NSF PR 01-33 - April 19, 2001
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Scientists Suggest New Index to Capture "Flavors"
of El Niño
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 Niño.
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 one temperature index does not give a complete
picture of the climate phenomenon, according to scientist
Kevin Trenberth of NCAR.
"El Niño 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 Niños, 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 Niño
index called the "Trans-Niño Index" or TNI.
This new mathematical equation calculates the difference
between sea surface temperature changes in the central
equatorial Pacific Ocean and those in waters along
the coast of South America. 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.
In his research Trenberth found that although El Niño
events tend to be locked to the annual cycle and typically
peak in the northern winter, the evolution of El Niño
has changed substantially. The TNI 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.
"Trenberth's and Stepaniak's study, which has resulted
in an innovative way of differentiating between El
Niños of various strengths and characters,
should lead to a better understanding of the dynamics
of El Niños--and their improved prediction,"
said Jay Fein, director of the National Science Foundation
(NSF)'s climate dynamics program, which funded the
research.
"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 Niño events,"
added Trenberth. "Our goal is to capture that character
so we can improve confidence in future predictions."
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