Estimating Probabilities of Extreme ENSO Events from Copernicus Seasonal Hindcasts

Multi-system seasonal hindcasts supporting operational seasonal forecasts of the Copernicus Climate Change Service (C3S) are examined to estimate probabilities that El Niño and La Niña episodes more extreme than any in the reliable observational record could occur in the current climate. With 184 total ensemble members initialized each month from 1993 to 2016, this dataset greatly multiplies the realizations of ENSO variability during this period beyond the single observed realization, potentially enabling a detailed assessment of the chances of extreme ENSO events. The validity of such an assessment is predicated on model fidelity, which is examined through two-sample Cramér–von Mises tests. These do not detect differences between observed and modeled distributions of the Niño 3.4 index once multiplicative adjustments are applied to the latter to match the observed variance, although differences too small to be detected cannot be excluded. Statistics of variance-adjusted hindcast Niño 3.4 values imply that El Niño and La Niña extremes exceeding any that have been instrumentally observed would be expected to occur with a > 3% chance per year on average across multiple realizations of the hindcast period. This estimation could also apply over the next several decades, provided ENSO variability remains statistically similar to the hindcast period.