Monsoon Mechanism: Traditional and Contemporary Theories (Air Mass Theory, Jet Stream Theory)

At a Glance

• Onset Normal date 1 June at Keralam (Kerala)
• Covers India Normal date 8 July
• Withdrawal Starts 17 September; ends about 15 October
• Normal rainfall 96 to 104 per cent of the long period average
• Long period average About 87 cm
• Exam link UPSC Prelims, Geography: Physical Geography of India

Where Does It Fit?

This topic follows the factors of India’s climate and takes its most important feature, the monsoon. It explains the old and new theories of its origin and the factors that control its strength and breaks. Previous topic: Factors Affecting India’s Climate. Next topic: Impact of El Niño and La Niña on the Indian Monsoon.

What Are the Theories of the Monsoon?

The modern view combines them. The monsoon is a part of the global circulation, as in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell, and the land-sea heating contrast, the ITCZ shift and the upper-air jets work together.

How Does the Southwest Monsoon Set In?

1. Heating and low pressure: from March to May the land heats up; a deep low pressure forms over northwest India and Pakistan, and the ITCZ, called the monsoon trough, shifts north. Hot dry winds like the loo blow, and the weather is dry, because the ridge of the subtropical westerly jet keeps the air over north India sinking, as in Jet Streams: Permanent and Temporary Types and Their Impact on Weather.
2. Jet stream shift: in early June the southern branch of the westerly jet breaks down and moves north of the Tibetan Plateau, and the Tropical Easterly Jet forms over the peninsula at 14 to 16 km. The upper-air pattern reverses from convergence to divergence, which turns the surface high into a low, and this change, rather than land heating alone, produces the sudden burst of the monsoon.
3. Tibetan Plateau: the plateau acts as a high-level heat source and as a barrier. It is several degrees warmer than the air around it, and the rising air creates a strong low and an easterly jet; a heavy winter snow cover on Tibet can delay its warming, and is linked to a weaker monsoon in some studies.
4. Cross-equatorial flow: the southeast trades of the southern hemisphere, strengthened by the Mascarene high near Madagascar, cross the equator and are deflected to the right by the Coriolis force to become the southwest monsoon, helped by the low-level Somali jet, as in Major Ocean Currents of the World and Their Effects.
5. Two branches: the Arabian Sea branch strikes the Western Ghats first, bringing heavy rain to the west coast, and then moves east; the Bay of Bengal branch moves north-west along the Ganga plain and is turned by the Himalaya, as in Factors Affecting India’s Climate.

The monsoon onset normally takes place over Keralam (the state’s official name since August 2026, formerly Keralam) on 1 June, covers the whole country by 8 July, and begins to withdraw from west Rajasthan on 17 September, with complete withdrawal by about 15 October. In 2026 the onset came on 4 June, three days late, after IMD had postponed its expected date several times.

What Controls Its Strength and Breaks?

• El Niño and La Niña: El Niño tends to weaken the monsoon and La Niña to strengthen it, as in El Niño and La Niña: ENSO and the Indian Ocean Dipole Effect.
• Indian Ocean Dipole: a positive IOD, with a warm western Indian Ocean, increases the rainfall and can offset El Niño, as in El Niño and La Niña: ENSO and the Indian Ocean Dipole Effect.
• Madden–Julian Oscillation: a 30 to 60 day eastward-moving pulse of cloud and rain that produces active and weak spells within the season.
• Break in the monsoon: a dry spell of a week or more when the monsoon trough moves to the foothills of the Himalaya, bringing heavy rain there and floods in the north-east but little over the plains and peninsula. Active and break spells are normal.
• Monsoon depressions: low pressure systems from the Bay of Bengal, about six or seven in a season, which move north-west along the trough and give much of the central Indian rainfall.

The IMD classifies seasonal rainfall as normal when it is between 96 and 104 per cent of the long period average (LPA) of about 87 cm, deficient below 90 per cent and excess above 110 per cent. In 2026, in a developing strong El Niño year, IMD first forecast about 92 per cent of the LPA in April and lowered it to 90 per cent at the end of May; a news report states that the season ended at about 87 per cent, a deficit of about 12 to 13 per cent and the lowest since 2015, with the largest deficits in the east and north-east and in the southern peninsula. The figure should be checked against IMD’s final season report, as in El Niño and La Niña: ENSO and the Indian Ocean Dipole Effect.

As an analytical point, forecasting has improved but the monsoon is still the chief uncertainty for Indian agriculture, and so IMD’s use of ensemble models, the spread of automatic rain gauges, and farm advisories matter as much as the headline forecast.

Why Is This Topic Important?

• Core topic Mechanism asked every year
• Current affairs 2026 El Niño-year monsoon
• Link Farming and economy

What Came Next?

The next topic discusses the impact of El Niño and La Niña on the Indian monsoon. Next topic: Impact of El Niño and La Niña on the Indian Monsoon.

Key People and Terms

• Monsoon trough ITCZ over the Indian subcontinent
• Burst Sudden onset of heavy rain
• Break Dry spell in the monsoon season
• LPA Long period average
• Tropical Easterly Jet Summer upper-level easterly jet
• MJO Madden–Julian Oscillation

Exam Corner

Mains Angle

Questions ask for the mechanism and the teleconnections. These points add depth.

• Mechanism Heating plus jet shift
• Drivers ENSO, IOD, MJO
• Forecast Seasonal uncertainty