Why do we have El Niño: quantifying a diabatic and nonlinear perspective using observations

Abstract

El Niño, due to its global impact on weather patterns, ecosystems, agriculture and public health, has become as commonly known to the public as the recent global warming. But why we have El Niño is not yet as well answered as it may have been assumed. Linear theories have been successful in explaining the transition from the warm phase to the cold phase of the eastern tropical Pacific that results from the rise and fall of El Niño, but failed to explain the asymmetry between the two phases. A nonlinear theory for El Niño has suggested that there exist two equilibrium states for the tropical Pacific—one is zonally symmetric (or nearly so) with the warm-pool extending all the way to the eastern Pacific, and the other is strongly zonally asymmetric with the warm-pool confined to the western half of the tropical Pacific. Under this hypothesis, ENSO results from the fact that under the current radiative heating, both states are unstable, resulting in the apparent “wandering” behavior in between these two states as seen in the observations. To test this hypothesis, the authors have obtained the best approximations for the two equilibrium states empirically using updated ocean assimilation data, and quantified the stability of these two empirically obtained equilibrium states using two stability analysis methods. The results suggest that the two states are unstable, offering support for the nonlinear view of why we have El Niño.