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The monsoon is a seasonal reversal of wind driven by the differential heating of land and ocean, shaped by the Earth's rotation and the position of the jet streams.
Monalisa DuttaThe word monsoon derives from the Arabic mausim, meaning season. The etymology is instructive, because it describes a seasonal wind rather than seasonal rain — and that distinction is the key to understanding the entire system.
Land and water respond very differently to solar heating. Land has a low specific heat capacity and heats rapidly; the heating is concentrated in a thin surface layer. Water has a high specific heat capacity, is transparent to some depth, and circulates, distributing absorbed heat through a considerable volume.
During the northern spring, as the sun moves north of the equator, the Indian subcontinent and the Tibetan Plateau heat intensely while the Indian Ocean remains comparatively cool. By May, surface temperatures across north-western India regularly exceed 45 °C while sea surface temperatures remain near 30 °C.
The heated land warms the air above it, which expands and rises, producing an area of low pressure over the subcontinent — the heat low, centred over north-west India and Pakistan. The relatively cool ocean retains higher pressure.
Air flows from high pressure towards low pressure. Consequently, moisture-laden air begins to move from the Indian Ocean towards the subcontinent. That flow is the summer monsoon.
Air moving from the southern Indian Ocean towards the Asian landmass crosses the equator. In doing so it passes from the southern hemisphere, where the Coriolis effect deflects motion to the left, into the northern hemisphere, where it deflects motion to the right.
South-easterly trade winds in the southern hemisphere therefore recurve upon crossing the equator, becoming the south-westerly flow which characterises the Indian summer monsoon. This is why the monsoon approaches Kerala from the south-west rather than directly from the south.
The flow is further concentrated by the Somali Jet, a low-level jet stream which forms along the East African highlands and channels a substantial proportion of the total monsoon moisture transport across the Arabian Sea towards the Indian coast.
The Tibetan Plateau, averaging above 4,500 metres over an area comparable to Western Europe, functions as an elevated heat source. During summer it heats the middle troposphere directly, at an altitude where that heating has considerable dynamical effect.
This produces the Tibetan anticyclone in the upper troposphere and helps establish the Tropical Easterly Jet, which flows from east to west across peninsular India at around 150 hPa. The strength of this upper-level easterly flow is well correlated with monsoon activity, and it forms part of the diagnostic picture forecasters examine.
Through the winter, the subtropical westerly jet stream sits south of the Himalaya, across northern India. The monsoon cannot advance while it remains in that position.
During late spring the jet migrates north of the Himalayan barrier. This displacement is a necessary precondition for monsoon onset, and its timing varies from year to year. A jet which retreats late tends to be associated with a delayed onset.
Once established, the monsoon organises around the monsoon trough — an elongated area of low pressure extending from the heat low over north-west India south-eastward across the Gangetic plain towards the head of the Bay of Bengal.
The position of this trough governs where rain falls. When it lies in its normal position across the plains, central and northern India receive widespread rainfall. When it shifts north towards the Himalayan foothills, the plains dry out while the foothills receive extremely heavy rain. This is the break monsoon situation.
A substantial proportion of monsoon rainfall over central India is delivered not by the general flow but by discrete low-pressure systems which form over the north Bay of Bengal and track west-north-westward across the subcontinent.
These systems, classified as low-pressure areas, depressions or deep depressions according to their intensity, may each deliver several days of heavy and widespread rain along their track. A season with frequent Bay of Bengal systems will generally be a wet season over central India; a season with few will generally not.
The Western Ghats run parallel to the west coast and force the incoming south-westerly flow to ascend. The rising air cools, condenses and produces the extraordinary rainfall totals recorded along the windward slopes, where annual accumulations exceeding 6,000 mm occur.
Having lost much of its moisture in the ascent, the air descends on the eastern side, warming and drying. This rain shadow produces the semi-arid conditions of interior Karnataka, western Andhra Pradesh and parts of Maharashtra, some of which lie less than 150 kilometres from among the wettest places on Earth.
The system reverses in autumn. As the land cools more rapidly than the ocean, the pressure gradient inverts and the flow becomes north-easterly, moving from land to sea.
For most of India this brings the dry season. For Tamil Nadu, coastal Andhra Pradesh and Kerala, however, the north-east monsoon of October to December is the principal rainfall season. Air travelling across the Bay of Bengal collects moisture before reaching the south-eastern coast, which is why Chennai receives the greater part of its annual rainfall in a period when Mumbai is entirely dry.
The mechanism is well understood in outline. Predicting any individual season remains genuinely challenging, because the outcome depends upon the interaction of several factors operating on different timescales: sea surface temperatures across the Pacific and Indian Oceans, the Madden–Julian Oscillation on a timescale of thirty to sixty days, snow cover over Eurasia, and the internal variability of the system itself.
Forecasting skill has improved considerably over recent decades, particularly for the seasonal total. Predicting the distribution of that total in space and time — which determines whether a season is beneficial or damaging for agriculture — remains substantially more difficult, and is where a great deal of current research is directed.
Meteorology consultant and former Squadron Leader in the Indian Air Force, with over eleven years of experience in operational aviation meteorology, weather forecasting and flight safety support. She now advises technology organisations on AI-driven weather forecasting and decision-support systems. Consultancy services →