A novel complex systems approach distinguishes sustained, large-scale monsoon onset from transient local rainfall, offering a potential tool for agriculture, water management and flood preparedness
When does the monsoon actually arrive at a particular location? Researchers at the Indian Institute of Technology Madras (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 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 organize 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 findings were published in Journal of Climate (DOI: https://doi.org/10.1175/JCLI-D-25-0384.1), a leading peer-reviewed journal of the American Meteorological Society, that publishes research on climate variability, climate processes, modelling, observations and climate change.
The paper was co-authored by Yogenraj Patil, Gaurav Chopra, Shruti Tandon and Prof. R.I. Sujith from the Centre of Excellence for Studying Critical Transitions in Complex Systems (CoE-CTCS), IIT Madras and Prof. B. N. Goswami, Gauhati University
The study treats geographical locations as nodes in a climate network. 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. R.I. Sujith, Head, CoE-CTCS, IIT Madras, said, “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. In physical terms, this indicates that the rainfall at that location has become embedded within the broader monsoon system rather than being driven only by a transient, local weather event. The researchers describe this transition as a phase transition in a climate network.”
Prof. R.I. Sujith, also an Institute Professor and D. Srinivasan Institute Chair Professor, Department of Aerospace Engineering, IIT Madras, added, “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.”
The distinction is particularly important for India, where local rainfall can occur well before the sustained monsoon system reaches a location. Conventional operational definitions rely on regional rainfall and weather observations and can therefore be influenced by transient weather events.
Further, Prof. B.N. Goswami, ST Radar Centre, Gauhati University, Assam, added, “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 IIT Madras-led study instead examines how rainfall onsets across different locations organise collectively.
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 identify 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. 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 identify 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.
84 YEARS OF RAINFALL DATA
Using 84 years of rainfall data, the researchers found that, climatologically, the large-scale onset occurs around 15th May over Northeast India and 9th June 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.
TRACKING THE 2026 MONSOON
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 22nd June 2026. The subsequent northward progression of rainfall was reflected in heavy rainfall episodes reported in Mumbai by 24th June and Delhi by 1st July.
The research brings together network science, complex systems and meteorology. The approach addresses a fundamental multiscale challenge: the monsoon is a planetary-scale phenomenon, but people need to know when it has arrived at their particular location. By linking local rainfall to the collective evolution of the monsoon, the climate-network framework provides a way to bridge these two scales. The study could eventually support more informed decisions in agriculture, water-resource management, flood preparedness and other activities that depend on the timing and persistence of monsoon rainfall.