Nobody anticipated that El Niño would be weak and prolonged in 1992, but brief and intense in 1997/98. Why are various El Nino episodes so different, and so difficult to predict? The answer involves the important role played by random atmospheric disturbances (such as westerly wind bursts) in sustaining the weakly damped Southern Oscillation, whose complementary warm and cold phases are, respectively, El Nino and La Niña. As in the case of a damped pendulum sustained by modest blows at random times, so the predictability of El Nino is limited, not by the amplification of errors in initial conditions as in the case of weather, but mainly by atmospheric disturbances interacting with the Southern Oscillation. Given the statistics of the wind fluctuations, the probability distribution function of future sea surface temperature fluctuations in the eastern equatorial Pacific can be determined by means of an ensemble of calculations with a coupled ocean–atmosphere model. Each member of the ensemble starts from t... Nobody anticipated that El Niño would be weak and prolonged in 1992, but brief and intense in 1997/98. Why are various El Nino episodes so different, and so difficult to predict? The answer involves the important role played by random atmospheric disturbances (such as westerly wind bursts) in sustaining the weakly damped Southern Oscillation, whose complementary warm and cold phases are, respectively, El Nino and La Niña. As in the case of a damped pendulum sustained by modest blows at random times, so the predictability of El Nino is limited, not by the amplification of errors in initial conditions as in the case of weather, but mainly by atmospheric disturbances interacting with the Southern Oscillation. Given the statistics of the wind fluctuations, the probability distribution function of future sea surface temperature fluctuations in the eastern equatorial Pacific can be determined by means of an ensemble of calculations with a coupled ocean–atmosphere model. Each member of the ensemble starts from t...