Assam: IIT Madras, Gauhati University researchers develop new method to pinpoint monsoon arrival

Guwahati: A spell of rain does not necessarily mean the monsoon has arrived. Researchers from the Indian Institute of Technology Madras (IIT Madras) and Gauhati University have developed a new method to identify when the monsoon establishes itself over a particular location, distinguishing sustained rainfall linked to the larger weather system from short-lived local showers.

The approach could help improve decisions on sowing, water-resource management, food security and flood preparedness by providing a more reliable indication of when the monsoon has actually set in.

The study, published in the Journal of Climate of the American Meteorological Society, uses a climate-network framework to track how rainfall onset at different locations across India becomes interconnected as the monsoon progresses.

The research paper is available at: https://doi.org/10.1175/JCLI-D-25-0384.1

The study was co-authored by Yogenraj Patil, Gaurav Chopra, Shruti Tandon and R.I. Sujith of IIT Madras’ Centre of Excellence for Studying Critical Transitions in Complex Systems (CoE-CTCS), along with B.N. Goswami of Gauhati University, Assam.

When does the monsoon really arrive?

In India, the first rainfall at a location may be caused by local weather conditions, including thunderstorms, rather than the arrival of the larger monsoon system.

This can make it difficult to determine when the monsoon has genuinely established itself. A brief spell of rain may be followed by a prolonged dry period, leaving farmers uncertain about when to begin sowing.

The researchers have developed a method that looks beyond the first rainfall event. Instead, it examines how rainfall onset across different parts of the country becomes connected as the monsoon system develops.

The team treats geographical locations as nodes in a climate network, with connections representing the relationships between rainfall-onset events at different places.

As the monsoon advances, these connections form clusters. The researchers identify the point at which rainfall onset at a particular location becomes part of the largest interconnected cluster as the arrival of the large-scale monsoon system at that location.

R.I. Sujith, head of CoE-CTCS at IIT Madras, said the approach offers a different way of understanding monsoon onset.

“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,” he said.

This indicates that rainfall at the location has become part of the broader monsoon system rather than being driven solely by a temporary local weather event.

The researchers describe this transition as a phase transition in a climate network.

Sujith, who is also an Institute Professor and D. Srinivasan Institute Chair Professor in the Department of Aerospace Engineering at IIT Madras, said the approach could help avoid false monsoon-onset signals.

“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 policymakers plan for food security and water-resource management,” he said.

How the monsoon spreads across India

The researchers used the network approach to explain how the monsoon becomes established over different regions.

Over Northeast India, small clusters of rainfall onset begin forming in April, partly due to thunderstorm activity. As the season progresses, these clusters grow and merge with another cluster associated with the Bay of Bengal region.

The researchers identify the abrupt growth of the largest interconnected cluster as the large-scale monsoon onset over Northeast India.

A similar process takes place over the Indian peninsula

A smaller rainfall-onset cluster develops over Kerala due to local rainfall activity. As the monsoon progresses, this cluster becomes connected to the larger rainfall-onset network advancing westwards from Northeast India.

The researchers identify this connection as the large-scale monsoon onset over the peninsula.

The method therefore helps distinguish between an isolated spell of rain over Kerala and the establishment of the larger monsoon system.

B.N. Goswami of the ST Radar Centre, Gauhati University, said the approach is intended to complement existing operational methods rather than replace 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.

What 84 years of rainfall data revealed

The researchers analysed 84 years of rainfall data to establish the climatological timing of large-scale monsoon onset.

Their findings indicate that the large-scale onset typically occurs around May 15 over Northeast India and June 9 over the Indian peninsula.

The onset is marked by a sudden increase in the size of the largest interconnected rainfall-onset cluster, followed by its continued expansion and northward movement across the country.

The study also found that large-scale onset over the Indian peninsula is followed by a strong northward progression 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 larger monsoon system rather than a temporary rainfall event.

Tracking the 2026 monsoon

The researchers also applied their method to monitor the 2026 monsoon.

According to the study, 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.

The findings highlight how the new approach could help connect rainfall observations at individual locations with the broader evolution of the monsoon.

The research brings together network science, complex systems and meteorology to address a key challenge in monsoon forecasting: while the monsoon operates on a vast geographical scale, its arrival has to be understood at the level of individual locations.

By tracking how local rainfall events become part of a larger interconnected system, the researchers have developed a framework that could offer a more meaningful indication of monsoon arrival.

If further developed for operational use, the approach could help farmers make more informed sowing decisions and assist authorities in planning for water availability, food security and flood risks.