El Niño, more predictable than previously thought

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Good news. It seems that the prevailing pessimism about the predictability of El Niño may be misplaced. The strong climatic, economic and societal impact of El Niño climate events mean that their prediction is important. But current forecasts, based on data from the past three decades, achieve useful prediction at lead times of only 6-9 months. Chen et al. have carried out retrospective forecasts of El Niño for the period 1856-2003 using an improved ocean-atmosphere coupled model, and were able to predict all the prominent El Niños up to two years in advance. The notorious El Niño events of the late nineteenth century, implicated in the deaths of millions in India, Asia and South America, were retrospectively predicted by a numerical model for the first time.

Predictability of El Niño over the past 148 years Dake Chen et al. Nature 428, 733 – 736 (15 April 2004);
Forecasts of El Niño climate events are routinely provided and distributed, but the limits of El Niño predictability are still the subject of debate. Some recent studies suggest that the predictability is largely limited by the effects of high-frequency atmospheric ‘noise’, whereas others emphasize limitations arising from the growth of initial errors in model simulations. Here we present retrospective forecasts of the interannual climate fluctuations in the tropical Pacific Ocean for the period 1857 to 2003, using a coupled ocean-atmosphere model. The model successfully predicts all prominent El Niño events within this period at lead times of up to two years. Our analysis suggests that the evolution of El Niño is controlled to a larger degree by self-sustaining internal dynamics than by stochastic forcing. Model-based prediction of El Niño therefore depends more on the initial conditions than on unpredictable atmospheric noise. Although westerly wind bursts do affect the exact onset time and perhaps the amplitude of El Niño, the gross features of ENSO seem to be coded in the large-scale dynamic state. Our results favour the interpretation that the enhanced wind burst activity in the boreal spring preceding large El Niño events is a consequence of those ongoing events. We conclude that throughout the past century, El Niño has been more predictable than previously envisaged.