Ranking the strongest ENSO events while incorporating SST uncertainty

The strength of El Nino-Southern Oscillation (ENSO) is often measured using a single, discrete value of the Nino index. However, this method does not consider the sea surface temperature (SST) uncertainty associated with the observations and data processing. On the basis of the Nino3.4 index and its uncertainty, we find that the strength of the three strongest ENSO events is not separable at 95% confidence level. The monthly peak SST anomalies in the most recent 2015-2016 El Nino is tied with 1997-1998 and 1982-1983 El Nino as the strongest. The three most negative monthly Nino values occur within the 1955-1956, 1973-1974, and 1975-1976 La Nina events, which cannot be discriminated by rank. The histograms of 1000-member ensemble analysis support the conclusion that the strength of the three strongest ENSO events is not separable. These results highlight that the ENSO ranking has to include the SST uncertainty.