Parameter Tree:
EARTH SCIENCE > CLIMATE INDICATORS > ATMOSPHERIC/OCEAN INDICATORS > SEA SURFACE TEMPERATURE INDICES > NINO 3 INDEX

Definition:
The Niño 3 index is an average of the sea surface temperatures in the region 150 degrees West - 90 degrees West (longitude) and 5 degrees North to 5 degrees South (latitude). When the index is positive (red) then waters are warmer than normal and when the index is negative (blue) then waters are cooler than normal. El Niños occur when the water is much warmer than normal for a sustained period of time. For example, El Niños occured in 1982-83, 1986-87 and 1997-98

Reference:
Applied Mathamatics, University of Colorado at Boulder:
http://amath.colorado.edu/courses/2460/2004Spr/Lab1/nino3.html