The Ensemble Oceanic Niño Index

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Abstract
We describe a new El Niño–Southern Oscillation (ENSO) index that we call the Ensemble Oceanic Niño Index (Ensemble ONI). Ensemble ONI uses monthly sea surface temperature (SST) anomalies in the Niño3.4 region from 32 input SST datasets as the basis for the ensemble, and our new monthly ENSO index extends from the present back to the year 1850. We use the input datasets to quantify the uncertainty in the monthly index values. The uncertainty is calculated from datasets that are not completely independent from each other, but we use other estimates of uncertainty in SST, well‐documented historical ENSO events, a detailed consideration of literature‐based definitions of ENSO events, and proxy‐based determinations of past ENSO events, to show that our uncertainty estimate from the ensemble of input datasets is comparable with other studies both in terms of average magnitude and the time‐varying trend. Similar to past research, we find that ENSO events occur every 4–5 years on average, and there have been six “Super” El Niños (1877–1878, 1888–1889, 1972–1973, 1982–1983, 1997–1998, 2015–2016) that statistically rise above all other El Niños since 1850. Finally, the time span of our work shows that El Niño events were most intense at ends of both the 19th century and the 20th century, with a lull in the mid‐1900s, corroborating previous instrumental, written, and proxy records.

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