): the monsoon trough weakens and withdraws from the north-west from about mid-September. The sky clears and humidity with heat produces "October heat". Low-pressure systems move to the Bay of Bengal, producing tropical cyclones that hit the east coast, and the north-east monsoon gives Tamil Nadu and south Andhra Pradesh most of their annual rain.

Breaks and variability

The monsoon does not rain continuously. When the monsoon trough shifts north to the foothills of the Himalaya, there are break spells – rain falls over the hills and the north-east while central India stays dry. Variability is high: IMD calls a year "normal" when rainfall is within 96–104 per cent of the Long Period Average (LPA), which is about 87 cm for 1971–2020. A deficit of more than 10 per cent across a large area is treated as an all-India drought year.

Climate change is making the pattern more uneven: studies show fewer moderate-rain days, more extreme rainfall events, longer dry spells and shifts in onset and withdrawal dates. This affects farming, flood risk and urban drainage.

Rainfall distribution and climatic regions

• Over 200 cm: western coast and Western Ghats, the north-east, Andaman and Nicobar.
• 100–200 cm: the eastern plateau, the middle Ganga plain, parts of Odisha and Bengal.
• 50–100 cm: the interior Deccan, the upper Ganga plain, eastern Rajasthan, Punjab and Haryana.
• Below 50 cm: western Rajasthan, Kachchh, Ladakh and the rain-shadow pockets of the Deccan.

Using the Köppen classification, India has climates such as Amw (monsoon with short dry season, Konkan), As (dry summer, wet winter on the Coromandel coast), Aw (tropical savanna, most of the peninsula), BWhw (hot desert, western Rajasthan), Cwg (monsoon with dry winter, Ganga plain) and Dfc / E (cold, high Himalaya).

The monsoon and the economy

A good monsoon raises kharif output (rice, pulses, oilseeds, cotton), refills reservoirs for rabi, boosts rural demand for goods and helps control food inflation. A weak monsoon does the reverse and hurts hydropower. This dependence is why India has invested in irrigation, crop insurance (PMFBY), weather-based advisories and better forecasting through the Monsoon Mission and supercomputers such as Arka and Arunika (installed in 2024–25).

Mains practice question

Explain the mechanism of the Indian monsoon. How do El Niño and the Indian Ocean Dipole influence its behaviour? (250 words)

Answer framework

• Intro: seasonal reversal; share of rainfall.
• Mechanism: differential heating, ITCZ shift, Coriolis, Tibetan plateau, jet streams, Somali jet.
• El Niño/La Niña and ENSO; positive IOD as offset (2019 example).
• Breaks, MJO, climate change.
• Conclusion: forecasting and adaptation.