Research suggests disturbance from North Atlantic could be reason why monsoon derails in August

Research suggests disturbance from North Atlantic could be reason why monsoon derails in August

New Delhi: A planetary wave from the North Atlantic is capable of derailing the Indian monsoon in August, a study by the Indian Institute of Sciences (IISc), Bengaluru, has suggested.

A team from the Centre for Atmospheric and Oceanic Sciences (CAOS) of the IISc carried out the research, supported in part by the Department of Science and Technology (DST) under their climate change programme.

The research showed that in the past century, Indian monsoon droughts that occurred in non-El Ni o years were sub-seasonal as against El Ni o droughts where the deficit persists throughout the season.

The research team analysed the daily rainfall during the two categories of droughts from 1900 to 2015 and noticed dramatic differences in the evolution of rainfall deficit.

Rainfall deficit in El Ni o droughts sets in early around mid-June and becomes progressively worse. By mid-August, the deficit is very high and spreads across the country, with no sign of recovery, the study revealed. During non-El Ni o droughts there is a moderate decrease in June rainfall, followed by signs of recovery during mid-July to mid-August ? the peak of the season.

However, in late August, there is an abrupt and steep fall in rainfall, resulting in drought conditions.

"We tried to trace this late August break to a forcing agent or system that influences the behaviour over India. We looked at the winds that were prevalent in these non-El Ni o drought years," a statement issued by the IISc quoted Jai Sukhatme, associate professor at CAOS and one of the senior authors, as saying.

The interaction between upper-level winds and deep cyclonic vorticity anomalies located above anomalously cold North Atlantic waters during late August to early September results in an atmospheric disturbance, explained V Venugopal, associate professor at CAOS and a co-author.

"This disturbance, a Rossby wave, curves in towards India and, apparently squeezed in by the Tibetan Plateau, disrupts the flow of the monsoon winds," he said.

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