Cooperative Institute for Research in Environmental Sciences

New study identifies driving force behind El Niño-induced drought

Sept. 7, 2006

When monsoon forecasts failed to predict the severe droughts that devastated Southeast Asian food supplies during 2002 and 2004, it was clear there that the climate models were inadequate. Now, scientists at NOAA and the Cooperative Institute for Research in Environmental Sciences (CIRES) have unveiled the mystery behind the Indian Monsoon failure, while also raising important questions about El Niño's influence on the United States and elsewhere.

Traditionally, climate models have depended on the strength of the El Niño cycle to predict the severity of drought over the Indian subcontinent. But this model not only failed to predict some of India's worst droughts, it also predicted drought for years in which monsoon rains were ample. These false predictions have hit India's agriculturally-based economy hard, affecting, among other things, the government's preparedness to combat short food supplies.

"What we've discovered," says Balaji Rajagopalan, one of the lead CIRES researchers on the project and a professor at the University of Colorado at Boulder, "is that the strength of the El Niño isn't the sole driving factor behind Indian Monsoon failure. It turns out that where in the tropical Pacific the El Niño sea surface temperature anomaly is greatest is more important than how hot it gets."

The El Niño Southern Oscillation, or ENSO, describes a shift in atmospheric circulation over the Pacific Ocean, which causes the easterly trade winds to relax or even reverse. As a result, warm surface waters appear in the Eastern Pacific, producing a variety of climatic changes worldwide. Typically, El Niño years produce droughts in Central America, the Philippines, South India, Indonesia, Africa, and Australia. In contrast, flooding is prevalent in the United States, Cuba, Western Europe, and parts of South America.

The CIRES research, titled "Unraveling the Mystery of Indian Monsoon Failure During El Niño" and authored by K. Krishna Kumar, B. Rajagopalan, M. Hoerling, G. Bates and M. Cane, shows that when the El Niño sea surface temperature anomaly forms closer to the central Pacific Ocean, the Indian Monsoon fails to produce sufficient rains. When the temperature anomaly forms closer to the Eastern rim of the equitorial Pacific, chances of Indian Monsoon failure are reduced. The research is set to be published in Science Express on September 7th.

"We hope that by identifying the driving force behind Indian Monsoon failure, we can aid forecasters in predicting future droughts more accurately for the benefit of society," says Rajagopalan. He also points out that an improved understanding of the various spatial expressions of the El Niño cycle could have significant impacts for El Niño climate predictions around the world.

For future research, the scientists are considering looking at a climate change scenario and determining which of the El Niño "flavors," and consequent climatic impacts, might be dominant.

CIRES is a joint institute of the University of Colorado at Boulder and the National Oceanic and Atmospheric Administration.

Contact: Adriana Bailey, 1.303.492.6289