Estimating Probabilities of Extreme ENSO Events from Copernicus Seasonal Hindcasts

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