How the Indian monsoon works (and why El Niño matters)

About three-quarters of India's annual rainfall arrives in four months, from June to September. Agriculture, reservoirs, electricity, inflation and rural incomes all depend on it. No wonder UPSC keeps asking how the monsoon works, why it fails in some years, and what faraway oceans have to do with it.

The monsoon is best understood in layers. Start with the simplest explanation, then add the pieces that explain its timing and its variability.

Layer 1: differential heating

The classical explanation, going back centuries, is differential heating of land and sea.

• In summer, the land heats up faster than the ocean. A low-pressure area develops over north-west India and Pakistan.
• The Indian Ocean stays relatively cooler, with higher pressure.
• Winds blow from high pressure over the sea to low pressure over the land, carrying moisture.
• In winter, the pattern reverses: winds blow from land to sea.

This is true as far as it goes, but it cannot explain why the monsoon arrives so suddenly, why it has breaks, or why it varies so much from year to year. For that, you need the modern understanding.

Layer 2: the shifting ITCZ

The Inter-Tropical Convergence Zone (ITCZ) is a belt of low pressure near the equator where the trade winds of the two hemispheres meet and air rises.

The ITCZ moves with the sun. In the northern summer, over the Indian subcontinent it shifts far north — to around 20–25°N, lying over the Indo-Gangetic plain. There it is called the monsoon trough.

As the ITCZ moves north, the south-east trade winds of the southern hemisphere cross the equator. The Coriolis force deflects them to the right in the northern hemisphere, so they arrive over India as south-westerly winds — the south-west monsoon.

Layer 3: the Tibetan plateau and the jet streams

The upper atmosphere matters as much as the surface.

• The Tibetan plateau heats intensely in summer, acting as an elevated heat source. This helps create high pressure in the upper troposphere over Tibet.
• The sub-tropical westerly jet stream flows south of the Himalaya in winter. In early summer it shifts north of the Himalaya. This shift is closely associated with the sudden onset — the "burst" — of the monsoon.
• The tropical easterly jet develops over peninsular India in summer, flowing from east to west in the upper troposphere. Its strength is linked to the monsoon's vigour.
• The Somali jet (also called the Findlater jet), a strong low-level wind off the coast of East Africa, carries moisture across the Arabian Sea towards India.

Onset, advance and withdrawal

• The monsoon's normal onset over Kerala is around 1 June.
• It advances in two branches:
  • the Arabian Sea branch, which hits the Western Ghats and moves north along the west coast;
  • the Bay of Bengal branch, which moves towards the north-east and is deflected west along the Himalaya into the Indo-Gangetic plain.
• By about the first half of July, it normally covers the whole country.
• Withdrawal begins from north-west India in September and is usually complete by mid-October.

The India Meteorological Department (IMD) revised its normal onset and withdrawal dates in 2020, so check its latest figures if a question depends on exact dates.

Breaks in the monsoon

The monsoon does not rain continuously. There are active spells and breaks. A break occurs when the monsoon trough shifts north to the Himalayan foothills: rainfall then decreases over most of the country but increases over the foothills and the north-east. Long breaks during crucial crop stages can cause serious damage even in a year of normal total rainfall.

Why the monsoon varies: ocean connections

El Niño and La Niña (ENSO)

• El Niño — an unusual warming of sea surface temperatures in the central and eastern equatorial Pacific. It shifts the zones of rising air and tends to weaken the Indian monsoon. Many (though not all) El Niño years have seen below-normal monsoon rainfall in India.
• La Niña — the opposite: cooler eastern Pacific waters, often associated with good monsoon rainfall.
• The atmospheric counterpart is the Southern Oscillation — a see-saw of pressure between the western and eastern Pacific. Together they are called ENSO.

The Indian Ocean Dipole (IOD)

The IOD is a difference in sea surface temperature between the western and eastern Indian Ocean.

• A positive IOD (warmer west, cooler east) tends to support the Indian monsoon.
• A negative IOD tends to weaken it.

A positive IOD can partly offset an El Niño. This is one reason the relationship between El Niño and Indian rainfall is not one-to-one.

The Madden–Julian Oscillation (MJO)

The MJO is an eastward-moving pulse of cloud and rainfall that travels around the tropics every 30 to 60 days or so. When it is over the Indian Ocean, it can enhance monsoon rainfall; its position influences active spells and breaks.

Measuring a "normal" monsoon

IMD compares seasonal rainfall with the Long Period Average (LPA) — the average rainfall over a fifty-year reference period. Based on the 1971–2020 period, the LPA for the June–September season is about 87 cm. Rainfall between 96 and 104 per cent of the LPA is classified as "normal".

The retreating monsoon and the north-east monsoon

As the monsoon withdraws in October–November, winds reverse and blow from the north-east. Crossing the Bay of Bengal, they pick up moisture and bring rain to Tamil Nadu and adjoining areas, which receive much of their annual rainfall in this season. This period is also a peak season for cyclones in the Bay of Bengal.

Practice questions

1. Prelims-type: Which of the following tends to strengthen the Indian monsoon — a positive IOD, El Niño, La Niña? What is the normal date of monsoon onset over Kerala?
2. Mains-type (150 words): Explain the role of the jet streams in the onset of the Indian monsoon.
3. Mains-type (250 words): "El Niño alone cannot explain the variability of the Indian monsoon." Discuss.