A study by IIT-Madras and Gauhati University reveals that the Indian monsoon system first forms over Northeast India around May 15, weeks before reaching the peninsula.
Using 84 years of rainfall data, researchers develop a climate-netwrok approach to highlight the merging of local rainfall clusters into a large-scale system
The approach could complement conventional models to offer insights for agricultural planning and monsoon monitoring
The large-scale Indian monsoon system first takes shape over Northeast India around May 15, nearly three weeks before it becomes established over the Indian peninsula, according to a new study by researchers from the Indian Institute of Technology (IIT), Madras and Gauhati University.
The study, published in the Journal of Climate last month, is based on an analysis of 84 years of daily rainfall data from 1940 to 2023. Using a climate-network approach, the researchers show that small clusters of rainfall onsets develop independently across the country before merging into a larger system.
In April, for instance, clusters begin forming over Northeast India, partly due to thunderstorm activity. Around May 15, this cluster merges with another rainfall system moving northwards from the southeastern Bay of Bengal. This creates a large, interconnected system that begins spreading westwards.
Meanwhile, a separate cluster develops over the Indian peninsula, particularly Kerala, in response to local rainfall.
Around June 9, the Northeast and peninsular clusters merge over the peninsula, marking the establishment of the large-scale monsoon system.
“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,” R I Sujith, Institute Professor and D Srinivasan Institute Chair Professor, Department of Aerospace Engineering, IIT Madras, and a co-author of the study, said in a statement.
“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,” he added.
The distinction between local rainfall and the establishment of the larger system also affects how the duration of the monsoon is measured. The study estimates that the large-scale rainy season in Northeast India and the southwestern peninsula is 20-30 days shorter than the locally defined rainy season in these regions.
The researchers also trace the large-scale withdrawal of the monsoon, and suggest that it begins over Northwest India around September 22 and retreats rapidly across much of the country by October 22. The withdrawal continues over the southeastern tip of the peninsula in December.
The researchers also applied the method to track the 2026 southwest monsoon, and identified June 22 as the date of large-scale onset over the Indian peninsula.
The system subsequently progressed northwards. Mumbai received heavy rainfall on June 24, two days after the identified onset over the peninsula, while Delhi experienced heavy rainfall by July 1.
The 2026 assessment illustrates the possibility of using the framework to monitor the monsoon as it develops, including at locations far from where the system first becomes established.
The researchers say this could help reduce the risk of treating isolated showers as the onset of the monsoon during agricultural planning. “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,” said Sujith.
However, they do not propose replacing the conventional operational definitions of 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,” added B N Goswami, ST Radar Centre, Gauhati University, Assam, and another author of the study.