NOAA uses SST index for identifying El Niño (warm) and La Niña (cool) events in the tropical Pacific. It is the seasonal SST anomaly for the Niño 3 and the Nino 3.4 region (i.e., 5oN-5oS, 120o-170oW). Events are defined above the +0.5o anomaly for warm (El Niño) events and at or below the -0.5 anomaly for cold (La Niña) events. The threshold is further broken down into Weak (with a 0.5 to 0.9 SST anomaly), Moderate (1.0 to 1.4) and Strong (≥ 1.5) events. For the purpose of this report for an event to be categorized as weak, moderate or strong. In this study
Monthly means were computed for each time series for the period 1950-1979. These means were then subtracted from their respective time series for the entire data set (1950-1999). Finally, a five-month running mean was applied to each time series.