  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.
