Monsoon’s first large-scale onset occurs over Northeast India, suggests IIT-Madras, Gauhati University study

Detailed summary

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 pen ...

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…

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