Niño 3 Index
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
Below is a 51 year time series of the monthly Niņo 3 index:
The data was provided by Climate Prediction Center of NOAA.