Around half of Indian droughts are due to North Atlantic atmospheric disturbances: IISc Study

10 out of 23 droughts that India faced in the past century have occurred during years when El Nino was absent

The dreaded El Nino originating in the Pacific is not the only culprit behind Indian droughts – it has a partner in the North Atlantic ocean.

Nearly half of the droughts that occurred during the Indian summer monsoon season in the last 100 years may have been driven by another atmospheric disturbance that originated in the North Atlantic region, weather scientists at Bangalore reported on Friday.

While El Nino, a recurring climate event during which abnormally warm equatorial Pacific waters pull moisture-laden clouds away from the Indian subcontinent, is generally blamed for droughts in India, as many as 10 out of 23 droughts that India faced in the past century have occurred during years when El Niño was absent.

What, then, could have caused these droughts?

Scientists at the Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science found that these droughts were a consequence of a sudden and steep drop in rainfall in late August, which was linked to another weather phenomenon originating in mid-latitude over the North Atlantic ocean that swoop in over the subcontinent and “derail” the monsoon.

“This is an important paper which explains the causes of droughts occurring over India during non-ENSO years like 1974, 1979 and 2014,” M Rajeevan, Secretary, Ministry of Earth Sciences, who is not associated with the study, told DH.

The IISc researchers looked closely at daily rainfall during both El Niño and non-El Niño drought years, and noticed stark differences in their patterns between June and September.

The droughts that happen during an El Niño year follow a standard pattern. The rainfall deficit sets in around mid-June and becomes progressively worse. By mid-August, the deficit spreads across the country and there is no going back from a drought.

Surprisingly, the droughts during the non-El Niño years also seemed to follow a common pattern. First, there was a moderate slump in June. Then between mid-July and mid-August - the peak of the season ‒ the monsoon showed signs of recovery. But around the third week of August, there was a sudden steep decline in rainfall, which resulted in drought conditions.

“The question was: why does the break occur this late in August?” said Jai Sukhatme, one of the authors of the study published in Science. That was when the researchers noticed such an unusual atmospheric disturbance in the mid-latitudes.

“These droughts occur due to adverse effects of atmospheric waves moving across north west India during the second half of the season especially. These waves have originated from the North Atlantic ocean,” Rajeevan said.

The unusual atmospheric disturbance emerged from winds in the upper atmosphere interacting with a deep cyclonic circulation above abnormally cold North Atlantic waters.

The resulting wave of air currents – called Rossby wave - curved down from the North Atlantic squeezed in by the Tibetan plateau ‒ and hit the Indian subcontinent around mid-August, suppressing rainfall and throwing off the monsoon that was trying to recover from the June slump.

“As early as the 1980s, people have looked at these droughts individually. But they have not collated and pooled them together, and deduced that these droughts may all have a different type of evolution than El Niño droughts, as well as a common cause,” said V Venugopal, another IISc scientist and one of the senior authors of the paper.