By late May, much of India has endured weeks of intense heat. Roads shimmer in the afternoon. Fields dry out, water levels fall, and people watch the sky for signs of change. The first signs may be subtle: a more humid morning, clouds building over the horizon, or a strong breeze before an evening storm. Soon, the change spreads across a much wider region.
The monsoon changes more than the clouds. Winds shift to a new seasonal pattern. Moist air travels inland from the surrounding seas, broad cloud bands grow, and rain reaches different parts of the country over several weeks. Some places receive heavy rain early, while others continue to wait. Even after the rainy season settles over India, wet spells alternate with quieter periods.
Why does summer heat change winds, moisture, clouds and rain across so much of South Asia? We will first follow the change from heating to rainfall in simple terms. Once that sequence is clear, we can name its parts and examine the upper winds and ocean–atmosphere links that affect it.
From summer heat to rain: the basic sequence
Why the land leads the seasonal change
During summer, land heats up faster than the ocean. The Indian subcontinent therefore becomes very hot during spring and early summer. The surrounding ocean also receives solar energy, but its water mixes and spreads heat through a much greater depth. Its surface temperature changes more slowly.
The hot land warms the air above it. That air expands, becomes less dense and helps create a broad region of relatively low pressure near the ground. Air pressure remains relatively higher over parts of the southern Indian Ocean. This pressure difference helps move air towards South Asia.
The broad tropical belt of cloud and rain also moves north during the season. It forms where warm, moisture-bearing air comes together and rises. The belt has no sharp edge, but by early summer it reaches South Asia and favours widespread cloud growth there.
How moist ocean air brings rain
The pressure difference moves air north from the southern Indian Ocean. Some of this air crosses the equator, the imaginary line halfway between the two poles. As it enters the Northern Hemisphere, Earth's rotation gradually turns it towards the right. The wind bends and reaches India mainly from the south-west, so we call it a south-westerly wind.
Before reaching India, the air travels over warm parts of the Arabian Sea and the Bay of Bengal. Water evaporates from the sea surface and enters the air as invisible water vapour. As the air crosses these seas, it gathers moisture and carries ocean water towards the land.
Rain forms when this moist air rises, expands and cools enough for its water vapour to condense into tiny droplets or ice particles. Mountains force winds up their slopes. Air streams meet and rise, while developing weather systems lift air across wider areas.
Inside the clouds, droplets and ice particles grow until some become heavy enough to fall. This rain completes the summer monsoon's basic sequence. The land heated, pressure changed, air crossed the equator, south-westerlies gathered moisture over the seas, and rising air cooled to form clouds and rain.
These four steps recap the sequence. Each one follows directly from the step before it:
- Summer heating changes the broad pressure pattern and shifts the tropical rain belt towards South Asia.
- Air travels north from the southern Indian Ocean, crosses the equator and gradually curves into south-westerlies.
- The moving air gathers water vapour over the Arabian Sea and the Bay of Bengal.
- Convergence and terrain lift the moist air; it cools, forms clouds and gives rain.
Keep this sequence in mind as we add detail. The monsoon links heating, pressure, winds, moisture, rising air and rainfall in one seasonal system. We will now name its main features and see why its rain changes within a season and from one year to another.
The moving rain belt and the pressure pattern
Giving the rain belt a name
Warm, moist air gathers and rises within a broad tropical belt. Meteorologists call this belt the Intertropical Convergence Zone, or ITCZ. “Intertropical” refers to the warm region between the tropics, while “convergence” means that air comes together. The rising moist air often produces deep clouds and heavy rain.
The ITCZ shifts with the seasons and spreads across a broad, uneven area rather than one fixed latitude. Continents, oceans, mountains and moving weather systems change its shape. During the South Asian summer, it extends far north of the equator. Over northern India, it forms a long zone called the monsoon trough.
A trough is a long area of relatively low pressure with winds turning around it. The monsoon trough stretches across northern parts of the subcontinent and often continues towards the Bay of Bengal. Weather maps usually draw its axis as a line, although the actual trough covers a wider area and keeps shifting. Air gathers around it and helps organise cloud and rain.
The pressure field on both sides of the equator
Strong summer heating creates a shallow area of low pressure near the surface over north-western parts of the subcontinent. Meteorologists call it the heat low. Differences in pressure from one place to another push air and create the pressure-gradient force. The heat low contributes to that difference, but its strength alone cannot determine rainfall across India.
A broad belt of higher pressure forms over the subtropical southern Indian Ocean. Meteorologists call one average centre within this belt the Mascarene High. Its position and strength change, so the wind responds to the broad high-pressure region rather than to one fixed point.
Air moves out of the southern high-pressure belt, crosses the equator and continues towards the heated continent and the northward-shifted rain belt. This broad pressure pattern supports the northward flow described earlier.
Earth's rotation gradually turns the moving air. Meteorologists call this the Coriolis effect. The effect is zero at the equator and grows as the air moves farther north. The wind therefore crosses the equator smoothly and then curves to its right in the Northern Hemisphere.
Land heating and ocean air also create a sea breeze near coasts, but that breeze changes within a day and covers a small area. The monsoon lasts for a season, reaches high into the atmosphere and links both hemispheres. Tropical convergence, deep clouds and upper winds make the monsoon much larger and deeper than a sea breeze.
The winds that bring moisture to India
The fast current near East Africa
Part of the cross-equatorial air gathers into a narrow band of fast wind near the East African coast. Meteorologists call it the Somali low-level jet. A jet is a concentrated stream of strong wind, while “low-level” places its core in the lower atmosphere. Its core often lies near the 850-hectopascal pressure level, roughly one to two kilometres above sea level depending on conditions.
The East African highlands focus and strengthen this wind near the Somali coast. The broader cross-equatorial flow can still form without the highlands, but it becomes weaker and less concentrated. Monsoon heating also helps set the jet's position. The jet carries fast-moving air and large amounts of water vapour towards the Arabian Sea and India. Its path shifts, so a stronger jet does not guarantee heavier rain at every Indian location.
The Somali low-level jet carries air. Below it, the Somali Current carries ocean water. Strong summer winds also bring cooler subsurface water towards the sea surface near the coast, a process called upwelling. A strong atmospheric jet can therefore blow above locally cooler ocean water.
Two broad moisture pathways
As the moist wind approaches India, it follows two broad pathways. The Arabian Sea branch reaches the western coast and then follows several inland routes. The Bay of Bengal branch curves towards eastern, north-eastern and northern India. These names help us read the map, although real air streams curve, mingle and exchange moisture.
Two short points recap the pathways. Both begin over warm seas and carry moisture towards the land:
- The Arabian Sea pathway first meets India's western coast and then feeds several inland routes.
- The Bay of Bengal pathway bends towards eastern and north-eastern India and helps supply the northern plains.
Pressure systems keep changing the direction of these winds. Some air crosses the peninsula, while other streams meet over the plains or bend around the monsoon trough. Air reaching one region may carry moisture from more than one part of the ocean. The branch names show the main routes without creating fixed boundaries in the atmosphere.
How India’s relief reshapes the moist flow
The Arabian Sea wind strikes the steep western slopes of the Western Ghats and rises. Air pressure falls with height, so the rising air expands and cools. Water vapour then condenses, clouds grow, and much of the windward side receives abundant rain.
After crossing the crest, the air descends towards the interior. Increasing pressure compresses and warms it, which makes cloud formation more difficult. Parts of the Deccan interior therefore lie in a rain shadow. The same wind produces a wet coastal slope and a much drier inland area because the mountains force it to rise and then descend.
Land and mountains guide the Bay of Bengal wind towards the north-eastern hills. Steep slopes lift the moist air and can produce very heavy rain. Farther west, Bay winds, the monsoon trough and travelling low-pressure systems carry moisture across the northern plains. The Himalayas form a great northern barrier that redirects the flow and helps air rise.
Mountains do not create water vapour. They produce heavy rain only when moist winds reach their slopes and rise. This is why one side of a range can be wet while the other is dry, and why rainfall changes when the wind changes direction or depth. Terrain redirects and lifts the moisture already carried by the monsoon.
Where the monsoon brings more and less rain
The monsoon reaches most of India, but it does not spread rain evenly. Three controls explain much of the broad pattern. A region needs moisture-bearing air, a process that lifts that air, and repeated visits from rain-bearing systems. Changes in these three controls create broad wet and dry belts, along with many local variations.
The two major high-rainfall belts
One major wet belt lies along the west coast and the windward side of the Western Ghats. Arabian Sea winds arrive with abundant moisture and strike the steep western face of the range. They rise sharply, cool and form deep clouds. The coastal belt and windward slopes therefore receive much more rain than large parts of the plateau immediately to the east.
The contrast can be strong over a short distance. A station on the western slope may receive repeated heavy rain while a place farther inland remains much drier. After crossing the crest, the air descends and warms. This reduces cloud growth and creates a broad rain-shadow region over parts of interior Maharashtra, Karnataka, Telangana and the adjoining Deccan. Gaps in the Ghats and travelling weather systems still carry rain inland, so the rain shadow is drier than the windward belt, not rainless.
The other major wet belt lies in the northeast and along parts of the eastern Himalayan foothills. Bay of Bengal winds carry moisture towards the region. Hills force the air to rise, while convergence and passing weather systems add further lift. The southern slopes of the Meghalaya Plateau face the moisture-bearing flow and receive exceptionally heavy rain.
Mawsynram in the Khasi Hills is commonly described as the place with the highest average annual rainfall. This record refers to a particular long-term average, not to the heaviest rain on every day or in every year. Sohra, also known as Cherrapunji, has a long official observatory record and is also among the world's rainiest places. A different averaging period, station record or boundary can alter the ranking. Missing observations and the method used to make a rainfall grid can alter it as well. Whatever the exact ranking, the south-facing Khasi Hills around Mawsynram and Sohra form India's most exceptional average-rainfall belt.
From the northern plains to central India and the dry west
Across much of the northern plains, average monsoon rainfall broadly decreases from east to west. Bay winds and many low-pressure systems enter the plains from the east or southeast. As the flow and its weather systems move inland, their moisture supply, track and contact with the monsoon trough keep changing. The western plains generally receive less frequent or less sustained monsoon rain than the eastern plains.
This westward decline is a broad tendency, not a rule for every district or every year. The monsoon trough may shift, a depression may follow an unusual path, and local relief can change the result. Successive rainfall also removes some moisture from the moving air, but the air does not simply become empty. System tracks, convergence and distance from the main moisture pathways matter as well.
Central India receives substantial summer rain when lows and depressions move inland from the Bay along or near the monsoon trough. These systems draw in moist air and spread rain across Odisha, Chhattisgarh, Madhya Pradesh, Vidarbha and adjoining regions along changing tracks. They help connect the Bay moisture pathway with the interior. Their uneven number, strength and route explain why central Indian rainfall can vary greatly within a season and between years.
The eastern coastal plains also receive summer rain from Bay winds and passing low-pressure systems. A continuous steep barrier does not face the summer flow there as it does along the western side of the Western Ghats. The east coast therefore does not form the same kind of narrow windward rain belt. Farther south, the rainfall calendar changes: summer rain still occurs, but October–December rain becomes relatively more important towards Tamil Nadu.
The drier northwest and arid west lie farther from the main Bay moisture supply and often receive weaker or less reliable monsoon rain. Western Rajasthan and adjoining dry regions also have high summer heat and strong evaporation. The orientation of the Aravalli Range influences lifting, but it cannot explain the whole pattern by itself. Moisture pathways, relief, continental position and the tracks of rain-bearing systems work together.
Trans-Himalayan areas such as Ladakh remain dry for a different reason. The great mountain barrier blocks most low-level monsoon moisture from reaching them. Their dry climate therefore reflects a high-mountain rain shadow rather than the same set of controls that creates the hot desert of western Rajasthan.
India's islands remain surrounded by moisture, but their rainfall still varies with wind direction, relief and passing systems. Lakshadweep lies within the Arabian Sea monsoon flow, while the Andaman and Nicobar Islands lie close to Bay and Andaman Sea moisture pathways. Their maritime setting favours a long rainy influence, but the amount and timing still differ from island to island.
One rainfall total cannot describe the rainy season
An annual rainfall map adds all rain received during the year. It does not show when the rain fell or how it arrived. Over much of India, June to September supplies the largest share of the annual total. On the southeast coast, including much of Tamil Nadu, October to December has much greater relative importance. We will follow that later-season mechanism when we examine withdrawal.
Rainfall must therefore be read in several ways. Total means the amount collected over a period. Seasonal share asks how much of the annual total falls in a particular season. Frequency counts rain events or rainy days, while intensity describes how heavily rain falls during an event. Timing shows when wet and dry spells occur.
Two further ideas tell us how dependable or changeable the rain is. Reliability asks how consistently useful rain arrives. Variability describes how much the amount or timing changes across places, within a season or between years. These measures describe different features of the same rainy season.
These measures can move differently. A region may finish a season near its usual total after a few extreme downpours separated by long dry spells. Another region may receive a smaller total through frequent moderate rain. The first season is not automatically more reliable because it was wetter in total.
Rainfall maps also depend on how they are made. A band marked “high rainfall” represents a chosen range, not a natural wall on the ground. A different normal period, station network, grid size or interpolation method can shift the boundary and alter a station ranking. The broad west-coast, northeast, interior and northwest patterns remain useful, but exact bands and superlatives must always be read with their period, dataset and method.
From surface winds to a deep atmospheric system
Clouds begin to influence the circulation
Monsoon clouds can rise many kilometres above the lower winds that bring in moisture. We must therefore follow the circulation through the full depth of the atmosphere. Lower winds carry moisture towards India, clouds lift air through a deep layer, and upper winds carry it away from the rainy region.
Inside a growing cloud, water vapour condenses into liquid droplets and sometimes freezes. These changes release heat that the water had absorbed during evaporation. Meteorologists call it latent heat. Deep clouds release this heat through a tall column, warming the surrounding air and helping sustain further ascent.
Cloud formation feeds back into the monsoon circulation. Converging winds supply moisture, condensation releases heat, and that heat changes pressure and winds through a deep layer. The changed winds bring in the next supply of moisture. This feedback helps sustain the monsoon even after clouds and wet soil reduce the extreme surface heating that came before the rain.
The Himalayas, Tibetan Plateau and heat released by clouds
The Himalayas force moist air up their slopes and guide lower winds around the mountain arc. They also restrict cold, dry air from spreading south through the lower and middle atmosphere. By limiting this inflow of dry air, the mountain barrier helps maintain warm, moist conditions over South Asia.
The Tibetan Plateau lies north of the Himalayas. Its high surface and slopes warm during summer and affect regional pressure and winds. Because this heating occurs high in the atmosphere, it supports the seasonal circulation. It does not drive the entire monsoon alone. Deep rain clouds south of the plateau also release enormous amounts of heat into the atmosphere.
Strong land heating begins the seasonal pressure change and northward shift of the rain belt. Once deep clouds grow, latent heat, mountains and large-scale winds also shape the circulation. Their continuing interaction sustains the mature monsoon even after clouds and wet ground reduce the strongest surface heating.
The upper branch of the circulation
In winter, a fast upper-air wind usually flows from west to east south of the Himalayan–Tibetan barrier. It often remains there into the early part of the summer transition. Meteorologists call it the subtropical westerly jet because it crosses subtropical latitudes. This southern position is a feature of South Asia's cool-season circulation.
As the land and the deep atmosphere warm during spring and early summer, the north–south temperature difference changes. The tropical rain belt moves north, deep clouds grow over South Asia, and pressure changes through a great depth of the atmosphere. During this transition, the subtropical westerly jet weakens over India and shifts north of the Himalayan–Tibetan barrier.
In summer, winds high above South Asia turn clockwise around a broad centre. Meteorologists call this circulation the upper anticyclone. Along its southern side, a strong band of wind often blows from east to west; this is the tropical easterly jet. The anticyclone covers the wider circulation, while the jet marks one fast-moving part of it. Both develop as summer heating and deep rain clouds alter pressure high above South Asia.
These upper winds mark a mature monsoon circulation. Heat released by deep clouds helps shape them, and the upper anticyclone carries air away from rising regions. The tropical easterly jet is part of the east-to-west flow on the anticyclone's southern side. It helps describe how air leaves the heated and rainy region high above the ground, but it does not pull lower air upward or create the monsoon by itself.
The northward movement of the subtropical westerly jet and the development of the upper anticyclone and tropical easterly jet are important parts of the summer change. They are also useful signs that the deep circulation has reorganised. The westerly jet does not simply disappear and switch the monsoon on. The surface pressure field, lower moisture flow, rain belt, deep clouds, latent heating and upper winds change together.
For the same reason, upper-jet movement alone is not the official rule for declaring onset over Kerala. Onset observers examine rain, lower westerly winds and deep cloud over the region. The upper winds help us understand the larger three-dimensional transition in which that onset occurs.
Meteorologists also compare temperatures through a deep layer over the continent and the southern ocean. A strong north–south difference can mark the large-scale monsoon transition. They measure it within the troposphere, the lowest major layer of the atmosphere and the home of almost all ordinary weather. This temperature measure captures one part of the transition, so local rain or an official onset declaration may begin on a different date.
The circulation has different parts at different heights. Near the ground, winds carry moisture towards the heated region where air converges. Moist and dry layers in the middle atmosphere affect whether clouds can grow deeply. The air rises in tall clouds, releases latent heat and then spreads outward high above the ground beneath the upper anticyclone and easterly winds.
How the monsoon arrives and advances
Onset means a sustained regional change
A place may receive an isolated thunderstorm before the seasonal monsoon arrives. The rain can be intense but short-lived. Monsoon onset begins only when the larger seasonal circulation takes hold across a region. The first shower therefore need not mark the start of sustained monsoon rain.
To identify onset over Kerala, weather observers examine three kinds of evidence. They look for widespread, persistent rain across a defined group of stations. They check whether westerly winds extend through a deep enough layer to show more than a local sea breeze. They also look for large areas of deep cloud that reveal organised rising air.
Three short points recap the onset evidence. No single item establishes onset by itself:
- Rain has become widespread and persistent across a defined group of stations.
- Westerly winds extend through a meaningful depth of the lower atmosphere, showing a change larger than a local sea breeze.
- Large areas of deep cloud reveal organised convection over the region.
Together, these conditions show that the seasonal circulation has begun to operate over a broad region. They record changes in rain, lower winds and deep clouds.
Satellites measure the energy leaving the top of the atmosphere and help observers track deep clouds. Tall, cold cloud tops emit less longwave radiation than warm ground or low clouds. A broad area of low outgoing longwave radiation therefore points to organised deep clouds. Rainfall, deep westerly winds and this cloud signal give a more reliable onset diagnosis when they occur together.
Weather services use exact thresholds so that they can declare onset consistently. The thresholds measure the change; they do not cause it. Scientists may use other wind or temperature measures to study onset across a larger region. These measures can produce different dates because they track different parts and scales of the same transition.
Organised rain and westerly winds can strengthen quickly around onset. Meteorologists call this rapid establishment the monsoon burst. It describes a broad change that lasts, rather than one dramatic storm. A single heavy shower, gust front or short-lived cyclone supplies too little evidence; rain, wind and cloud organisation must change together.
How the monsoon advances across India
After onset, the monsoon advances across India. Daily weather maps mark its northern edge as the Northern Limit of Monsoon. This line shows how far the seasonal circulation has spread; it does not mark the edge of every rain cloud. Areas behind it can have dry days, while pre-monsoon storms can reach places ahead of it.
Meteorologists average many years of onset observations to map the monsoon's usual progress. This long-period description of typical climate is a climatology. Lines called isochrones connect places with similar average onset dates. Actual advance in a particular year can still be early, late, fast, slow or interrupted.
Normal-date maps can change when meteorologists change the reference period, observations or method. The maps summarise past averages; they do not announce this year's dates. The monsoon often advances in surges separated by pauses. Moving low-pressure systems and favourable cloud patterns can speed it up, while dry air and weak cloud growth can delay it.
Dry air above the moist lower winds can slow the monsoon's advance towards north-western India. Continued cloud growth gradually moistens this middle layer, while earlier rain wets the land. These changes help rainy conditions spread farther north-west.
The lower atmosphere must carry moisture, the middle atmosphere must allow deep clouds to grow, and passing systems must lift the air. Wet ground can make later cloud growth easier. The advance line moves when these conditions spread together through the atmospheric column.
Why the rainy season contains wet and quiet spells
Trough movement and travelling low-pressure systems
Rain continues to vary after the monsoon covers most of India. The position of the monsoon trough causes some of this change. When its main axis lies across the plains, moist winds and travelling weather systems often produce widespread rain over the monsoon core. When the trough shifts towards the Himalayan foothills, rain can weaken over much of the core and increase along the foothills and in parts of north-eastern India.
Short-lived circulations also form within the monsoon. Pressure falls towards their centre and winds turn around them, so meteorologists call them low-pressure systems. Many develop over the Bay of Bengal and travel inland along or near the monsoon trough. They gather moisture, lift air and spread rain along their path.
Low-pressure systems vary in strength. Meteorologists generally call the weaker systems lows and the stronger ones depressions, with further categories for greater intensification. Many lows weaken before they become depressions or cyclones. Their number, tracks and strength differ greatly from one season to another.
These systems distribute rain unevenly. Moisture supply, changes in wind with height, terrain and the system's movement often place the heaviest rain away from its centre. A mature monsoon depression can tilt with height and may contain warmer air aloft over cooler air below. Its structure therefore changes with height and direction.
Low-pressure systems contribute a large share of rain in many monsoon regions. Broad convergence, mountain lifting, offshore features and moving tropical cloud bands also create wet spells. Slower changes within the monsoon influence all of them, so we must now look beyond individual weather systems to patterns that last several weeks.
Active spells and breaks
Meteorologists describe several unusually wet days over a chosen monsoon region as an active spell. They call a sustained period of below-normal rain over that region a break. Both terms depend on the region and the calculation. An active spell in one area therefore does not mean heavy rain everywhere in India.
Weather reports may call rain “active” within a small subdivision when it is unusually heavy and widespread there. Scientists may define an active spell from average rainfall across a much larger monsoon core. The same word therefore has different meanings at different scales. We must always state the region and rainfall measure.
Other parts of India may remain wet during a break over the central monsoon zone. A northward trough can increase rain near the Himalayan foothills, while coastal or north-eastern regions receive rain from other flows. A wet local week likewise tells us little about the whole country. India can experience contrasting monsoon conditions at the same time.
Patterns that organise several weeks
Active spells and breaks are forms of intraseasonal variability, or change within the same season. Individual weather systems cause some variations that last a few days. Other patterns last roughly two weeks or several weeks. These timescales overlap.
Large areas with more or less cloud than usual often move north from the equatorial Indian Ocean towards the subcontinent. This movement helps alternate active and break-like conditions. Meteorologists group many such changes under the monsoon intraseasonal oscillation, or MISO. Here, “oscillation” means a recurring rise and fall, not an exact repetition.
During the Northern Hemisphere's summer, some large cloud and wind patterns move both east and north. Meteorologists call one important pattern the Boreal Summer Intraseasonal Oscillation, or BSISO. “Boreal summer” simply means Northern Hemisphere summer. A place may receive more or less rain than usual as the pattern passes. Meteorologists call this departure from the usual value an anomaly: a wet anomaly has above-normal rain, while a dry anomaly has below-normal rain.
Another large pattern of tropical clouds and winds usually travels east around the tropics. Meteorologists call it the Madden–Julian Oscillation, or MJO. During the Asian summer, related cloud patterns often develop a stronger northward movement. The MJO and BSISO interact and partly overlap, but the names describe different seasonal structures. Neither follows exactly the same path in every event.
Scientists divide each MJO or BSISO cycle into phases according to the position of its cloud and wind pattern. A phase shows where tropical cloud is more or less extensive than usual. It changes the chance of heavier Indian rain or more favourable conditions for Bay low-pressure systems during the following period. It cannot tell us whether one town will receive rain on one day. Scientists continue to study how these patterns restart, turn north and interact with the ocean and land.
Several dry weeks can balance several very wet weeks and leave the seasonal total near normal. That total cannot show when the rain fell, how many extreme events occurred or what happened in each region. Farmers, reservoirs and cities respond to the sequence of rain as well as the final sum.
Why one monsoon season differs from another
A national total cannot describe every outcome
Monsoon seasons differ from one year to another. Meteorologists call this interannual variability. One year may have an early onset and uneven later rain; another may start late and recover during strong wet spells. The all-India total, regional distribution, onset, withdrawal, active and break spells, and extreme events can all change differently.
Meteorologists compare a season's rain with a long-period average. They call the difference an anomaly. An all-India rainfall anomaly compresses a very large country into one number, so it cannot show a flood in one region and dry conditions in another. Maps and regional records reveal the pattern hidden by the national average.
Remote oceans change the background odds
Ocean and atmospheric changes far from India can alter the monsoon's chances. They do so by changing large-scale winds, areas of rising and sinking air, and the paths followed by moisture and weather systems. Meteorologists call such a distant relationship a teleconnection. It changes the background odds rather than fixing one unavoidable outcome.
The best-known example begins in the equatorial Pacific Ocean. When the Pacific is near its usual state, easterly trade winds help keep the warmest surface water and the strongest tropical cloud growth towards the western Pacific. Cooler water reaches the surface farther east. The ocean temperatures, winds, air pressure and cloud belt support one another, so a change in one can spread through the whole tropical circulation.
Sometimes the central and eastern equatorial Pacific remain warmer than usual while the trade winds and tropical cloud pattern also change. Meteorologists call this warm phase El Niño. The coupled cycle contains this warm phase and its opposite cold phase. Its name is the El Niño–Southern Oscillation, shortened to ENSO.
During El Niño, a larger share of Pacific cloud and rising air often shifts east from its usual position. Sinking and rising motion elsewhere in the tropics then reorganise, and the large-scale winds linked to the Indian monsoon can weaken or change position. El Niño therefore often raises the probability of a weaker Indian summer monsoon or a deficient all-India rainfall total.
The cold phase, La Niña, develops when the central and eastern equatorial Pacific become cooler than usual within a matching wind and cloud pattern. Stronger trade winds often keep more warm water and cloud growth towards the western Pacific. This background often raises the probability of a stronger or wetter Indian summer monsoon.
Neither phase commands India's rain. El Niño does not produce drought in every Indian season, and drought can occur without it. La Niña does not guarantee a wet season, a flood or evenly spread rain. The strength, location and timing of the Pacific change all matter. Conditions in the Indian Ocean, active and break spells, and the number and paths of monsoon low-pressure systems can alter the result.
The outcome measure also matters. ENSO may influence the all-India seasonal total while different regions experience different rainfall. Onset, withdrawal, active and break spells, and individual extreme-rain events can change in other ways. One national number cannot describe all these outcomes.
The equatorial Indian Ocean can also develop a temperature contrast between its western and eastern sides, along with changes in winds. Meteorologists call this pattern the Indian Ocean Dipole, shortened to IOD. In its positive phase, the western side is warmer and the eastern side cooler than their usual conditions. The contrast can shift cloud growth and change moisture and winds around the ocean.
The IOD interacts with ENSO and the Indian monsoon, but it follows its own ocean–atmosphere pattern. A positive IOD can sometimes favour parts of the monsoon circulation. It cannot automatically cancel El Niño or rescue a weak season. Timing, location, strength and other atmospheric conditions all matter, so two years with the same broad label can produce different regional rainfall.
The MJO and BSISO mainly help explain changes over days to several weeks. ENSO and the IOD describe more of the seasonal background. These timescales still interact: a season influenced by El Niño can have a powerful active spell, and a favourable short-term phase can occur within a weak season. At any moment, the monsoon reflects both its slowly changing background and the weather systems developing within it.
How the summer monsoon withdraws and rain shifts south
Withdrawal begins in the northwest
By late summer, the Northern Hemisphere moves away from its period of strongest summer heating. Solar heating weakens over northern India, the continental low-pressure pattern becomes less prominent, and the monsoon trough and rain belt start moving south. The lower winds and the moisture carried through the atmosphere change with them.
Withdrawal usually begins in northwestern India. Rain decreases there for a sustained period, moisture falls through a deep layer, and lower winds begin turning clockwise around a region of relatively high pressure. Meteorologists call this an anticyclonic pattern. Its spreading drier flow replaces the deep, moist monsoon conditions over a growing area.
One dry day does not establish withdrawal. Drying must persist while winds and moisture also change. The withdrawal boundary then moves south-east as the seasonal circulation retreats from adjoining areas. Local showers can still occur behind it, just as isolated rain can occur before onset.
Withdrawal follows a different process from onset because the land, seas and upper atmosphere have entered a different seasonal state. The soil and ocean retain heat from the summer, the rain belt follows a different path south, and weather systems continue to form over warm seas. The monsoon trough weakens or moves away, lower-level south-westerlies lose their dominance, and drier continental winds spread across the north and interior. Tropical systems can still bring rain to some regions during this retreat.
This withdrawal forms part of India's post-monsoon or retreating-monsoon transition. Those broad seasonal names describe the countrywide change after the summer monsoon begins to retreat. They do not mean that a regional northeast monsoon has started everywhere.
Why easterly winds can bring rain to Tamil Nadu
As the main convergence and rain belt shifts south, lower winds over the southern Bay of Bengal and the southern peninsula increasingly blow from the east or northeast. The name of a wind tells us where it comes from, not whether it is dry or moist. Air may begin over the continent, cross the Bay of Bengal, gather water vapour and then approach the southeast coast full of moisture.
We must also describe wind relative to the coastline. An onshore wind blows from sea to land, an offshore wind blows from land to sea, and an alongshore wind runs roughly parallel to the coast. A northeast or easterly wind can become onshore when it crosses the Bay and reaches the east-facing Coromandel and Tamil Nadu coast. Compass direction alone cannot tell us how much sea the air crossed or whether terrain will lift it.
The Bay supplies moisture, while the southward-shifted convergence zone and passing disturbances help the air rise. Lows and depressions over the Bay can organise large areas of cloud and rain. Smaller easterly disturbances can also travel west with patches of enhanced cloud. Tropical cyclones sometimes produce very heavy rain, but ordinary rain spells do not all require a cyclone.
The regional rainy circulation that develops over the southern peninsula during this part of the year is called the northeast monsoon. It is related to the wider post-monsoon transition, but it is not another name for southwest-monsoon withdrawal. One describes the retreat of the summer circulation across India; the other describes a later rain-bearing wind, convergence and weather-system regime over a particular region.
Tamil Nadu receives some rain during the southwest-monsoon months as well. Yet much of the state lies east of the Western Ghats and receives less rain from the Arabian Sea flow than the windward west coast. During October to December, Bay-crossing easterly and northeasterly winds approach its east-facing coast from the sea. This later period supplies a much larger share of the annual rain in many parts of Tamil Nadu and the adjoining southeast coast. Its share is much smaller across most of India.
The balance differs within the region. Coastal exposure, nearby hills, the exact wind path and the tracks of Bay systems change the amount and timing. Tamil Nadu is neither completely dry during the southwest monsoon nor uniformly wet during every northeast-monsoon season.
Clearer skies can allow the land to warm during parts of the early retreat, while moist air keeps conditions uncomfortable. This is often called October heat. It is a regional transition experience rather than the cause of the southeast-coast rain.
The southwest monsoon first withdraws as persistent drying, lower anticyclonic flow and moisture decline spread from the northwest. The national post-monsoon transition follows. Farther south, a different regional circulation brings Bay moisture towards the southeast coast and produces the northeast-monsoon rainy period. These are connected stages of the seasonal change, but they are not three names for the same process.
How all parts of the monsoon fit together
We can now return to the observation with which we began: a hot early-summer sky giving way to humid winds, large cloud bands and widespread rain. Summer heating helps change the broad pressure pattern and move the tropical rain belt towards South Asia. Air crosses the equator, curves into south-westerlies, gathers ocean moisture, rises over terrain and convergence zones, and produces rain.
The ITCZ and monsoon trough are names for the shifting convergence region. The Mascarene High, heat low and pressure gradients help drive air across the equator. The Somali jet concentrates part of that flow, while the Arabian Sea and Bay of Bengal pathways carry moisture around the subcontinent.
Mountains change where the moist air rises and where rain falls. The Western Ghats lift Arabian Sea air and create a rain shadow inland. The Khasi Hills help create the exceptional Meghalaya rainfall belt. Across the plains and central India, the paths of the monsoon trough and travelling systems redistribute rain, while the northwest remains generally drier. The Himalayas guide the flow, favour ascent and limit the easy entry of dry northern air. The Tibetan Plateau influences regional heating, while deep monsoon clouds release latent heat and help sustain the mature circulation.
The circulation also changes with height. Moist lower winds feed rising clouds, released latent heat warms the deep column, and upper air spreads away through the anticyclone and easterly flow. The subtropical westerly jet moves north of the mountain barrier as summer develops, while the tropical easterly jet forms high above South Asia. These upper winds show that the monsoon extends far above the surface.
Onset and advance describe how this circulation takes hold across India. Moving troughs, low-pressure systems and larger cloud patterns produce active spells and breaks after it arrives. El Niño, La Niña and the IOD change the seasonal background without deciding every outcome. Withdrawal begins when the summer pressure, wind and moisture pattern weakens and retreats. Later, Bay-crossing easterlies help create a distinct northeast-monsoon rainy period over the southeast.
This is why “strong monsoon” is incomplete unless we ask: strong in which place, over what period and according to which measure? Fast onset, high national rainfall, a wet central region and many extreme events are different outcomes. A careful reader asks where air and moisture are moving, where air is rising, at what height, during what period and over which region.
The Indian monsoon is a seasonal circulation that carries water from the ocean through the atmosphere to the land. The land, ocean, mountains and clouds continually influence one another. The many technical names in this chapter describe particular parts or variations of that connected system.