Is El Niño Sporadic or Cyclic ?

■ Abstract Is El Niño one phase of a continual, self-sustaining natural mode of the coupled ocean-atmosphere that has La Niña as the complementary phase? Or is El Niño a temporary departure from “normal” conditions “triggered” by a random disturbance such as a burst of westerly winds? A growing body of evidence—stability analyses, studies of the energetics, simulations that reproduce the statistics of sea surface temperature variations in the eastern equatorial Pacific—indicates that reality corresponds to a compromise between these two possibilities: The observed Southern Oscillation between El Niño and La Niña corresponds to a weakly damped mode that is sustained by random disturbances. This means that the predictability of El Niño is limited by the continual presence of “noise” so that forecasts should be probabilistic. The Southern Oscillation is also subject to decadal modulations. How it will be influenced by global warming is a matter of considerable uncertainty.