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Spring 2001

A new index to capture El Niño's flavors

Just as the Federal Reserve uses more than one index to measure the health and state of the economy, NCAR scientists Kevin Trenberth and David Stepaniak believe it is essential to have at least two climate measures to capture all "flavors" of El Niño. Writing in the 15 April issue of the Journal of Climate, the authors propose a second El Niño index called the Trans-Niño Index, or TNI.

Climate scientists have long used changes in sea-surface temperatures in specific regions of the Pacific Ocean to characterize El Niño events. But using just that approach does not give a complete picture of the climate phenomenon, according to Trenberth, head of NCAR's Climate Analysis Section. "El Niño comes in many different 'flavors," he says. "Each has a different and distinct character. An index of average sea-surface temperature variations in some parts of the Pacific Ocean does not allow us to differentiate between major, moderate, and minor El Niños, or between the entire nature of the event and its evolution."

The new index proposed by Trenberth and Stepaniak calculates the difference between sea-surface temperature changes in the central equatorial Pacific Ocean and those in waters along the coast of South America. This type of index, showing different developments across the Pacific, allows scientists to see how and where El Niño events have developed over the last 50 years and to detect changes that may be occurring on a decadal time scale.

Although El Niño events tend to be locked to the annual cycle and typically peak in the northern winter, Trenberth has found that the evolution of El Niño has changed substantially. The TNI index shows that El Niño events between 1950 and 1976 tended to develop first along the coast of South America and then spread westward. More recent El Niño events developed in the central Pacific and spread eastward.

"We want to explore whether we can use the relationships of temperature variations among the different parts of the Pacific to evaluate numerical climate models on how well they simulate El Niño events," added Trenberth. "Our goal is to capture that character so we can improve confidence in future predictions."

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Thu Jun 21 18:56:13 MDT 2001