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[NEW DELHI] Scientists have solved the riddle of why some El Niño events cause the Indian monsoon to fail while others do not, which may lead to more accurate forecasts of drought.

The extremes of weather associated with El Niño are caused by the periodic warming of the surface of the Pacific Ocean.

Severe droughts in India have always occurred in El Niño years, yet every El Niño does not cause monsoon failure and drought — a mystery that researchers have been struggling to crack.

Accurate monsoon prediction is crucial to India's economy: nearly one-fifth of the country's gross domestic product comes from agriculture. Even moderate crop failures have severe economic and societal impacts. 

Research published online by Science today (7 September) shows that it depends on whether the surface of the equatorial Pacific Ocean is warmest in the east, along Latin America, or closer to the centre.

Martin Hoerling of the National Oceanic and Atmospheric Administration Earth System Research Laboratory in Boulder, United States, and his colleagues say India is more prone to drought when the warm Pacific temperatures typical of El Niño extend westwards into the central Pacific Ocean.

The team analysed 23 strong El Niño years and their links to 13 droughts and 10 drought-free years in India, using satellite observations of sea surface temperatures and historical data of rainfall over central India.

Having found that drought was associated with warm water in the central Pacific, they used computer models to mimic the patterns, which confirmed their findings.

The researchers suggest that the "two flavours of El Niño" might affect the Indian monsoon differently through the tropical Walker circulation — an east-west wind over the Pacific.

The scientists say their research does not rule out the possibility that other factors, such as Indian ocean temperatures, also play a role.

And changes in ocean temperatures brought about by human-induced climate change could also affect the intensity of the Indian monsoon, they add.

Link to full paper in Science

Reference: Science doi:10.1126/science.1131152 (2006)