El Niño and La Niña events are driven by changes in the equatorial Pacific Ocean. During El Niño events, sea surface temperatures in the central and eastern Pacific become warmer than normal, while La Niña events are essentially the opposite, with cooler than normal sea surface temperatures in the same regions.
In order to monitor the Pacific Ocean for signs of El Niño or La Niña, climatologists often use several "NINO" indices. These indices simply refer to the difference from the long term mean of the sea surface temperature in several regions located along the equatorial Pacific. These regions are called NINO1 and NINO2 (which lie on the South American coast), NINO3, and NINO4 (which occupy the eastern and central equatorial Pacific) and NINO3.4 (which partially overlaps both the NINO3 and NINO4 areas).
The NINO regions (shown in Figure 1) cover the following areas:
For monitoring of El Niño-Southern Oscillation (ENSO) events, the value of the NINO indices are often used in conjunction with other data (e.g., sub surface ocean temperatures, cloudiness, winds, the Southern Oscillation Index). Australian climatologists often cite monthly NINO values above +0.8 as typical of El Niño conditions, with values of below -0.8 as of a La Niña. These values are approximately one standard deviation from the long term mean (i.e., around 70% of monthly NINO3 values, for example, lie between -0.8 and +0.8).
Figure 1 NINO regions in the Pacific Ocean