CHENNAI: The first spell of rain does not necessarily signal the arrival of the monsoon. Researchers from the IIT Madras and Gauhati University have developed a new scientific method to distinguish isolated rainfall from the establishment of the largescale monsoon system, offering a way to identify its onset more accurately at individual locations.

The climate-network approach tracks how clusters of local rainfall onsets across India evolve, expand and merge into a larger, interconnected system. Researchers say the method could complement conventional monsoon onset analysis and help farmers make informed sowing decisions, besides supporting policymaking on food security, water-resource management and flood preparedness. The findings were published in the

a peer-reviewed journal of American Meteorological Society. The study was co-authored by Yogenraj Patil, Gaurav Chopra, Shruti Tandon and Prof RI Sujith of IIT-M’s Centre of Excellence for Studying Critical Transitions in Complex Systems, and Prof BN Goswami of Gauhati University, Assam.

Researchers developed a climate network in which geographical locations were treated as nodes, with links representing spatial connections between locations experiencing rainfall onset. By tracking the formation, expansion and merging of these clusters, they identified when rainfall at a location becomes part of the larger monsoon system.

“The insight from our research is that the monsoon’s arrival at a location is not necessarily the first rainfall experienced there. Instead, our study defines large-scale onset at a location as the point when its local rainfall onset becomes part of the largest, planetary-scale cluster of interconnected rainfall onsets,” said Prof Sujith, head of the centre. “Transition indicated that rainfall had become embedded in the broader monsoon system rather than being driven by a transient local event. This is a phase transition in a climate network.”

Prof Sujith, also an Institute Professor and D Srinivasan Institute Chair Professor in the Department of Aerospace Engineering, said that the approach could help avoid false onset signals, giving farmers time to plan sowing and policymakers a better basis for decisions on food security and water resources.

Prof Goswami of the ST Radar Centre, Gauhati University, said, “The method iss intended to complement, rather than replace, conventional operational definitions. Its USP is that it linked rainfall observations at individual locations to the larger-scale organisation of the monsoon.”

An analysis of 84 years of rainfall data showed that the climatological large-scale onset occurs around May 15 over Northeast India, and also June 9 over the Indian peninsula.

The process begins with smaller clusters of rainfall developing over Northeast India, including those associated with thunderstorm activity. As the season progresses, these clusters expand and merge with another cluster associated with the Bay of Bengal. The abrupt growth of the largest cluster marks the large-scale onset over the region.

A similar transition occurs over the peninsula when a rainfall cluster over Kerala connects with the larger system advancing westwards from Northeast India. This helps researchers to distinguish isolated showers over Kerala from the establishment of the wider monsoon system.

The study also found that the large-scale onset over the peninsula is followed by robust northward propagation of rainfall associated with the first pulse of the monsoon intra-seasonal oscillation, providing a more physical indication of the establishment of the monsoon system.

Applying the method this year, researchers identified June 22 as the date of large-scale onset over the Indian peninsula. The subsequent northward progression of rainfall was reflected in heavy rainfall in Mumbai by June 24 and Delhi by July 1.

Monsoon’s arrival at a location is not necessarily the first rainfall experienced there. Our study defines large-scale onset at a location as the point when its local rainfall onset becomes part of the largest, planetary-scale cluster of interconnected rainfall onsets — Prof RI Sujith, head, IIT-M’s Centre of Excellence