Chennai: Researchers at the Indian Institute of Technology Madras and Gauhati University, Assam, have developed a method to tell a short-lived local rain spell from the arrival of the sustained, large-scale monsoon at a given place, according to a statement from IIT Madras.The study uses a “climate network”, in which locations are treated as points, or nodes, and links connect locations where local rainfall has begun. The findings, published in the Journal of Climate, a peer-reviewed journal of the American Meteorological Society, describe how the researchers tracked clusters of these local rainfall starts as they grow, move and join up across India.Using 84 years of rainfall data, the researchers found that the large-scale monsoon onset typically occurs around May 15 over Northeast India and June 9 over the Indian peninsula.The method defines onset at a location as the point when its local rainfall joins the largest, planet-scale cluster of connected rainfall onsets. “The monsoon’s arrival at a location is not necessarily the first rainfall experienced there,” said R I Sujith, Head of the Centre of Excellence for Studying Critical Transitions in Complex Systems at IIT Madras. The rain is then part of the broader monsoon system, rather than being driven only by a passing local weather event. The team calls this shift a “phase transition”, meaning an abrupt change in the network’s behaviour.The process works in stages. In April, small clusters form over Northeast India, some from thunderstorms. They grow and merge with a cluster linked to the Bay of Bengal. The sudden growth of the largest cluster marks onset over Northeast India. A similar process follows in the peninsula, where a small cluster forms over Kerala. When it connects with the large cluster moving west from the Northeast, onset is declared.The peninsular onset was followed by a strong northward push of rain tied to the first pulse of the monsoon intraseasonal oscillation (MISO), a cycle of active and weak rain spells within the season.Applying the method to 2026, the team found large-scale onset over the peninsula on June 22. Heavy rain followed in Mumbai by June 24 and Delhi by July 1.Sujith said avoiding false onsets could give farmers time to sow and help policymakers plan food security and water management. B N Goswami of Gauhati University said the method complements, and does not replace, current operational definitions.
