Why Indian monsoon derails in August? Research suggests disturbance from North Atlantic can be a reason

By: |
December 17, 2020 5:01 PM

Oceanic and atmospheric Rossby waves -- also known as planetary waves -- naturally occur largely due to the Earth's rotation. These waves affect the planet's weather and climate.

Three of the storms (Amphan, Nivar and Burevi) formed in the Bay of Bengal and the other two (Nisraga and Gati) in the Arabian Sea. Amphan, Nivar and Nisarga hit the Indian coasts as cyclonic storms.Three of the storms (Amphan, Nivar and Burevi) formed in the Bay of Bengal and the other two (Nisraga and Gati) in the Arabian Sea. Amphan, Nivar and Nisarga hit the Indian coasts as cyclonic storms.

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 Nino years were sub-seasonal as against El Nino 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 Nino 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 Nino 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 Nino 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.

Oceanic and atmospheric Rossby waves — also known as planetary waves — naturally occur largely due to the Earth’s rotation. These waves affect the planet’s weather and climate.

“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 telltale signs in the Pacific,” the statement said.

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