Mechanism of the Indian Monsoon

Monsoon arises from the differential heating of land and sea, together with shifts in global pressure belts and upper air currents. Several theories explain it; examiners expect you to know the classical (thermal) and dynamic concepts plus El Nino, IOD and jet streams.

Classical (thermal) theory

• In summer, land heats faster than sea. A low pressure forms over north-west India and the Gangetic plain.
• The south-east trade winds from the southern hemisphere are pulled across the equator, are deflected to the right by the Coriolis force and become south-west monsoon winds.
• The monsoon trough (the shifted ITCZ) brings moist air deep into the land.

Dynamic (air mass) theory

• Monsoon is a seasonal shift of the ITCZ and wind belts, not just land-sea heating.
• The Tibetan plateau heating in summer creates a strong upper-air high that helps the Tropical Easterly Jet and removes the westerly jet north of the Himalayas.

Sequence of onset

1. The sub-tropical westerly jet withdraws north of the Himalayas by about early June.
2. The Tropical Easterly Jet strengthens over the peninsula.
3. The monsoon "bursts" on the Kerala coast, often with thunder and heavy showers.
4. The Arabian Sea branch gives heavy rain on the Western Ghats; the Bay of Bengal branch hits the north-east and then turns west along the Himalayas.

The two branches

El Nino, La Nina and IOD

• El Nino: a warm ocean current appears off Peru every few years. It raises sea temperature in the east Pacific, weakens the Walker circulation, and usually causes a weak monsoon / drought in India.
• La Nina: cold phase, usually a good monsoon.
• Southern Oscillation: the pressure see-saw between the east and west Pacific; together called ENSO.
• IOD (Indian Ocean Dipole) or Indian Nino: positive phase brings more rain; negative phase less rain.
• The IMD forecasts monsoon using models and issues the first forecast in April and a second in late May or early June.

Monsoon forecasting basics

• Long Period Average (LPA) for all-India monsoon rain is about 87 cm (868.6 mm) for 1971-2020. Rainfall is categorised: deficient (below 90 percent of LPA), below normal (90-95), normal (96-104), above normal (105-110), excess (above 110).
• Mission Mausam (launched 2024) aims to improve forecasting.