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Cracked: El Nino monsoon mystery

New Delhi, Sept. 7: An Indian-American research team has unravelled a hitherto unknown connection between the Indian monsoon and the weather event called El Nino, a rise in sea surface temperatures in the Pacific Ocean.

The findings, announced today in the US journal Science, could explain why some monsoon forecasts have been embarrassingly wrong in recent years and could also lead to more reliable predictions of severe droughts in India.

Weather scientists have known for long that El Nino can influence the behaviour of the monsoon. India’s 132-year-old rainfall history has shown that severe droughts have always been accompanied by El Nino events. But El Nino events have not always produced severe droughts.

This has baffled meteorologists who’ve been struggling to fathom the El Nino-monsoon link.

Now, K. Krishna Kumar at the Indian Institute of Tropical Meteorology in Pune and his colleagues at the University of Colorado and other institutions have shown that the exact location of El Nino warming determines whether the monsoon will fail.

“Conventional wisdom told us that El Nino would mean a drought for India — but conventional wisdom was shattered by what happened in 1997, 2002 and 2004,” Krishna Kumar told The Telegraph.

The year 1997 was marked by severe El Nino warming and scientists had predicted a drought in India, but monsoon rainfall was abundant then.

During 2002 and 2004 — in the presence of a weak El Nino in the eastern Pacific — meteorologists had predicted normal rainfall.

In both these years, India experienced severe drought for which, the researchers said, no contingencies were in place. But El Nino in the central Pacific was substantial then, said Martin Hoerling of the Earth System Research Laboratory in Colorado.

“The question we really have to ask is where in the Pacific is the El Nino warming taking place,” Krishna Kumar said.

The El Nino warming may occur in the eastern Pacific — off the coast of Peru, or thousands of kilometres westward in the central Pacific.

The study has shown that severe droughts in India are likely when the warming occurs in the central Pacific.

“The two flavours of El Nino orchestrate significantly different responses in the Indian monsoon,” the researchers said in their report in Science.

Their study was based on a historical analysis of 23 El Nino years as well as a series of computer simulations to find out how sea surface temperature rises in different parts of the Pacific Ocean would affect the monsoon in India.

The simulations have shown that a westward shift in El Nino warming weakens the Indian monsoon severely.

“The central Pacific is closer to India, and mechanisms involving air circulation can explain how the warming there might produce drought here,” said Madhavan Rajeevan, director of the National Climate Centre at the Indian Meteorological Department in Pune.

The warm waters in the central Pacific lead to a descending motion of air over India that deters the formation of clouds and rain-forming systems.

Traditional statistical forecasts of the monsoon take into account the influence of distant weather events — including El Nino — on overall monsoon performance, Rajeevan said.

The new studies suggest that such traditional methods of monsoon forecasting that factor in El Nino without regard for the actual location of the El Nino warming event are unlikely to be successful.

Incorporating the location of the warming in the statistical models should improve monsoon forecasts, Krishna Kumar and his colleagues have said.

The researchers said it is unclear which of the two flavours of El Nino might dominate in the future as climate change resulting from global warming changes the atmosphere.

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