Asymmetry between El Niño and La Niña in a Global Coupled GCM with an Eddy-Permitting Ocean Resolution

Abstract

Observations show the asymmetric nature of El Niño and La Niña sea surface temperature (SST) anomalies. Warm events are often stronger than cold events. This asymmetric behavior is an important feature that can be used to validate coupled models to test their ability to represent the climate system. The asymmetry of El Niño and La Niña SST anomalies has been investigated in a simulation of the Hadley Centre eddy-permitting coupled general circulation model. It is found that the asymmetric behavior is captured by the model with SST anomalies associated with strong El Niño events being greater than those associated with strong La Niña events. The pattern of the SST asymmetry also bears some similar characteristics to those based on observations despite the deficiency that SST anomalies associated with both El Niño and La Niña extend too far westward in the model. Through a heat budget analysis of the ocean mixed layer, it is shown that nonlinear dynamic heating (NDH) is important in generating intense El Niño and the SST asymmetry between El Niño and La Niña events, especially in the eastern tropical Pacific Ocean. This nonlinear dynamic heating enhances the amplitude of El Niño and reduces the amplitude of La Niña, and therefore leads to the asymmetry between El Niño and La Niña events, with El Niño being stronger. However, the skewness and asymmetry in the model are relatively weak, being consistent with a relatively weak nonlinear dynamical heating. It is also shown that the eastward-propagating feature of subsurface anomalies provides a favorable phase relationship between temperature and current anomalies that results in strong nonlinear dynamical heating that tends to produce stronger El Niños. In addition, in the model simulation, the nonlinear nature of zonal wind stress anomalies between El Niño and La Niña events also plays an important role in the central tropical Pacific. These different mechanisms work constructively to determine the asymmetry between El Niño and La Niña events in the model, and they are similar to those proposed in recent studies based on observations. The ability of the model to simulate this asymmetric feature is encouraging and offers hope to the challenge of predicting the amplitude of strong El Niño events.