(Un)predictability of strong El Ni\~no events

TL;DR

This paper investigates the unpredictability of strong El Niño events by analyzing their frequency, regularity, and the underlying chaotic dynamics, revealing mechanisms that limit their predictability in climate models.

Contribution

It introduces a conceptual 'recharge oscillator' model showing how noise and seasonal forcing cause irregular El Niño strength and timing, highlighting limits to predictability.

Findings

Strong El Niño events are irregular and unpredictable on decadal scales.

A 'recharge oscillator' model explains the switching between predictable and chaotic ENSO states.

Noise and seasonal forcing induce irregular El Niño occurrences and limit forecast accuracy.

Abstract

The El Ni\~no-Southern Oscillation (ENSO) is a mode of interannual variability in the coupled equatorial Pacific coupled atmosphere/ocean system. 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 discusses the frequency and regularity of strong El Ni\~no events in the context of chaotic dynamical systems. We discover a mechanism that limits their predictability in a conceptual "recharge oscillator" model of ENSO. Weak seasonal forcing or noise in this model can induce irregular switching between an oscillatory state that has strong El Ni\~no events and a chaotic state that lacks strong events, In this regime, the timing of strong El Ni\~no events on decadal time scales is unpredictable.