Disturbance from North Atlantic derails Monsoon in August: Research

A planetary wave from the North Atlantic is capable of derailing the Indian monsoon on which the Indian economy is heavily dependent, suggests a study published in the journal Science.

The findings suggest that modelling efforts ought to focus on including the influence of mid-latitudes, in addition to the Pacific and Indian oceans, for getting a better handle on predictability of the monsoon, its variability as well as droughts.

A team from the Centre for Atmospheric and Oceanic Sciences (CAOS), Indian Institute of Science (IISc), who carried out the research, supported in part by DST under their climate change programme, showed that, in the past century, Indian monsoon droughts that occurred in non-El Niño years were sub-seasonal, as against El Niño droughts, where the deficit persists throughout the season.

The research team analysed 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 Niño droughts sets in early around mid-June and becomes progressively worse. By mid-August, the deficit is very high and spread across the country, with no sign of recovery.

During non-El Niño 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 Niño drought years,” said Jai Sukhatme, Associate Professor at CAOS, and one of the senior authors, in an IISC statement.

“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. This disturbance, a Rossby wave, curves in towards India and, apparently squeezed in by the Tibetan Plateau, disrupts the flow of the monsoon winds,” V Venugopal, Associate Professor at CAOS, and a co-author explained.

The atmospheric tele-connection studied in this paper whose first author was a PhD student Pritam Borah with DST inspire fellowship, offers an avenue for improved predictability of droughts, especially in the absence of tell-tale signs in the Pacific.

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