Scientists have developed a new tool for objectively defining the onset and demise of the Indian Summer Monsoon—a colossal weather system that affects millions of people annually.

The researchers from Florida State University in the US developed a method that uses rainfall rates to mark the span of the Indian Summer Monsoon (ISM) at any given location throughout the affected region.

For generations, scientists have struggled to produce a model for reliably defining the duration of the monsoon. The researchers said that no existing system has allowed researchers to reliably define the parameters of the season at this fine a scale.

“Current weather forecasting and monitoring protocols focus attention on monsoon onset at one location—specifically the state of Kerala in the southwest corner of the country—and extrapolate for the rest of the region,” said Vasu Misra, associate professor at Florida State University.

“We have gone down to specific locations, we have covered the whole country, and we have objectively defined the onset and demise dates for any given year,” said Misra, lead investigator of the study published in the journal Climate Dynamics.

The lack of a clear, granular and objective benchmark for ISM onset and demise for all areas of the country has been a longtime source of consternation for people, researchers said.

In some parts of the country, the torrents of rain that characterise monsoon season account for more than 90 per cent of the total annual precipitation, they said.

Researchers said that many rhythms of Indian political and agricultural life can be destabilised by dubious or false claims of monsoon onset.

“That leads to tremendous amounts of frustration and confusion for the general public and for the people who are trying to monitor the monsoon because nobody has really gotten down to do it at a granular scale,” Misra said.

The new system, which ties the onset of the monsoon to location-specific rainfall thresholds, can work to allay that frustration, he said.

Up until now, regional meteorological departments have relied on their own ad hoc criteria for determining ISM onset, which can often lead to contradicting claims.

A more inclusive method will allow officials and researchers throughout the country to define the monsoon season using a standardised system that, through rigorous testing, has been shown to capture ISM evolution comprehensively.

“Anchoring the definition of onset and demise solely in local rain rates eliminates the need to rely on less accessible atmospheric variables,” researchers said.

This streamlined approach makes it considerably easier to monitor monsoon evolution, they said. “Our research enables quite easy real-time monitoring of the onset and demise of the Indian monsoon,” Misra said.

“We have tested this for 105 years of available data, and this criterion has not failed once for any location over India,” he said.

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