(Un)predictability of strong El Nino events

Abstract

The El Niño-Southern Oscillation (ENSO) is a mode of interannual variability in the coupled equa- torial Pacific coupled atmosphere/ocean system. El Niño describes a state in which sea surface temper- atures in the eastern Pacific increase and upwelling of colder, deep waters diminishes. El Niño events typically peak in boreal winter, but their strength varies irregularly on decadal time scales. There were exceptionally strong El Niño 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 Niño 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 Niño 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 Niño events and a chaotic state that lacks strong events, In this regime, the timing of strong El Niño events on decadal time scales is unpredictable. (c) The Author(s) 2017. Published by Oxfold University Press.