El Nino and la Nina amplitude asymmetry caused by atmospheric feedbacks

Interannual variability of tropical Pacific sea surface temperatures (SST) has an asymmetry with stronger positive events, El Ni??o, and weaker negative events, La Ni??a, which is generally attributed to processes in the ocean. Here we present evidence from a new hybrid coupled model that the asymmetry and seasonality of El Ni??o can be caused by nonlinear and seasonally varying atmospheric feedbacks. The model consists of the ECHAM5 global atmospheric general circulation model (GCM) coupled to the 2-dimensional El Ni??o linear recharge oscillator ocean model in the tropical Pacific and a mixed layer ocean elsewhere. Despite the models simplistic and, by construction, linear representation of the ocean dynamics, it is able to simulate the main statistical features of El Ni??o including period, seasonality, skewness, and kurtosis. Analyses of the model show that a nonlinear relationship between the zonal wind stress and SST is causing the El Ni??o-La Ni??a asymmetry.