NORTH AMERICAN COUNTRIES REACH CONSENSUS ON EL NIÑO DEFINITION
NOAA announced that the NOAA National Weather Service, the Meteorological Service of Canada and the National Meteorological Service of Mexico have reached a consensus on an index and definitions for El Niño and La Niña events.
The index is defined as a three-month average of sea surface temperature departures from normal for a critical region of the equatorial Pacific (Niño 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 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.
operational definitions for El Niño and La Niña, based
on the index, are:
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 0.5 degrees C (0.9 degrees Fahrenheit), averaged over three consecutive months.
(NOAA News Online, Feb. 23, 2005)