On the Role of Off-Equatorial Oceanic Rossby Waves during ENSO

Recent theoretical and numerical modeling studies of the coupled tropical atmosphere-ocean system suggest that equatorial ocean wave dynamics may play an important role in the evolution of ENSO (El Niño/Southern Oscillation). These studies emphasize that the oceanic wave signal is confined to within a narrow equatorial band (within 6° of the equator). In this study we use a coupled atmosphere–ocean model to investigate the role of off-equatorial Rossby waves observed in the western North Pacific Ocean during the ENSO cycle. We find that these off-equatorial Rossby waves (found poleward of 6° from the equator) are formed through both eastern boundary reflection of the equatorial Kelvin wave signal generated in a warm event (El Niño), and changes in the off-equatorial wind stress curl. Our results indicate that, independent of the generation mechanism, off-equatorial Rossby waves should be thought of as the product and not the triggering mechanism for an ENSO event.