A cyclone spared Mumbai on Wednesday after a day of high alert. Reuters picture shows a boy trying to hold on to his umbrella during strong winds.

New Delhi, Nov. 11: More than Aila, a heated tussle between two regions of a great ocean caused the drought of 2009.

A tug of war between a warm Bay of Bengal but an unusually warmer Indian Ocean killed clouds prematurely in the Bay for about three weeks in June this year, leading to the drought, scientists have said.

The three-week hiatus in June led to such a massive early rainfall deficit that the rest of the monsoon season could not compensate, researchers said in the first investigation into the causes of the overall 23 per cent lower than normal rainfall.

India’s monsoon rainfall is sustained by cloud formation over a warm Bay. Normally, the sea surface temperatures in the Bay are slightly higher than sea surface temperatures in the equatorial Indian Ocean region.

But an analysis of the temperatures this year shows that the equatorial Indian Ocean region was warmer by about 1 degree Celsius than the Bay through early June.

“The equatorial Indian Ocean region had an edge — it kept killing any cloud systems which were forming over the Bay,” said Sulochana Gadgil, professor of atmospheric and oceanic sciences at the Indian Institute of Science. “Cloud formation was favoured over the equatorial Indian Ocean at the expense of the Bay region,” she told The Telegraph. “I would call it unusual — it’s the first time we’ve observed a warming phenomenon prolonged for up to three weeks in June.”

The findings by Gadgil and her colleague P.A. Francis appear in the journal Current Science today.

The scientists say it is unclear why the equatorial Indian Ocean was warmer this year. “There appears nothing else that could explain the poor rainfall in June,” said K. Krishna Kumar, a senior scientist at the Indian Institute of Tropical Meteorology, Pune, who was not associated with the study.

The 2009 drought marked by a 23 per cent monsoon deficit has been one of the worst droughts in recent decades, comparable to 24 per cent deficit in 1974 and 22 per cent deficit in 2002.

Some meteorologists had speculated that cyclone Aila in May may have led to a cooling of the Bay. Gadgil said while Aila would have led to a cooling, historical behaviour shows that the Bay has the ability to recover quickly from such cooling. “The (cyclone’s) effect would not have lasted three weeks,” she said.

The effect was a 48 per cent monsoon deficit during June.

Although rainfall during the remaining three months of the monsoon season could have made up, a slight warming of sea surface temperatures in the Pacific Ocean during August and September could have contributed to deficits during the last two months of the season.

The only other year when the Bay lost to the equatorial Indian Ocean was in 1995, but the effect lasted only a few days, leading to a 24 per cent below normal June rainfall.