The Southwest Monsoon Explained

The southwest monsoon is India’s main rainy season, running from June to September, when winds reverse direction and carry moisture from the warm Indian Ocean over the heated subcontinent, delivering roughly three-quarters of the country’s yearly rainfall. The India Meteorological Department (IMD) treats these four months as the official southwest monsoon season, and almost every part of the country except the far south-east and the Himalayan north-west depends on it for water, crops and power. This explainer was last updated on 30 October 2026.

The southwest monsoon is not a single storm but a large-scale seasonal circulation. It arrives first over Kerala, advances in two streams over the Arabian Sea and the Bay of Bengal, covers the whole country by early July in a normal year, and then withdraws from north-west India from mid-September. Understanding how it works explains why Indian farming, reservoirs and even economic forecasts are watched so closely every summer.

Key Takeaways

• The southwest monsoon brings India roughly 75 per cent of its annual rainfall between June and September.
• The southwest monsoon is driven by the contrast between a rapidly heating Asian landmass and the cooler Indian Ocean, which reverses surface winds and pulls moist air inland.
• The southwest monsoon splits into an Arabian Sea branch, which rains heavily on the Western Ghats, and a Bay of Bengal branch, which travels up the Gangetic plain.
• The IMD classifies seasonal southwest monsoon rainfall as “normal” when it falls within 96 to 104 per cent of the long-period average.
• Farm output, reservoir levels, hydropower and rural incomes in India are all closely linked to the performance of the southwest monsoon.
• Within a single southwest monsoon season, rain arrives in active spells and break spells, so a normal seasonal total can still hide floods and dry gaps.

What Is the Southwest Monsoon?

The southwest monsoon is the summer phase of the Indian monsoon system, in which moist winds blowing from the southwest bring heavy, widespread rain to most of India from June to September. The word “monsoon” comes from the Arabic word mausim, meaning season, and originally described the seasonal reversal of winds that Arab sailors used to cross the Arabian Sea.

In India the monsoon has two phases. The southwest monsoon is the wetter and far more important one, while the northeast monsoon is the smaller winter phase that mainly affects the south-eastern coast. When people in India say “the monsoon” without any qualifier, they almost always mean the southwest monsoon.

The southwest monsoon is also a lifeline for neighbouring countries. Sri Lanka, Bangladesh, Nepal, Myanmar and Pakistan all receive a large share of their annual rain from the same circulation, which is part of the wider South Asian summer monsoon.

Why Does the Southwest Monsoon Happen?

The southwest monsoon happens because land heats up faster than the ocean, creating a low-pressure zone over northern India and a high-pressure zone over the cooler southern Indian Ocean. Air always flows from high pressure to low pressure, so moist ocean air is drawn towards the subcontinent.

Several ingredients combine to produce this effect:

• Land-sea heating contrast: By May, the Indian plains and the Thar region are intensely hot, while the ocean to the south remains cooler. The hot land air rises, leaving a deep area of low pressure.
• Shift of the Intertropical Convergence Zone (ITCZ): The ITCZ is a belt of rising air and cloud near the equator. In summer it moves north over the Indian subcontinent, where it is often called the monsoon trough, pulling the rain belt with it.
• Wind reversal: South-east trade winds from the southern hemisphere cross the equator and, because of the Earth’s rotation (the Coriolis effect), bend to the right and become southwesterlies over the Arabian Sea.
• Tibetan Plateau heating: The high plateau warms strongly in summer, strengthening the rising air over Asia and helping to shift the high-altitude jet stream north of the Himalaya.
• Somali Jet: A fast, low-level wind stream off the coast of East Africa, also called the Findlater Jet, carries a great deal of moisture across the Arabian Sea towards India.

How Does the Southwest Monsoon Move Across India?

The southwest monsoon moves across India in a northward and westward sweep that normally begins at Kerala on about 1 June and reaches the whole country by about 8 July. The advance is not smooth, and it often stalls for days before surging again.

In a typical year the monsoon first appears over the Andaman Sea and the Nicobar Islands around the third week of May. It then reaches Kerala, advances along the west coast to Goa and Mumbai by roughly the second week of June, and spreads over the north-east and the Bay of Bengal states at about the same time. By late June the monsoon normally reaches Delhi, and it covers western Rajasthan, the last region to be included, in early July.

Meteorologists describe the progress using a “northern limit of monsoon” line, which IMD updates daily. Surges in the advance are often linked to low-pressure systems forming over the Bay of Bengal, or to an intraseasonal pulse called the Madden-Julian Oscillation (MJO) travelling across the Indian Ocean.

What Are the Two Branches of the Southwest Monsoon?

The southwest monsoon has two branches, the Arabian Sea branch and the Bay of Bengal branch, which bring rain to different parts of India and meet over the north-west. Both are driven by the same southwesterly flow, but geography directs them differently.

The Arabian Sea branch

The Arabian Sea branch is the stronger stream, striking the Western Ghats head-on and giving the west coast its heaviest rain. Moist air is forced to rise along the mountain slopes, cools, and drops very heavy rain on the windward side in a process called orographic rainfall. Coastal Kerala, Karnataka, Goa and Konkan receive some of the highest seasonal totals in the country, while the Deccan plateau on the leeward side lies in a rain shadow and stays comparatively dry.

The Bay of Bengal branch

The Bay of Bengal branch curves north-westward after crossing the Bay of Bengal and the Bangladesh coast. It rains heavily on the north-eastern hills, with places such as Mawsynram and Cherrapunji in Meghalaya famous for extreme rainfall, and then travels along the Himalayan foothills and the Indo-Gangetic plain. Low-pressure systems that form over the northern Bay of Bengal steer much of this rain to eastern, central and northern India.

What Is the Monsoon Trough and Why Does It Matter?

The monsoon trough is an elongated area of low pressure stretching from north-west India to the head of the Bay of Bengal, and its position largely decides where the heaviest rain falls on any given day. It is the local expression of the ITCZ during the summer monsoon.

When the trough lies in its normal position over the Gangetic plain, rain is widespread across central and eastern India. When it shifts northward towards the Himalayan foothills, central India dries out, while the Himalayan states and the north-east receive intense rain. This situation is known as a break in the monsoon, and it can last a week or more.

The opposite pattern, in which the trough lies south of its normal position and low-pressure systems form frequently, is called an active monsoon spell. Alternating active and break spells are a natural feature of the southwest monsoon, and they explain why one district can flood while another a few hundred kilometres away stays parched.

How Is Southwest Monsoon Rainfall Measured and Classified?

Southwest monsoon rainfall is measured by IMD as a percentage of the long-period average (LPA), which is the average seasonal rainfall over a fixed multi-decade baseline, currently about 87 cm for the country as a whole. IMD issues forecasts in April and updates them in late May and again for the second half of the season.

The IMD uses the following categories for the all-India seasonal total:

• Deficient: less than 90 per cent of the LPA
• Below normal: 90 to 95 per cent of the LPA
• Normal: 96 to 104 per cent of the LPA
• Above normal: 105 to 110 per cent of the LPA
• Excess: more than 110 per cent of the LPA

The country is also divided into meteorological subdivisions, and rainfall in each is reported separately. A season can be “normal” nationally even when several subdivisions are deficient, which is why local data often matters more to a farmer than the headline figure.

What Controls the Strength of the Southwest Monsoon?

The strength of the southwest monsoon is controlled mainly by ocean-atmosphere patterns, especially the El Nino-Southern Oscillation (ENSO) in the Pacific and the Indian Ocean Dipole (IOD) in the Indian Ocean. These patterns change sea-surface temperatures and wind flow, which then change rainfall over India.

• El Nino: A warming of the central and eastern Pacific that often, though not always, weakens the Indian monsoon and raises the risk of drought.
• La Nina: The cooler counterpart, which is generally favourable for good monsoon rainfall in India.
• Indian Ocean Dipole: A positive IOD, with a warmer western Indian Ocean, tends to support the monsoon and can offset the damage of an El Nino year.
• Eurasian snow cover and Himalayan heating: Heavy winter snow over Eurasia has often been linked with a weaker monsoon, though the relationship is not perfectly reliable.
• Madden-Julian Oscillation: A 30 to 60 day eastward-moving pulse of cloud and rain that helps produce the active and break spells inside the season.

Because so many factors interact, forecasting the southwest monsoon remains difficult. Seasonal forecasts are reasonably good at the national level but much less precise for individual districts.

Why Does the Southwest Monsoon Matter for India?

The southwest monsoon matters because it supplies most of India’s fresh water and sets the rhythm of agriculture, which still supports a large share of the population. A substantial portion of Indian farmland is rainfed, so crops such as rice, pulses, cotton, soybean and groundnut grown in the kharif season depend directly on the timing and spread of the rain.

The effects extend well beyond the farm:

• Water supply: Reservoirs, rivers and groundwater tables are recharged during the monsoon and are then drawn down through the dry months.
• Energy: Hydropower output rises with good inflows, and electricity demand for cooling falls when the rains arrive.
• Economy: Rural spending, tractor and fertiliser sales, and food inflation all tend to follow the monsoon’s performance.
• Health: Standing water increases the risk of mosquito-borne diseases such as dengue and malaria, and flooding spreads waterborne infections.
• Ecosystems: Forests, wetlands, rivers and grasslands, including the Western Ghats biodiversity hotspot, flourish in the wet season.

How Is Climate Change Affecting the Southwest Monsoon?

Climate change is making the southwest monsoon more erratic, with a trend towards fewer gentle rainy days and more short, extreme rainfall events, according to a large body of climate research. A warmer atmosphere holds more moisture, so when conditions favour heavy rain, the downpours can be more intense.

Studies of Indian rainfall records have noted a rise in extreme rain events over central India and a lengthening of dry spells inside the season. At the same time, the Arabian Sea has warmed, which has been linked with an increase in intense cyclonic systems and heavy rain bursts along the west coast. Warming of the Indian Ocean also affects how moisture reaches the land.

Scientists expect total seasonal rainfall to remain highly variable, but they expect the rain to arrive in more concentrated bursts. For cities and farmers, this raises the importance of drainage, flood forecasting, water storage and climate-resilient crops.

How Does the Southwest Monsoon End?

The southwest monsoon ends through a gradual withdrawal that normally begins from west Rajasthan around 17 September and is complete across the country by about 15 October. The withdrawal is declared region by region once rainfall has stopped for several days and the winds in the lower atmosphere have turned dry and northerly.

As the sun moves south after the September equinox, the low-pressure area over north India weakens and the wind system reorganises. The skies clear, temperatures rise for a spell, and the rain belt shifts to the south-east coast, where the northeast monsoon takes over from October. Because the withdrawal timing can vary by weeks from year to year, a late-ending monsoon can bring floods to the northern plains even as other regions begin to dry.

Bottom Line

The southwest monsoon is a seasonal wind reversal that powers India’s water, farming and economy for four months each year. Its two branches, shifting trough and links to ocean patterns make it variable from season to season and place to place, and a warming climate is making that variability sharper.

Frequently Asked Questions

What is the southwest monsoon in simple words?

The southwest monsoon is the summer rainy season in India, lasting from June to September, when winds from the southwest bring moisture from the Indian Ocean. It delivers around three-quarters of the country’s yearly rainfall.

Which months does the southwest monsoon cover in India?

The southwest monsoon covers June, July, August and September in India. It normally starts over Kerala around 1 June and begins withdrawing from west Rajasthan around 17 September.

Which parts of India get the most rain from the southwest monsoon?

The Western Ghats and the west coast, along with Meghalaya and the north-east, receive the heaviest southwest monsoon rain. Places such as Mawsynram and Cherrapunji in Meghalaya are among the wettest in the world.

Why does the southwest monsoon bring so much rain?

The southwest monsoon brings so much rain because moist winds from the warm Indian Ocean are drawn inland by a heat low over the subcontinent. When this air meets hills and the monsoon trough, it rises, cools and condenses into heavy rain.

What is the difference between the southwest monsoon and the northeast monsoon?

The southwest monsoon blows from the sea to the land between June and September and brings rain to most of India, while the northeast monsoon blows from land to sea between October and December. The northeast monsoon mainly rains on Tamil Nadu, coastal Andhra Pradesh and parts of Kerala and Karnataka.

How does El Nino affect the southwest monsoon?

El Nino often weakens the southwest monsoon by disturbing the winds that carry moisture to India, which can lead to below-normal rainfall. The effect is not automatic, because a favourable Indian Ocean Dipole can partly offset it.

{“@context”:”https://schema.org”,”@graph”:[{“@type”:”FAQPage”,”mainEntity”:[{“@type”:”Question”,”name”:”What is the southwest monsoon in simple words?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”The southwest monsoon is the summer rainy season in India, lasting from June to September, when winds from the southwest bring moisture from the Indian Ocean. It delivers around three-quarters of the country’s yearly rainfall.”}},{“@type”:”Question”,”name”:”Which months does the southwest monsoon cover in India?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”The southwest monsoon covers June, July, August and September in India. It normally starts over Kerala around 1 June and begins withdrawing from west Rajasthan around 17 September.”}},{“@type”:”Question”,”name”:”Which parts of India get the most rain from the southwest monsoon?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”The Western Ghats and the west coast, along with Meghalaya and the north-east, receive the heaviest southwest monsoon rain. Places such as Mawsynram and Cherrapunji in Meghalaya are among the wettest in the world.”}},{“@type”:”Question”,”name”:”Why does the southwest monsoon bring so much rain?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”The southwest monsoon brings so much rain because moist winds from the warm Indian Ocean are drawn inland by a heat low over the subcontinent. When this air meets hills and the monsoon trough, it rises, cools and condenses into heavy rain.”}},{“@type”:”Question”,”name”:”What is the difference between the southwest monsoon and the northeast monsoon?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”The southwest monsoon blows from the sea to the land between June and September and brings rain to most of India, while the northeast monsoon blows from land to sea between October and December. The northeast monsoon mainly rains on Tamil Nadu, coastal Andhra Pradesh and parts of Kerala and Karnataka.”}},{“@type”:”Question”,”name”:”How does El Nino affect the southwest monsoon?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”El Nino often weakens the southwest monsoon by disturbing the winds that carry moisture to India, which can lead to below-normal rainfall. The effect is not automatic, because a favourable Indian Ocean Dipole can partly offset it.”}}]}]}