Determining an El Niño event by SST

(SST = Sea Surface Temperature)

There are many indicies that are formed from measuring sea surface temperatures from buoys in the Pacific Ocean in a certain region. The sea surface temperatures are averaged over the entire region. The above picture shows the region the Japan Meteorological Agency uses to calculate their index (the JMA index). If the average sea surface temperature is 0.5 F warmer than normal for six consecutive months, it's an El Niño ; according to some definitions, if the sea surface temperature is 0.5 F degrees colder than normal, it's a La Niña.