Researchers at the Indian Institute of Technology (IIT) Madras and Gauhati University, Assam, have developed a climate-network approach that can distinguish between a short-lived local rain spell and the arrival of the large-scale, sustained monsoon system. The research identifies when monsoon can arrive at a location, the engineering institute said.
In a press release shared with NDTV, IIT Madras noted that the study introduces an objective method to identify the large-scale onset of the monsoon at individual locations by tracking how clusters of local rainfall onsets across India organise into a much larger, interconnected system.
The approach could complement conventional monsoon onset definitions and potentially provide more reliable information for decisions related to sowing, water resources, food security and flood preparedness, the researchers added.
According to the official document, the study treats geographical locations as nodes in a climate network. As per the explanation given, links between the nodes represent spatial connections among locations experiencing local rainfall onset. The researchers tracked growth and movement of clusters of these local onsets.
Elaborating on the key findings, Prof RI Sujith, Head, Critical Transitions in Complex Systems (CoE-CTCS), IIT Madras, said that "by avoiding false onsets, our new perspective of defining large-scale onset of monsoon at every location will allow farmers time for sowing and help the policymakers to plan for food security and water resource management."
Further, Prof BN Goswami, ST Radar Centre, Gauhati University, Assam, added that "this method is intended as a complement to conventional operational definitions, rather than as a replacement for them. Its distinctive feature is that it connects a local observation to the larger-scale organisation of the monsoon, allowing the same framework to identify large-scale onset at different locations."
The researchers noted that during April, for example, small clusters of rainfall onsets develop over Northeast India, including those due to thunderstorm activity. As the season progresses, the cluster over Northeast India grows and merges with another cluster associated with the Bay of Bengal region. The researchers identified the resulting abrupt growth of the largest cluster as the large-scale onset over Northeast India.
A similar process subsequently occurs over the Indian peninsula, as per the study. A smaller cluster develops over Kerala due to local rainfall activity. When this cluster becomes connected to the largest rainfall-onset cluster propagating westwards from Northeast India, the researchers identified this transition as the large-scale onset over the Indian peninsula. This enables the methodology to separate an isolated local rain spell over Kerala from the establishment of the larger monsoon system, the study noted.
Using 84 years of rainfall data, the researchers found that, climatologically, the large-scale onset occurs around May 15 over Northeast India 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 study also found that the large-scale onset over the Indian peninsula is followed by a robust northward propagation of rainfall associated with the first pulse of the monsoon intraseasonal oscillation (MISO). This provides an additional physical indication that the identified transition represents the establishment of the large-scale monsoon rather than a transient rainfall event, the study noted.
The researchers also applied their new definition to monitor the 2026 monsoon. The researchers found that the large-scale onset over the Indian peninsula occurred on June 22, 2026. The subsequent northward progression of rainfall was reflected in heavy rainfall episodes reported in Mumbai by June 24 and Delhi by July 1.
