‘Droughts in India influenced by North Atlantic air currents’
The findings of researchers at IISc. underscore the importance of considering influences on Indian monsoon from outside the tropics
The Indian monsoon is a phenomenon that has inspired great interest among scientists, authors and others over generations. To a large number of Indians, the rainfall between June and September is a matter of life and death. The monsoon’s failure heralds drought, which is often blamed on El Niño, a recurring climate event during which abnormally warm equatorial Pacific waters pull moisture-laden clouds away from the Indian subcontinent. But 10 out of 23 droughts that India faced in the past century have occurred during years when El Niño was absent.
A new study by researchers at the Centre for Atmospheric and Oceanic Sciences (CAOS), Indian Institute of Science (IISc.) has found that nearly half of the droughts that occurred during the Indian summer-monsoon season in the past century may have been driven by atmospheric disturbances in the North Atlantic region. The study has been published in ‘Science’.
The study shows that droughts, which were a consequence of a sudden and steep drop in rainfall in late August, were linked to an atmospheric disturbance in the mid-latitude region over the North Atlantic Ocean, creating a pattern of atmospheric currents that “swoop in over the subcontinent and derail the monsoon”.
“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, which is this mid-latitude influence,” V. Venugopal, Associate Professor at CAOS and one of the senior authors of the paper, was quoted as saying in an IISc. release.
The research team 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 ‒ departure from a long-term average ‒ sets in early 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, said the release.
“Surprisingly, the droughts during the non-El Niño years, when analysed together, also seemed to follow a common pattern. First, there was a moderate slump in June. Then, during mid-July to mid-August ‒ the peak of the season ‒ the monsoon showed signs of recovery and rainfall increased. However, around the third week of August, there was a sudden steep decline in rainfall, which resulted in drought conditions,” the release explained.
The team began to question why the break occurred this late in August and tried to see if they could trace it back to a forcing agent or system that influences the behaviour over India, looking at the winds that were prevalent in the non-El Niño drought years.
That was when they noticed an unusual atmospheric disturbance in the mid-latitudes. It 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 a 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. The wave’s usual course is to go from west to east, but not towards the equator, explained Jai Sukhatme, Associate Professor at CAOS and another author.
The IISc. release said these findings underscore the importance of also considering influences on the Indian monsoon from outside the tropics, which current forecast models focus heavily on.