The Non-linearity in the Pattern and Time Evolution of El Niño Southern Oscillation

In this study it is shown that ENSO skewness is not only a characteristic of the amplitude of events but also of the spatial pattern and time evolution. It is demonstrated that these non-linearities can be related to the non-linear response of the zonal winds to sea surface temperature (SST) anomalies. It is shown in observations as well as in coupled model simulations that significant differences in the spatial pattern between positive (El Niño) vs. negative (La Niña) and strong vs. weak events exist, which is mostly describing the difference between central and east Pacific events. The Bjerknes feedbacks and time evolution of strong ENSO events show strong asymmetries, with strong El Niños being forced more strongly by zonal wind stress than by thermocline depth anomalies and are followed by La Niña events. In turn strong La Niña events are preceded by El Niño events and are more strongly forced by thermocline depth anomalies than by wind stress anomalies. Further, the zonal wind stress response to sea surface temperature anomalies during strong El Niño events is stronger and shifted to the east relative to strong La Niña events, supporting the eastward shifted El Niño pattern and the asymmetric time evolution. Based on the simplified hybrid coupled RECHOZ model of ENSO it can be shown that the non-linear zonal wind stress response to SST anomalies causes the asymmetric forcings of ENSO events. This also implies that strong El Niños are mostly wind driven and less predictable and strong La Niñas are mostly thermocline depth driven and better predictable, which is demonstrated by a set of 100 perfect model forecast ensembles.