Monsoon

A monsoon is a major wind system that seasonally reverses its direction, blowing for approximately six months from the northeast and six months from the southwest. More broadly, monsoons are seasonal changes in the direction of the prevailing, or strongest, winds of a region, and they cause wet and dry seasons throughout much of the tropics. The most prominent monsoons occur in South Asia, Africa, Australia, and the Pacific coast of Central America, while the Gulf Coast of the United States and central Europe experience only monsoonal tendencies rather than true monsoons.

The primary cause of monsoons lies in different warming trends over land and sea, so monsoon winds typically travel from sea to land in summer and from land to sea in winter. Monsoons always blow from cold to warm regions, and the summer and winter monsoons together determine the climate for most of India and Southeast Asia. The basic cause of the Asian monsoon is that the Eurasian landmass and the Indian Ocean warm and cool at different rates through the seasons: in summer the land surface gets hotter than the ocean, hot air above the land rises, and moist ocean air flows in to replace it, while in winter air over the land becomes colder and denser, warmer air over the ocean rises, and cold air from the north blows in to replace it. Land heats up faster than water, so as the sun intensifies over the Indian subcontinent between March and May the land becomes much hotter than the surrounding seas, and this contrast makes warm air rise and creates a low-pressure zone that draws in cooler, moisture-laden air from the Indian Ocean. A lack of snow allows the land surface to heat up more in summer, and the resulting widening temperature contrast between land and ocean strengthens the monsoon.

The heat source for the Indian summer monsoon resides over the Plateau of Tibet and the eastern foothills of the Himalayas, while for the Australian summer monsoon the heat source lies over the Maritime Continent and the heat sink over Siberia. The Tibetan High is a warm anticyclone located over the Tibetan Plateau in the middle and upper troposphere during the monsoon period, marked at the 300 hPa level with its centre near 30 N, 90 E, and in July its outflow concentrates into an easterly jet stream centred near the latitude of Chennai at 150 hPa.

The Inter-Tropical Convergence Zone appears as a band of clouds consisting of showers and occasional thunderstorms that encircles the globe near the equator and may extend for many hundreds of miles. It forms because the northeast trade winds from the Northern Hemisphere converge with the southeast winds from the Southern Hemisphere, forcing air up into the atmosphere.

In AR6 the global monsoon is defined as the area where the annual range of precipitation (local summer minus local winter) exceeds 2.5 mm per day. The global monsoon represents the leading empirical orthogonal function (EOF) mode of the annual variations of precipitation and circulation in the global tropics and subtropics, a forced response of the coupled climate system to the annual cycle of solar insolation. Variability of the global monsoon represents, to a large extent, changes in the Intertropical Convergence Zone and the associated Hadley circulation.

The regional monsoon systems recognised in AR6 comprise the South and South East Asian, East Asian, West African, North American, South American and Australian–Maritime Continent monsoons. The definition of regional monsoons was slightly modified in AR6, because some continental areas identified by the global precipitation metric have a seasonality not necessarily of monsoon origin, and the qualification of regions such as South Africa, Central America and equatorial South America as monsoons remains a subject of discussion. The East Asian winter monsoon weakened significantly in the mid-1980s, a change linked to atmospheric quasi-stationary planetary waves and external forcings, and it was observed to recover from that weak epoch in the mid-2000s.

The South American Monsoon System has well-defined wet seasons from October to March and dry seasons from June to August despite the lack of a seasonal reversal in wind direction. The Working Group on American Monsoons works with the CLIVAR/GEWEX Monsoons Panel on the American Monsoon system, comprising the South American, Central American and North American monsoons, while the Asian-Australian Monsoon Working Group addresses the South Asian, East Asian and Australian monsoons. Both groups pursue three key aims: promoting engagement among research, operational prediction and stakeholders; providing authoritative information on process understanding, model fidelity and forecast skill; and identifying key research priorities.

The South Asian monsoon is the most prominent of the world's monsoon systems, and its influence is felt across India and the surrounding water bodies. Its defining mechanism is a seasonal reversal of winds: the northeast winds of the cooler months give way to southwest winds during the warmest months, which deliver large amounts of rain in June and July. In Southwest Asia this cycle dominates the climate, with summer winds blowing from ocean to land and bringing heavy rain to India, Pakistan, Bangladesh and Myanmar, while in winter the flow reverses and blows from land to sea. During the boreal summer, Southern Hemisphere winds cross the equator and bring abundant monsoon rains over South, Southeast and East Asia, a region home to nearly 60% of the world's population, where agriculture is the main source of subsistence.

The advance of the system follows a seasonal progression. By May the southwest monsoon is well established over Sri Lanka; it typically begins over southwestern India around late May or early June and advances north-westward to reach the northernmost regions by mid-July. The Himalayas act as a natural barrier that halts its advance and causes intense rainfall in the northeastern foothills, where Mawsynram receives nearly 12 metres of rain annually. In the winter monsoon, the surface flow is deflected by the Coriolis force into a northeasterly flow, producing an October to December rainy season for the extreme southeast of the Deccan, including the Madras coast, and for eastern Sri Lanka. Midlatitude upper air flowing along the south face of the Himalayas can bring drought to northern districts and dry spells known as breaks of the monsoon, while waves of low pressure appear in the body of monsoonal air mainly in July and August, with fully developed depressions appearing once or twice per month.

The seasonal reversal has dramatic effects on the Arabian Sea, which is landlocked to the north and isolated from large-scale ocean circulation; summer southwest winds push surface waters toward Asia, and cold water wells up along the coast of Somalia and the Arabian Peninsula. The monsoon climate produces high annual rainfall totals, exceeding 1.5 m (5 ft) in many places. The southwest monsoon accounts for 75 to 90 per cent of annual rainfall in most parts of South Asia, making it the lifeblood of national economies, agricultural production and food security.

The southwest monsoon normally sets in over Kerala around 1 June and advances northwards, usually in surges; the northern limit of monsoon is the northernmost limit up to which it has advanced on any given day. IMD declares the onset of the monsoon over Kerala when 60 per cent of the 14 listed stations report rainfall of 2.5 mm or more for two consecutive days, with the onset dated on the second day provided the wind field and outgoing longwave radiation criteria are also met. The wind field criterion requires the depth of westerlies to be maintained up to 600 hPa in the box from the equator to 10 N between 55 E and 80 E, with zonal wind speed over 5-10 N, 70-80 E of the order of 15 to 20 knots at 925 hPa. The third criterion is that INSAT-derived outgoing longwave radiation should be below 200 wm-2 in the box confined by 5-10 N and 70-75 E.

The criteria IMD uses for declaring withdrawal of the monsoon from the extreme north-western parts of the country were adopted in 2006 and are considered only after 1 September. They cover cessation of rainfall over the area for 5 continuous days, establishment of an anticyclone in the lower troposphere and considerable reduction in moisture content as seen in satellite water vapour imageries.

The long period average is the rainfall recorded over a region for an interval such as a month or season, averaged over a long period like 30 or 50 years; the all-India south-west monsoon long period average based on 1961-2010 stands at 880.6 mm. Against this average, rainfall is classified as large excess at 60 per cent or more, excess at 20 to 59 per cent, normal between -19 and +19 per cent, deficient at -59 to -20 per cent and large deficient at -99 to -60 per cent.

The dominant driver of year-to-year variations in the Asian-Australian monsoon is the El Nino Southern Oscillation, in which El Nino warming and La Nina cooling occur in the central and eastern equatorial Pacific, with El Nino normally suppressing monsoon rainfall and La Nina enhancing it.

IMD releases seasonal or long-range forecasts for the southwest monsoon in two stages: a first forecast in mid-April giving an overall estimate of the rain the country is expected to receive, and a second at the end of June that updates it and adds expected July rainfall and likely regional rainfall patterns. Over the four years from 2021 to 2024 IMD maintained 100 per cent accuracy in forecasting the all-India southwest monsoon rainfall, with each year's forecast falling within the accepted margin of error of plus or minus 5 per cent of the long period average.

In 2021 the southwest monsoon current arrived over the south Andaman Sea and Nicobar Islands on 21 May, set in over Kerala on 3 June four days later than normal, covered the entire country on 13 July, and withdrew from the entire country on 25 October. North-west India and central India received normal rainfall at 96 and 104 per cent of the long period average, the south peninsula received above normal rainfall at 111 per cent while east and north-east India received below normal rainfall at 88 per cent, and 20 of the 36 meteorological subdivisions covering 58 per cent of the country's area recorded normal seasonal rainfall.

Rainfall is likely to be below normal during the June to September 2026 southwest monsoon season across much of South Asia, with the strongest signal over central regions.

The North American Monsoon is a seasonal change in the atmospheric circulation that occurs as the summer sun heats the continental land mass. It is a summer shift in wind patterns caused by intense solar heating, reversing the flow from dry land areas to moist ocean areas. During much of the year the prevailing wind over northwestern Mexico, Arizona and New Mexico is westerly and dry. Low-level moisture is transported primarily from the Gulf of California and eastern Pacific, and Sierra Madre Occidental forests add further moisture once they green up from the first monsoon rains. The monsoon happens once a year in the middle of summer, when warm, moist air from the Gulf of California and from the Gulf of Mexico meet over the Sierra Madre Occidental mountains in central Mexico.

The core of the North American Monsoon is located in northwestern Mexico and extends into the southwestern United States across much of Arizona and New Mexico. The circulation pattern typically develops in late May or early June over southwest Mexico and arrives in mid to late June over northwest Mexico and early July over the southwest United States. The monsoon season in northern Arizona officially runs from 15 June through 30 September, but monsoonal moisture typically does not reach the region until the first week of July.

Rainfall from the North American Monsoon supplies northwestern Mexico with upwards of 75 per cent of its average annual precipitation and Arizona and New Mexico with more than 50 per cent during July through September. 40 to 50 per cent of northern Arizona's annual precipitation falls during the monsoon season, refilling reservoirs and supporting farmers and ranchers. As monsoon moisture increases and upper-level focusing mechanisms strengthen, flash flooding becomes the main concern. Flash flooding is the biggest threat in northern Arizona, most flood fatalities happen in vehicles, and 12 inches of water can sweep a car off the roadway. Lightning poses a serious threat to outdoor activities and no outside location is safe when thunderstorms are near, while safe locations include a substantial building or an enclosed metal-topped automobile.

Wetter-than-average North American monsoons are slightly more common during La Nina years, while drier-than-average monsoons are slightly more common during El Nino years. Monsoon conditions, though less intense than the Asian monsoon, also occur in northern Australia, parts of Africa and the Americas.

Nearly 64 per cent of Indians depend on agriculture, which mainly relies on the southwest monsoon, and with only about 55 per cent of the net sown area irrigated the rest of the farmland depends on rain-fed systems and is vulnerable to changes in rainfall.

A lackluster monsoon can seriously impact food prices and India's overall economic growth. In one poor monsoon year, sugar, legume and potato prices rose and many estimated that India's gross domestic product growth rate could drop by a full percentage point.

In the low-lying deepwater areas of western Bangladesh and eastern India, farmers have adopted sequences using appropriately selected Boro rice cultivars that take advantage of slowly receding post-monsoon water, with persistent shallow groundwater accessible for late-season supplementary watering.

The season's performance is tracked against the long period average, and in 2024 the June to September season recorded 934.8 mm of rainfall for the country, or 108 per cent of the 868.6 mm long period average based on the 1971-2020 average ^{10Monsoon (Press Note) · Press Information Bureau, Government of India ↗}.

• 21 May 2021
  Arrival over the south Andaman Sea and Nicobar Islands
  The southwest monsoon current arrived over the south Andaman Sea and Nicobar Islands. ^{11Monsoon Report 2021 (IMD Met. Monograph MoES/IMD/SYNOPTIC MET/01(2022)26) · India Meteorological Department, Pune ↗}
• 3 June 2021
  Onset over Kerala
  The monsoon set in over Kerala, four days later than normal. ^{11Monsoon Report 2021 (IMD Met. Monograph MoES/IMD/SYNOPTIC MET/01(2022)26) · India Meteorological Department, Pune ↗}
• 13 July 2021
  Coverage of the entire country
  The monsoon covered the entire country. ^{11Monsoon Report 2021 (IMD Met. Monograph MoES/IMD/SYNOPTIC MET/01(2022)26) · India Meteorological Department, Pune ↗}
• 25 October 2021
  Withdrawal from the entire country
  The monsoon withdrew from the entire country. ^{11Monsoon Report 2021 (IMD Met. Monograph MoES/IMD/SYNOPTIC MET/01(2022)26) · India Meteorological Department, Pune ↗}
• 2026
  Seasonal outlook
  Rainfall is likely to be below normal during the June to September southwest monsoon season across much of South Asia, with the strongest signal over central regions. ^{12South Asia is expected to receive below average monsoon rainfall · World Meteorological Organization ↗}