New index to capture El Nino 'flavors'

April 16, 2001

BOULDER--Just as the Federal Reserve uses more than one index to measure the health and state of the economy, scientists at the National Center for Atmospheric Research (NCAR) believe it is essential to have at least two climate measures to capture all "flavors" of El Nino.

Climate scientists have long used changes in sea surface temperatures in specific regions of the Pacific Ocean to characterize El Nino events. But using just that one index does not give a complete picture of the climate phenomenon, according to Kevin Trenberth, head of the Climate Analysis Section at NCAR.

"El Nino comes in many different 'flavors,'" said Trenberth. "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 Ninos, or between the entire nature of the event and its evolution."

Writing in the April 15 issue of the American Meteorological Society's Journal of Climate, Trenberth and colleague David Stepaniak, propose a second El Nino index called the "Trans-Nino Index" or TNI. The new index is a mathematical equation that 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 Nino events have developed over the last 50 years and to detect changes that may be occurring on a decadal time scale.

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

"We want to explore whether we can use the relationships of temperature variations between the different parts of the Pacific to evaluate numerical climate models on how well they simulate El Nino events," added Trenberth. "Our goal is to capture that character so we can improve confidence in future predictions."
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of 66 universities offering Ph.D.s in atmospheric and related sciences. NCAR's primary sponsor is the National Science Foundation.

Founded in 1919, the AMS is the nation's leading professional society for scientists in the atmospheric and related sciences. The Society publishes well-respected scientific journals, sponsors scientific conferences, and supports public education programs across the country.Contacts:
UCAR Communications
Telephone: 303-497-8604

Stephanie Kenitzer
American Meteorological Society 425-432-2192
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