Every year, somewhere around the first of June, the India Meteorological Department (IMD) announces that the monsoon has reached Kerala, and much of the country exhales. Over the next four months the southwest monsoon delivers roughly three quarters of India's annual rainfall, enough to shape crop output, reservoir levels and, in a bad year, food prices.
It feels like a calendar event. It's really a giant, slightly unpredictable heat engine. Here's how it works.
Land heats up and cools down much faster than water. Leave a steel plate and a bucket of water in the April sun and you'll feel the difference within an hour.
By May, the plains of northwest India and Pakistan and the high Tibetan Plateau are baking. Hot air rises, and a broad zone of low pressure forms over the subcontinent. The Indian Ocean to the south stays comparatively cool, with higher pressure above it. Air flows from high pressure towards low, so a vast, steady wind starts blowing from the ocean towards the land.
That wind begins in the southern Indian Ocean, crosses the equator and, because of the Earth's rotation, curves to its right. It arrives at India's west coast as a southwesterly wind, which is where the name southwest monsoon comes from. On its long run over warm water it soaks up enormous amounts of moisture.
Near the equator there's a belt where the trade winds of the two hemispheres meet and air rises, building towering clouds and heavy rain. Meteorologists call it the Intertropical Convergence Zone, or ITCZ. It isn't fixed. It follows the sun's heating, drifting north during the northern summer and south during the northern winter.
In summer the convergence zone is pulled far north, and over India it takes the form of the monsoon trough, a long band of low pressure stretching roughly from Rajasthan to the head of the Bay of Bengal. Much of the season's rain is organised around it. When the trough sits over central India, rain is widespread. When it shifts north to the Himalayan foothills, much of the country gets a dry spell, called a break in the monsoon, while the hills and the northeast get pounded.
The normal onset date over Kerala is around 1 June, give or take about a week. IMD doesn't declare it on the first heavy shower. It uses set criteria: sustained rainfall across a group of stations in Kerala and nearby, the strength and depth of the westerly winds, and the cloud cover seen by satellites.
Once the moist winds hit the west coast, they run into the Western Ghats, a near-continuous wall of hills from Gujarat down to Kerala. The air is forced upward, cools as it rises, and its moisture condenses into cloud and rain. This is orographic rainfall, and it's why the Konkan coast, coastal Karnataka and Kerala get drenched, with some spots along the Ghats receiving several metres of rain in a season.
By the time the air crosses the crest and sinks down the other side, it has dropped much of its moisture and warms as it descends. So the inland side sits in a rain shadow. That's why Pune gets far less rain than Mumbai, though the two are only a few hours apart by road, and why large parts of interior Maharashtra and Karnataka are prone to drought.
The southwest monsoon reaches India along two main paths.
The Arabian Sea branch strikes the west coast head-on and produces that intense rain along the Ghats.
The Bay of Bengal branch heads northeast towards Myanmar and Northeast India. There it meets the hills of Meghalaya, where the shape of the terrain lifts the air sharply. That's why Mawsynram and Cherrapunji (Sohra) rank among the wettest places on Earth. The Himalayas then block the winds from going further north and turn them west, so this branch travels up the Gangetic plain through West Bengal, Bihar and Uttar Pradesh towards Delhi and Punjab.
As that air moves west across the plains it keeps losing moisture, one reason rainfall generally falls off from east to west across north India, with west Rajasthan the driest. Low-pressure systems that form over the Bay and travel inland along the monsoon trough bring much of central India's rain, so a quiet Bay often means a weak spell far inland.
The two branches merge over north India, and the monsoon normally covers the whole country by early July. It begins withdrawing from the northwest in September.
Tamil Nadu is the odd one out. During the southwest monsoon it sits in the rain shadow of the Western Ghats, so much of the state gets fairly modest rain from June to September.
From October the system reverses. Now the land cools faster than the sea, pressure rises over the subcontinent, and winds blow from the northeast, off the land and across the Bay of Bengal. Crossing the Bay, they pick up moisture and drop it on the Tamil Nadu coast, along with Puducherry, coastal Andhra Pradesh and parts of Kerala. This is the northeast monsoon, from October to December. Depressions and cyclones that form in the Bay in these months add to the totals, which is why Chennai's heaviest downpours and floods usually come in November or December, not July.
The monsoon doesn't depend only on what happens over India. Conditions in faraway oceans tilt the odds.
El Niño is a periodic warming of the surface waters of the central and eastern tropical Pacific. It shifts where the great columns of rising air over the tropics sit, and in many El Niño years the Indian monsoon is weaker than usual. Its opposite, La Niña, tends to come with normal or good monsoons. The link is strong but not absolute; some El Niño years have still seen near-normal rain.
The Indian Ocean Dipole (IOD) is a see-saw within our own ocean. In a positive IOD, the western Indian Ocean near Africa is warmer than usual compared with the eastern side near Indonesia, and that tends to support rain over India. A positive IOD can partly offset an El Niño. A negative IOD tends to do the opposite.
IMD issues its seasonal forecast in April and updates it around the end of May. It's expressed as a percentage of the long period average (LPA), the average June to September rainfall over several decades, with a margin of error. Seasons are classed like this:
| Category | Rainfall as % of LPA |
|---|---|
| Deficient | Below 90% |
| Below normal | 90 to 95% |
| Normal | 96 to 104% |
| Above normal | 105 to 110% |
| Excess | Above 110% |
IMD also gives the probability of each category. A forecast saying normal or above-normal rain is more likely than not isn't hedging. The atmosphere is chaotic: tiny differences today grow into big ones weeks later, so a season can only be forecast as a set of odds.
A national figure also hides a lot. A normal monsoon can still mean floods in Assam and a dry spell in Marathwada in the same month, or half a month's rain falling in three days.
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