Researchers Unravel the Mystery of El Niño’s Link to the Indian Monsoon

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Balaji Rajagopalan

Civil Engineering

For more than 100 years scientists have connected the variability of the summer monsoon rains in India to El Niño, a rise in sea surface temperatures in the equatorial Pacific having significant consequences for weather around the globe. But events in the last 25 years seem to have reduced forecasters’ ability to predict the monsoon to a mere roll of the dice.

During the 1980s and 1990s, the correlation between El Niño events and the monsoon fell to near zero. While El Niño events became more frequent during this period, the summer rains remained normal in India. Then, in 1997, a widely heralded prediction of drought based on real-time data from buoys placed in the Pacific Ocean proved to be totally false.

While flooding events have certainly damaged crops and claimed lives, drought is of even more concern in India, says CU-Boulder Assistant Professor Balaji Rajagopalan, because the impact is so long-lasting. The country lacks large reservoirs that can moderate a drought’s impact on food supplies. The fact that the monsoon season is evolving at the same time as El Niño each year makes it all the more difficult to predict conditions and avert disaster.

Following the 1997 prediction failure, Rajagopalan joined with colleagues Krishna Kumar and Mark Cane, all of whom were at Columbia University at the time, to analyze the apparent weakening relationship between El Niño and the monsoon. In a 1999 article in Science, they traced the breakdown to a southeastern shift in the position of El Niño along with increased surface temperatures over Eurasia brought on by the continental warming trend. The warming of the land mass appeared to essentially balance or negate the effects of the El Niño disturbance.

Two subsequent prediction failures, resulting in unexpected and devastating droughts in 2002 and 2004, prompted the researchers to continue their work. In the September 2006 issue of Science, they published a new paper with colleagues Martin Hoerling and Gary Bates of the National Oceanic and Atmospheric Admin-istration showing that the spatial location of El Niño warming is actually more important than its relative strength in predicting the monsoons.

The researchers analyzed historical data of rainfall over central India along with satellite observations of sea surface temperatures in the Pacific and were able to identify two distinct patterns or “flavors” of El Niño. Warming in the central Pacific correlated with drought conditions in India, whereas warming in the eastern Pacific correlated with normal monsoon conditions. When El Niño warming is located farther to the west, Rajagopalan says, the physical effect is to cause an increased ascent of moisture over the ocean anomaly, which then descends broadly over southeast Asia and suppresses the monsoon rains.

The authors confirmed their findings in three climate models, and efforts are under way by the Indian Meteorological Department to incorporate this information into the monsoon forecasting system.

Rajagopalan and his colleagues will continue their work by creating a new forecasting model, while also looking at climate warming trends and trying to determine which “flavor” of El Niño is likely to dominate in the years to come.

Although some researchers believe that conditions in the Indian Ocean may play an independent role in determining the strength of the monsoons, Rajagopalan maintains they are more a response to El Niño than a separate factor. “El Niño is still the 600-pound gorilla that moves the monsoon,” he says.

Rajagopalan studies a variety of climatic and hydrologic phenomena and their impacts on water resources management and decision support as a faculty member in the department of civil, environmental and architectural engineering and a fellow of the Cooperative Institute for Research in Environmental Sciences.

“The monsoon is my passion,” he says. “With even a little more understanding, we can impact up to one billion people.”

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