India's 2026 Monsoon Brought 87 Percent of Normal Rain, and Here Is What Scientists Do and Do Not Yet Know About How It Works

India’s weather department closed the 2026 monsoon season on 30 September with 759.4 mm of rain averaged over the country. The long period average (LPA) it measures against, taken from 1971 to 2020, is 868.6 mm, so the season came in at 87 percent of normal. The India Meteorological Department (IMD) ranks that fourth lowest since 2001 and 13th lowest since 1901, and its scale calls anything under 90 percent deficient.

87 percent of normal, and where the shortfall fell

June was the weakest month at 65 percent of its normal. July recovered to 101 percent, August dropped to 84 and September reached 92. IMD’s season report says no low-pressure system formed over the country in June, a month that averages three, and links that to the month’s deficit. Over the whole season there were 14 low-pressure systems and 77 low-pressure days against a normal of 57.

Central India received 97 percent of its normal and northwest India 94. The south peninsula got 76 percent, the fourth lowest there since 1901. East and Northeast India got 74 percent, its lowest total in IMD’s record back to 1901. That region includes the Khasi Hills, which hold the wettest place India measures. Of 36 meteorological subdivisions, 18 had normal rain, 17 were deficient and one, Odisha, was in excess. The core monsoon zone, where most rain-fed farming happens, got 779.9 mm, just under IMD’s normal band of 782.8 to 881.2 mm.

A rain belt that follows the sun north

The usual classroom account says land heats faster than sea in summer, pressure falls over the land, and moist air from the ocean is drawn in. IMD’s own FAQ carries part of that. In June, it says, the dry ground of northwest India, Pakistan and the Middle East passes about 160 watts per square metre of heat into the air, and a shallow trough called the heat low forms. An intense heat low acts as a suction device for moist air, IMD says, and it points to 1987, a drought year, when the heat low was weak.

A 2020 review in Reviews of Geophysics by Ruth Geen, Simona Bordoni, David Battisti and Katrina Hui describes a shift in the field. Monsoons were long treated as giant sea breezes driven by land-sea contrast, the authors write. They argue that theory and observation now support a different picture, in which a monsoon is the seasonal migration of the Intertropical Convergence Zone, the band where air flows together and rises into rain. The Ministry of Earth Sciences’ 2020 assessment of climate change over the Indian region uses that framing too, describing the annual cycle as the shift of the rain band from the southern to the northern Indian Ocean. It still lists the land-sea temperature gradient among the factors that matter.

The sources here describe the same annual sequence. How best to explain it is still argued, and Geen and her colleagues name the effect of mountains and continent shapes as something their picture does not yet include.

The jet that switches on in two weeks and the wall that holds the rain

The low-level wind that carries the moisture is the Somali jet, also called the Findlater jet. IMD’s FAQ places it 1 to 1.5 km above sea level, crossing the equator along the East African coast from near Mauritius and northern Madagascar, reaching India’s west coast in June and peaking in July. William Boos and Kerry Emanuel found in 2009 that it arrives abruptly. In a composite of 27 years of reanalysis data, its speed went from winter to summer values in under two weeks, while a smooth seasonal curve fitted to the same data took about seven weeks. They also write, citing a 1969 study by Findlater, that about half of the world’s low-level flow across the equator in northern summer passes through the region of this jet, which spans only 10 percent of the Earth’s circumference. They discuss several proposed explanations for the abruptness and do not rank them.

Mountains then sort the rain. The ministry’s assessment says the windward side of the Western Ghats gets the heaviest falls and the leeward side sits in a strong rain shadow. It says the Himalaya blocks warm, moist air on its south-facing slopes.

The part played by the Himalaya and the Tibetan Plateau is contested. The plateau had long been held to act as an elevated heat source, Boos and Zhiming Kuang noted in Nature in 2010. In their model, removing the plateau but keeping the narrow Himalaya and neighbouring ranges left the large-scale monsoon circulation largely unchanged. Further results, they wrote, suggest the mountains matter because they insulate warm, moist air over India from the cold, dry air to the north. The evidence is model experiments with supporting observations, and the authors called for a reinterpretation of how South Asian climate responded to the uplift of the mountains.

El Niño, the Indian Ocean and why dry years differ

The monsoon’s year-to-year swings track the ocean. A 132-year rainfall record shows that severe Indian droughts have always come with El Niño, the periodic warming of the equatorial Pacific, according to a 2006 paper in Science by Krishna Kumar and colleagues. El Niño has not always produced a severe drought, though. The paper found that events with the warmest water in the central Pacific were more effective at producing drought-causing sinking air over India than events centred in the eastern Pacific. The ministry’s assessment says the link between El Niño and the monsoon has weakened in recent decades, and that droughts have become more frequent and more intense, some of them without an El Niño. The assessment says it has been suggested that global warming could explain the weaker link, so the cause is still open. Two earlier failures, in 1876 and in 1965 and 1966, are told in the story of the grain that left India during a famine and the story of how India learned to feed itself.

The Indian Ocean has its own switch, the dipole, a difference in sea temperature between its western and south-eastern parts. Karumuri Ashok, Zhaoyong Guan and Toshio Yamagata reported in 2001 that over 1958 to 1997 the monsoon’s link to the dipole strengthened when its link to El Niño weakened, and the reverse. They found that in 1997 a positive dipole replaced El Niño’s sinking air with converging winds, which yielded a normal monsoon despite a strong El Niño.

The 2026 season fit the El Niño pattern in part. IMD’s report says a weak El Niño developed in June, strengthened through the season and was strong by its end. The dipole stayed neutral all season, although the April forecast had expected a positive phase near the end. IMD says its El Niño and dipole forecasts were broadly consistent with what happened.

Kerala’s date: 1 June, give or take a week

The monsoon normally sets in over Kerala on 1 June, with a standard deviation of about seven days, IMD says. A 2017 paper in the Journal of Earth System Science by P. N. Preenu, P. V. Joseph and P. K. Dineshkumar covers the record from 1870 to 2014. Its earliest onset was 11 May 1918 and its latest 18 June 1972. The long-term mean was 1 June and the authors saw no linear trend in the date over that period.

IMD declares onset by rule. Since 2016 the test has been that, after 10 May, 60 percent of 14 stations from Minicoy to Mangalore report 2.5 mm of rain or more on two days running, that the westerly winds reach up to the 600 hPa level, and that outgoing longwave radiation, a satellite measure of cloud, falls below 200 watts per square metre in a defined box over the southeast Arabian Sea.

In 2026 the monsoon reached the Andaman Sea on 16 May, six days before its normal date. It set in over Kerala on 4 June, three days late, and covered the country on 9 July against a normal of 8 July. On 15 May IMD had forecast 26 May with a margin of plus or minus four days. The gap was nine days by the calendar, though IMD’s report calls it eight. The report says this was only the second forecast since the series began in 2005, after 2015, that did not hold. In the six years before, the forecast date was off by one to four days.

How often the seasonal forecast was right

IMD says it brought back the all-India seasonal forecast in 1988 and introduced new models in 2003 and 2007. By its own account, the largest errors over 1988 to 2019 came in 1994, at 21 percent of the LPA, and 2002, at 20 percent. The average absolute error was 8.91 percent of the LPA over 1995 to 2006 and 6.25 percent over 2007 to 2019.

Published evaluations were harsher. In Current Science in 2005, Sulochana Gadgil, M. Rajeevan and Ravi Nanjundiah wrote that neither IMD’s empirical models nor national and international dynamical models had predicted the 13 percent deficit of 2004, as with 2002. Their analysis of IMD’s operational predictions from 1932 suggested the skill had not improved over seven decades. A 2015 paper in Nature Communications, with Rajeevan among its authors, measured the correlation between IMD’s operational statistical forecasts and the rainfall at minus 0.12 for 1989 to 2012. It named 1994, 2002, 2004 and 2009 as extreme years the official forecasts missed.

The ministry credits a multi-model ensemble, which combines forecasts from several coupled climate models, with a better record. In a Rajya Sabha reply of 13 August 2026, it gave an average absolute error of 7.8 percent of the LPA for 2016 to 2020, against 3.1 percent for the first-stage forecast of 2021 to 2025 and 2.2 percent for the update. Those figures come from the ministry, and the peer-reviewed evaluations above cover earlier years.

For 2026, IMD’s April forecast was 92 percent with a margin of 5, and the late-May update was 90 percent with a margin of 4. The outcome was 87. IMD says both correctly anticipated below-normal rain for the country as a whole. Its regional and monthly forecasts did less well. It had expected normal rain in northeast India, which got 74 percent, and below-normal rain in central India, which got normal. July and September were forecast below normal and came in normal.

Mission Mausam, the Bharat Forecast System and what they do not forecast

The Union Cabinet approved Mission Mausam on 11 September 2024 with Rs 2,000 crore (Rs 20 billion) over two years. The Ministry of Earth Sciences runs it through IMD, the Indian Institute of Tropical Meteorology (IITM) in Pune and the National Centre for Medium-Range Weather Forecasting. Its listed parts include next-generation radars and satellites and improved earth system models. A ministry reply of 23 July 2026 says IMD now receives data from 50 Doppler weather radars and is adding 29 C-band, 45 X-band and 12 S-band radars, along with 19 wind profilers, 25 microwave radiometers and 60 radiosonde stations.

The Bharat Forecast System was unveiled on 26 May 2025. Developed at IITM, it narrows the grid of the weather model from 12 km to 6 km. At the launch the minister, Jitendra Singh, cited a 30 percent improvement in extreme-rainfall forecasts, a figure this piece could not check against an outside evaluation. The ministry’s July 2026 reply lists the system with the models that produce short-, medium- and extended-range weather forecasts. The seasonal outlook comes from a separate system, the multi-model ensemble of coupled climate models.

What the IPCC and Indian assessments say about the trend

The IPCC’s Sixth Assessment Report, in its chapter on the water cycle, says observed monsoon changes over South Asia, East Asia and West Africa were limited over much of the 20th century because increases from greenhouse warming were cancelled out by decreases from cooling by human-made aerosols (high confidence). The Asia fact sheet says the South and Southeast Asian monsoon weakened in the second half of the 20th century (high confidence), with aerosols as the dominant cause. It expects near-term changes to be dominated by internal variability, meaning natural swings (medium confidence), and South Asian monsoon rainfall to increase over the 21st century, with greater year-to-year swings (medium confidence).

The Indian assessment, edited at IITM, agrees on direction. Its chapter on precipitation reports a falling trend in all-India summer monsoon rainfall over 1951 to 2015, especially over the Indo-Gangetic Plains and the Western Ghats, with aerosols appearing to have played a role (medium confidence). Localised heavy rain has become more frequent (high confidence). The chapter lists seven suggested contributors to the drying, including aerosols, Pacific and Atlantic ocean cycles, western Indian Ocean warming and land-use change. It also notes that over 1986 to 2015 the decline is not statistically significant.

For the rest of the century the assessment projects more rain on average and more heavy-rain events over most of India (medium confidence) and a rise in year-to-year variability of summer monsoon rainfall (high confidence).