The role of the southward wind shift in both, the seasonal synchronization and duration of ENSO events

Abstract

Near the end of the calendar year, when El Niño events typically reach their peak amplitude, there is a southward shift of the zonal wind anomalies, which were centred around the equator prior to the event peak. Previous studies have shown that ENSO’s anomalous wind stresses, including this southward shift, can be reconstructed with the two leading EOFs of wind stresses over the tropical Pacific. Here a hybrid coupled model is developed, featuring a statistical atmosphere that utilises these first two EOFs along with a linear shallow water model ocean, and a stochastic westerly wind burst model. This hybrid coupled model is then used to assess the role of this meridional wind movement on both the seasonal synchronization as well as the duration of the events. It is found that the addition of the southward wind shift in the model leads to a Christmas peak in variance, similar to the observed timing, although with weaker amplitude. We also find that the added meridional wind movement enhances the termination of El Niño events, making the events shorter, while this movement does not appear to play an important role on the duration of La Niña events. Thus, our results strongly suggest that the meridional movement of ENSO zonal wind anomalies is at least partly responsible for seasonal synchronization of ENSO events and the duration asymmetry between the warm (El Niño) and cool (La Niña) phases.