Researchers from the Indian Institute of Technology (IIT) Madras and Gauhati University have developed a method to identify when the monsoon has actually reached a particular location, distinguishing short-lived local rainfall from the establishment of the larger monsoon system.
The study uses a climate-network approach to track how rainfall onset at different locations across India develops into a connected system as the monsoon advances. Published in the Journal of Climate of the American Meteorological Society on September 15, 2026, the research could help improve decisions related to agricultural sowing, water management, food security and flood preparedness.
The researchers analysed 84 years of rainfall data and found that the large-scale monsoon onset occurs, on average, around May 15 over northeast India and June 9 over the Indian peninsula. Applying their method to the 2026 monsoon, they identified June 22 as the date of large-scale onset over the peninsula.
The research was co-authored by Yogenraj Patil, Gaurav Chopra, Shruti Tandon and Professor R I Sujith from IIT Madras, along with Professor B N Goswami of Gauhati University.
How does the method work?
The method addresses a common difficulty in identifying the arrival of the monsoon. A location may receive rainfall due to thunderstorms or other local weather conditions before the larger monsoon system reaches it. Such showers can be followed by a dry spell, making the first rainfall an unreliable indicator of sustained monsoon activity.
The researchers approached the problem by treating geographical locations as nodes in a climate network. Connections between these nodes represent relationships between locations experiencing the onset of rainfall. They then tracked how these clusters formed, expanded and merged as the monsoon progressed.
Under their definition, large-scale monsoon onset at a location occurs when its local rainfall-onset cluster becomes part of the largest interconnected cluster across the country. This indicates that rainfall at that location has become connected to the broader monsoon system rather than remaining an isolated weather event.
"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," Professor Sujith said.
The researchers describe this transition as a phase transition in a climate network, referring to the sudden growth of the largest interconnected rainfall cluster.
What did the 84-year analysis reveal?
The researchers observed that small clusters of rainfall onset begin developing over northeast India during April, including those associated with thunderstorms. As the season progresses, the cluster expands and merges with another rainfall cluster associated with the Bay of Bengal region.
The resulting abrupt increase in the size of the largest cluster marks the large-scale onset over northeast India under the new method.
A similar process takes place over the Indian peninsula. A smaller rainfall cluster develops over Kerala due to local rainfall activity. When this cluster connects with the larger system advancing westwards from northeast India, the researchers identify the transition as the large-scale monsoon onset over the peninsula.
The analysis placed the average onset around May 15 over northeast India and June 9 over the peninsula. These are climatological estimates derived from the historical rainfall record, not fixed dates for the monsoon's arrival every year.
The study also found that the large-scale onset over the peninsula is followed by a strong northward progression of rainfall associated with the first pulse of the Monsoon Intraseasonal Oscillation (MISO), a pattern of active and weak rainfall phases within the monsoon season. This provides an additional indication that the identified transition corresponds to the establishment of the larger monsoon system.
What did the method identify in 2026?
The researchers applied their definition to the 2026 monsoon and identified June 22 as the date of large-scale onset over the Indian peninsula.
The subsequent northward progression of rainfall was followed by heavy rainfall episodes in Mumbai by June 24 and Delhi by July 1, according to the study.
The findings could be useful for agricultural planning, particularly in deciding when to begin sowing. Farmers who act on isolated early showers risk planting crops before sustained rainfall sets in. A method that identifies the establishment of the larger monsoon system could provide a more reliable basis for such decisions.
It could also support water-resource planning and help authorities prepare for periods of sustained rainfall and potential flooding.
Professor B N Goswami of Gauhati University said the method was designed to work alongside existing ways of identifying monsoon onset.
"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," he said.
The researchers have presented the approach as a new way to study the large-scale organisation of the Indian monsoon. Its potential use in operational forecasting and agricultural advisories will depend on further assessment and integration with existing weather-monitoring systems.