Unique CCNY study of extreme Indian rainfall upends conventional wisdom

New research led by City College of New York scientist Spencer A. Hill challenges generations-old beliefs about how El Niño events influence rainfall during the Indian summer monsoon. Entitled “More extreme Indian monsoon rainfall in El Niño summers,” the study appears in the journal Science.

“Our key finding is that you tend to get more days with extreme amounts of rainfall within India, not less, in El Niño summers.  An El Niño event means that ocean surface temperatures in the eastern equatorial Pacific Ocean are warmer than usual,” said Hill, assistant professor, in CCNY’s Department of Earth and Atmospheric Sciences. “This finding was unexpected, because it has been known for over a century that El Niños do precisely the opposite, meaning they promote drought, for total rainfall summed over the rainy season, June through September.”

Hill, whose affiliations include the CUNY Graduate Center Departments of Earth and Environmental Sciences and of Physics, as well as  Columbia University’s Lamont-Doherty Earth Observatory, however pointed out that these changes are not distributed uniformly within India.  “The increases in extreme daily rainfall under El Niño compared to La Niña are concentrated in central India and in the southwestern coastal band, whereas in the southeast and northwest the signal is opposite, meaning daily extreme rainfall is less likely in El Niño summers,” he noted.

Highlighting the importance of the study, Hill said that extreme rain events come every summer, destroying infrastructure and killing people through flooding and landslides.  The World Bank estimates that some 80 million people live in extreme poverty in the world’s most populous country of more than 1.45 billion.  “Better predictions of when and where extreme rainfall events are likely to occur give society better chances to prepare, such as perhaps by earlier and better warnings or pre-mobilizing aid.”

And this novel work will continue beyond this study thanks to a new three-year $408,862 grant awarded to Hill this fall by the National Science Foundation [NSF].  “In this new NSF grant we will investigate how and why the type of storms responsible for much of this extreme rainfall, called monsoon low-pressure systems, change depending on whether there are El Niño or La Niña conditions,” said Hill.