Scientists report that droughts in India are associated with a particular type of El Niño, the climate phenomenon marked by increased sea-surface temperatures in the tropical Pacific.
The rainy season in India occurs in June, July, and August. Between 1871 and 2002, central India experienced 10 severe summertime droughts, says Martin Hoerling, a meteorologist with the National Oceanic and Atmospheric Administration in Boulder, Colo. Every one of those dry spells occurred during an El Niño, he notes. However, not all El Niños during that 132-year period caused droughts—in 13 cases, summer rainfall during an El Niño was at or slightly above normal.
The explanation, says Hoerling, is that not all El Niños are the same. While some primarily warm up the eastern Pacific near South America, the hot spots for other El Niños appear mainly in the central Pacific. Indian droughts seem to result from only the latter type, Hoerling and his colleagues report in the Oct. 6 Science.
Warmer-than-normal ocean temperatures in the central Pacific send large amounts of moist, warm air up to high altitudes there. This shift in the atmosphere causes air masses to move downward over central India, climate models suggest. Such downwelling tends to suppress rainfall.
Few studies have scrutinized central-Pacific El Niños, says Hoerling. A better understanding of that type may enable scientists to develop an early-warning system for Indian droughts, he notes.