New Delhi, July 24: India's summer monsoon, which brings 80 per cent of the annual rainfall on the subcontinent, has strengthened over the past 15 years, scientists said today, attributing the trend to simultaneous changes in land and sea temperatures.
Scientists at the Massachusetts Institute of Technology in the US have used observational data to show that the daily monsoon rainfall has increased over north central India at 1.34 millimetre per decade since 2002, a seeming revival after a 50-year drying period.
Their analysis found that between 1950 and 2002, north central India experienced a decrease in the daily rainfall on average of 0.18mm per decade during the monsoon season. But since 2002, the daily rainfall has increased along with strong warming of the subcontinent.
"The Indian monsoon is considered a textbook, clearly defined phenomenon, and we think we know a lot about it, but we don't," said Chien Wang, a research scientist at MIT's Centre for Global Change, who led the study. "We identify a phenomenon that was mostly overlooked," he said in a media release.
Wang and his colleague Qinjian Jin, who have published their findings today in the journal Nature Climate Change, believe the current strengthening monsoon trend is the result of higher land temperatures along with slower warming trends in the ocean temperatures.
The scientists have attributed the dry period during the 2015 monsoon season that caused widespread droughts across the subcontinent to a severe El Nino, a phenomenon in which sea surface temperatures in parts of the Pacific rise and lead to decreased rainfall in India and elsewhere.
Weather observations show India has experienced strong land surface warming of about 0.36 degrees Celsius per decade between 2002 and 2014. The scientists said this "drastic" land warming could be the result of reduced cloud cover at low altitudes or which allow more sunlight to fall on land.
But it could also have other explanations such as changes in land use and land cover, they said.
An independent study by Indian and Japanese scientists had last year shown that the number of intense heat waves in India between March and June has been increasing over the past decade, underscoring a need to unravel the mechanisms driving such warming.
A senior Indian meteorologist said an increase in rainfall would be in line with earlier predictions of how monsoon behaviour might change under the influence of global warming. "When the atmosphere warms up, its water-holding capacity increases. This is expected to lead to more rainfall," said Kanduri Jayaram Ramesh, director general of the India Meteorological Department.
However, the increase in the quantum of rainfall is not expected to be uniform across space or time.
"It is expected the frequency of high intensify rainfall days will increase -- there may be more number of days with extreme heavy spells of rain," Ramesh said. "This will mean the country will need to prepare for different strategies to manage surface water."