Title: (Un)predictability of strong El Niño events,
Abstract: The El Niño-Southern Oscillation (ENSO) is a mode of interannual
variability in the coupled equatorial Pacific coupled atmosphere/ocean system.
El Niño describes a state in which sea surface temperatures 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.