An El Nino event is defined when the three month running mean of (ERSST.v4) sea surface temperature anomalies in the Niño 3.4 region climb above the threshold of 0.5o for five consecutive overlapping three month periods.In other words, we take the average ocean temperature anomaly in the 3.4 region over a 3 month period, say September through January. Take the next five month overlapping periods--October through December and November through January--(the first two for example) and find these averages. That is your five consecutive, 3 month running mean. If this period's average is at or above +0.5 degrees, we classify this as an El Nino. The ERSST.v4 dataset goes back to 1850. Using this methodology, the top 10 strongest E Ninos are easy to find. Hat tip to Eric Webb for compiling the ERSSTv4 data.
|1877-78 El Nino|
|1888-89 El Nino|