A short spell of rain does not necessarily mean the monsoon has arrived. Sometimes, the rain is caused by local weather conditions and may stop soon. Researchers from the Indian Institute of Technology (IIT) Madras and Gauhati University have developed a new method that can distinguish between a short spell of local rain and the arrival of the larger monsoon system that brings regular rainfall across the country.The method could help farmers decide when to sow their crops and help governments prepare for water storage, food security and possible floods.The findings have been published in the Journal of Climate, a scientific journal of the American Meteorological Society. The researchers explained that the monsoon has truly arrived when rainfall in that area becomes connected to the larger system of rainfall spreading across the country.Prof R I Sujith, Head of the Centre of Excellence for Studying Critical Transitions in Complex Systems at 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.”How does the method workThe researchers explain the process through the example of Northeast India and Kerala.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 in the Southern part of India where 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.The researchers studied 84 years of rainfall data to understand when this large-scale monsoon onset usually takes place. Their analysis found that the large-scale onset generally occurs around May 15 over Northeast India and June 9 over the Indian peninsula.They also found that after the monsoon becomes established over the peninsula, rainfall begins moving northwards across the country. This movement is linked to the first phase of a weather pattern known as the Monsoon Intraseasonal Oscillation (MISO).Prof B N Goswami of Gauhati University said the new method would support existing ways of identifying monsoon onset rather than replace them.“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.