IIT Madras, Gauhati University team finds new way to pin monsoon onset

Researchers from IIT Madras and Gauhati University have developed a new way to work out when the monsoon has really arrived at a place, separating sustained monsoon rain from short-lived local showers that often mislead farmers.

The study has been published in the Journal of Climate of the American Meteorological Society, NorthEast Now reported.

The problem with the first rain

In much of India, the first rain of the season at a location can come from thunderstorms or other local weather rather than the large monsoon system. A brief wet spell can then be followed by a long dry gap, leaving farmers unsure when to sow.

The new approach looks beyond the first rainfall. The team treats locations across India as points in a climate network, linking them according to how their rainfall-onset events are related. As the monsoon advances, these links form clusters. The monsoon is said to have arrived at a place when its rainfall onset joins the largest connected cluster.

R.I. Sujith, who heads IIT Madras's Centre of Excellence for Studying Critical Transitions in Complex Systems, said avoiding false onsets would give farmers time to plan sowing and help policymakers with food security and water management.

B.N. Goswami of the ST Radar Centre at Gauhati University said the method is meant to complement existing operational methods, not replace them.

What it says about the North East

Over Northeast India, small clusters of rainfall onset begin forming in April, partly because of pre-monsoon thunderstorms. These later grow and merge with a cluster linked to the Bay of Bengal, and the researchers mark that sudden growth as the large-scale monsoon onset over the region.

Using 84 years of rainfall data, the team found that large-scale onset typically happens around May 15 over the North East and around June 9 over the Indian peninsula. On the peninsula, a smaller cluster over Kerala becomes connected to the network spreading westward from the North East.

Applied to this year, the method placed large-scale onset over the peninsula on June 22, 2026, followed by heavy rain in Mumbai by June 24 and Delhi by July 1.

Why it matters for the region

For the North East, where early-season storms and floods are common, a clearer signal of when the monsoon has truly set in could help with planning for paddy sowing, reservoir operations and flood preparedness.

The paper was co-authored by Yogenraj Patil, Gaurav Chopra, Shruti Tandon and R.I. Sujith of IIT Madras with B.N. Goswami. The researchers said the framework would need further development before it could be used operationally.