‘Curving waves’ from Atlantic may be drying up monsoon, say scientists.
Droughts in India have historically been associated with El Nino, an anomalous warming of equatorial Pacific.
Droughts in India have historically been associated with El Nino, an anomalous warming of the equatorial Pacific, but scientists from Bangalore suggest other culprits too.
The study that appears in journal Science says that nearly six out of 10 droughts, in non El Nino years, that occurred during the Indian summer- monsoon season in the past century may have been driven by atmospheric disturbances from the North Atlantic region.
The study was led by researchers at the Centre for Atmospheric and Oceanic Sciences (CAOS), Indian Institute of Science (IISc), Bengaluru.
In an El Niño year, abnormally warm equatorial Pacific waters pull moisture-laden clouds away from the Indian subcontinent, but the IISc study shows that in non-El Nino years, these droughts are a consequence of a sudden and steep drop in rainfall in late August.
“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”, V Venugopal, Associate Professor at CAOS, says in a press statement by the IISc.
In an El Nino year, the rainfall deficit ‒ departure from a long-term average ‒ will set in early around mid-June and progressively worsen. However, in an non-El Nino-year, droughts will see rainfall and will similarly weaken in mid-June. But from mid-July to mid-August ‒ usually the rainiest period of the monsoon‒ the monsoon will appear to recover. However, around the third week of August, there was a sudden steep decline in rainfall, which resulted in drought conditions.
“We tried to see if we could trace this [August] drought back 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.”
It seems, the researchers note, that winds in the upper atmosphere are interacting with a deep cyclonic circulation above the 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, explains Jai Sukhatme, Associate Professor at CAOS and co-author. “This inward curving was the peculiar thing that we noticed during these particular years,” the IISc statement notes.
Thus beyond looking at the Pacific Ocean it is important to consider other influences on the Indian monsoon from outside the tropics. “The Indian Ocean and Pacific Ocean seem to be at the forefront of all discussions surrounding Indian monsoon droughts,” says Mr. Venugopal. “It is perhaps time to focus just as much on midlatitude influences, which might aid in getting a better handle on enhanced predictability of monsoon variability.”
Independent scientists say that while the link in the research paper is “novel” it is yet early days to be able to predict an impending drought in India from this paper alone. The Indian Ocean, says Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology (IITM), too plays an important role in determining the performance of the monsoon and several other factors will have to be considered to develop a reliable model to forecast a potential shortfall in the absence of an El Nino.
In 2014, India saw a 14% rainfall deficit—or a drought—that wasn't linked to an El Nino and before that in 1986 and 1985. “The sheer size of the Pacific means that it influences global climate much more than the Atlantic or other oceans. Therefore, the latter links aren’t easily discoverable. But this study shows that we do need to look at such links closely. (Source: The Hindu)
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