Intrinsic unpredictability of strong El Ni\~no events

TL;DR

This paper reveals an intrinsic limit to predicting strong El Niño events, showing that their occurrence is governed by a switching mechanism between oscillatory and chaotic states influenced by weak seasonal forcing or noise.

Contribution

It introduces a new robust mechanism explaining the unpredictability of strong ENSO events through state switching induced by minimal seasonal forcing or noise.

Findings

Strong El Niño events are limited by a switching mechanism between oscillatory and chaotic states.

Weak seasonal forcing or noise can induce state switching, reducing predictability.

The mechanism explains the irregular occurrence of strong El Niño events.

Abstract

The El Ni\~no-Southern Oscillation (ENSO) is a mode of interannual variability in the coupled equatorial ocean/atmosphere Pacific. El Ni\~no describes a state in which sea surface temperatures in the eastern Pacific increase and upwelling of colder, deep waters diminishes. El Ni\~no events typically peak in boreal winter, but their strength varies irregularly on decadal time scales. There were exceptionally strong El Ni\~no events in 1982-83, 1997-98 and 2015-16 that affected weather on a global scale. Widely publicized forecasts in 2014 predicted that the 2015-16 event would occur a year earlier. Predicting the strength of El Ni\~no is a matter of practical concern due to its effects on hydroclimate and agriculture around the world. This paper presents a new robust mechanism limiting the predictability of strong ENSO events: the existence of an irregular switching between an oscillatory state that has strong El Ni\~no events and a chaotic state that lacks strong events, which can be induced by very weak seasonal forcing or noise.