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Study says the monsoon first takes shape over Northeast India

Researchers from IIT Madras and Gauhati University propose a new way to tell when the monsoon has really arrived, as opposed to a passing local shower.

By Delulublog Desk · · 4 min read

TL;DR in 30 sec

  • The large-scale monsoon first forms over Northeast India around 15 May, nearly three weeks before it is established over the Indian peninsula, according to a study by IIT Madras and Gauhati University reported by Down To Earth.
  • The study, in the Journal of Climate, analysed 84 years of rainfall data, The Tribune and Deccan Chronicle reported.
  • Its method separates a short local spell of rain from the arrival of the wider monsoon system, The Tribune reported.
  • For 2026, the method placed the large-scale onset over the peninsula on 22 June, NE Now reported; the India Meteorological Department had declared the onset over Kerala on 4 June.
Rain clouds form over green hills on the left and over a coastal palm grove on the right, joined by a chain of linked dots above a farm field with young seedlings.
Illustration

What happened

Researchers from IIT Madras and Gauhati University in Assam have published a new way of working out when the monsoon has really arrived, The Tribune reported. The study appeared in the Journal of Climate, a journal of the American Meteorological Society.

The authors are Yogenraj Patil, Gaurav Chopra, Shruti Tandon and R.I. Sujith of IIT Madras and B.N. Goswami of Gauhati University, NE Now reported.

The team used a “climate network”, NE Now reported. Think of it as a map of dots. Each location is a point, and the links between points stand for connections between the dates on which rain starts there.

A location counts as having its large-scale onset when its local rain joins the largest connected cluster of such onsets, according to The Tribune.

Looking at 84 years of rainfall data, the researchers found that large-scale onset typically comes around 15 May over Northeast India and around 9 June over the Indian peninsula, The Tribune and Deccan Chronicle reported. Down To Earth said the data covered 1940 to 2023.

Clusters of rain onset begin forming over Northeast India in April, partly from thunderstorms, Down To Earth reported. Around 15 May, that cluster merges with a system moving north from the southeastern Bay of Bengal and starts spreading west. A separate cluster forms over the peninsula, mainly Kerala, and the two merge around 9 June, according to Down To Earth.

The method put the large-scale onset over the peninsula in 2026 on 22 June, NE Now reported. Heavy rain followed in Mumbai by 24 June and in Delhi by 1 July, according to NE Now.

Why it matters for India 🇮🇳

The researchers say their method could help farmers time sowing and help governments plan water storage, food security and flood preparation, The Tribune reported.

“The monsoon’s arrival at a location is not necessarily the first rainfall experienced there,” Sujith said, as quoted by Deccan Chronicle.

The study suggests that the large-scale rainy season in Northeast India and the southwestern peninsula is 20 to 30 days shorter than the rainy season defined by local rain alone, Down To Earth reported.

Background

The India Meteorological Department (IMD) announces the monsoon’s onset over Kerala each year. In 2026 it declared the onset on 4 June, three days after the normal date of 1 June, according to its press release.

Before declaring it, IMD checked several conditions: more cloud over the southeast Arabian Sea, westerly winds reaching up to 4.5 km above sea level at about 20-25 knots, and widespread rain over Kerala in the previous two days, its press release said.

An earlier version of the study, posted by the authors on arXiv in March 2025, says the monsoon begins in Northeast India and spreads west and north. That differs from the usual picture of a monsoon that starts in the south and moves north and west, according to the preprint.

The study also looked at the end of the season, Down To Earth reported. Large-scale withdrawal begins over Northwest India around 22 September and moves quickly across most of the country by 22 October. Over the southeastern tip of the peninsula, it continues into December.

Different perspectives

Goswami said the method is meant to complement the conventional ways of defining onset, not replace them, according to The Tribune and Deccan Chronicle.

So why do the dates differ? IMD’s 4 June and the study’s 22 June measure different things: IMD’s onset over Kerala and the study’s large-scale onset over the whole peninsula, according to the IMD press release and NE Now.

The sources carried no response from IMD to the study, and no comment from scientists outside the research team.

Open questions 🤔

  • Will the India Meteorological Department use the new method alongside its own onset declaration?
  • Can the method warn of the monsoon’s arrival in advance, or only identify it once the rain data is in?
  • Will farmers’ sowing advice change if the method is adopted?

What to watch next

No follow-up study or official response has been announced in the sources.

Sources & Credits

This article is based on the following reporting and official sources. Credit to each original publisher.

  1. Monsoon's first large-scale onset occurs over Northeast India, suggests IIT-Madras, Gauhati University study · Down To Earth · accessed
  2. When does monsoon really arrive? IIT Madras, Gauhati University researchers find way to tell · The Tribune · accessed
  3. Assam: IIT Madras, Gauhati University researchers develop new method to pinpoint monsoon arrival · NE Now · accessed
  4. IIT-M study offers new way to identify when monsoon actually arrives · Deccan Chronicle · accessed
  5. Southwest Monsoon has set in over Kerala, today the 4th June, 2026 · India Meteorological Department · accessed
  6. Climatic Phase Transitions Unravel the Onset and Withdrawal of Indian Monsoon · arXiv · accessed