"Before now, no widely accepted operational definition of El Niño or La Niña existed," said retired Navy Vice Admiral Conrad C. Lautenbacher, Jr., Ph.D., under secretary of commerce for oceans and atmosphere and NOAA administrator. "This effort provides for greater consistency of communication between scientists, who monitor and study the ENSO cycle and more uniformity in our message to the public through the media," he added.
To come up with the definitions, NOAA scientists conferred with experts at the Scripps Institute of Oceanography, the Center for Ocean-Land-Atmosphere Studies (COLA), the Center for Ocean-Atmospheric Prediction Studies (COAPS), the International Research Institute (IRI) for Climate Prediction, the National Center for Atmospheric Research (NCAR), and the University of Washington.
NOAA's new, official definitions for El Niño and La Niña are:
El Niño: A phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the 1971-2000 base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5C, averaged over three consecutive months.
La Niña: A phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal (for the 1971-2000 base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5C, averaged over three consecutive months.
The definitions are based on an index of El Niño and La Niña water temperature extremes for the El Niño/Southern Oscillation (ENSO) cycle.
The index is defined as three-month averages of sea surface temperature departures from normal for a critical region of the equatorial Pacific (Nino 3.4 region; 120W-170W, 5N-5S). This region of the tropical Pacific contains what scientists call the "equatorial cold tongue," a band of cool water that extends along the equator from the coast of South America to the central Pacific Ocean.
Departures from average of sea surface temperatures in this region are critically important in determining major shifts in the pattern of tropical rainfall, which influence the jet streams and patterns of temperature and precipitation around the world.
NOAA began using the index and definitions operationally for monitoring and predicting El Niño and La Niña conditions on September 1, 2003. NOAA issues assessments of ENSO's status in the Monthly Climate Diagnostic Bulletin, the ENSO Diagnostic Discussion, and the Weekly ENSO update. Currently, NOAA is engaged in dialogue with the international meteorological community for global acceptance of the index and definitions.