Guwahati: Researchers at IIT-Madras and Gauhati University have developed a method to identify the sustained arrival of the monsoon at specific locations, potentially improving farming, water management and flood planning.The climate-network approach distinguishes sustained monsoon rainfall from brief, localised showers. It could complement conventional onset definitions and support planning for agriculture, water resources, food security and flood preparedness, researchers said.The findings were published in Journal of Climate, a peer-reviewed journal of the American Meteorological Society.Prof RI Sujith of IIT-Madras said the method could provide farmers with more reliable sowing information and help policymakers plan food security and water-resource management.“The key 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 Sujith, head of the Centre of Excellence for Studying Critical Transitions in Complex Systems at IIT-Madras.The study treats geographical locations as nodes in a climate network, with links representing spatial connections between locations experiencing rainfall onset. Researchers tracked the growth and movement of these clusters to determine when rainfall at a location became part of the larger monsoon system.The study, co-authored by Yogenraj Patil, Gaurav Chopra, Shruti Tandon and Sujith of IIT-Madras, and B N Goswami of Gauhati University, analysed 84 years of rainfall data. They found that large-scale monsoon onset occurs climatologically around May 15 over the northeast and June 9 over the Indian peninsula.“The onset is marked by an abrupt increase in size of the largest cluster of rainfall onsets, followed by its continued spatial growth and northward progression across India,” the researchers said.Goswami said the method was intended to complement, not replace, conventional operational definitions. It links local observations to the larger-scale organisation of the monsoon and can identify large-scale onset at different locations.The study found that the peninsula’s large-scale onset is followed by northward propagation of rainfall associated with the first pulse of the monsoon intraseasonal oscillation (MISO), indicating the establishment of the monsoon rather than a transient event.Using the method, researchers identified June 22 this year as the onset date of the large-scale monsoon over the Indian peninsula. Its northward progression was reflected in heavy rainfall in Mumbai by June 24 and Delhi by July 1.
