The effect of El Niño on intraseasonal Kelvin waves

Intraseasonal Kelvin waves represent an important aspect of tropical ocean dynamics. There has been considerable speculation about whether intraseasonal Kelvin waves generated by intraseasonal wind events can trigger El Nino Southern Oscillation (ENSO) events, and therefore play a potentially important role in the prediction of El Nino. This study considers the different but related issue of how ENSO conditions influence intraseasonal Kelvin waves. Understanding this issue is an important part of clarifying the total role of scale interactions in the ENSO cycle. Numerical experiments carried out with an ocean general circulation model forced with European Centre for Medium-Range Weather Forecasts re-analysis surface fluxes show that both the west-to-east energy transmission and speed of propagation of intraseasonal Kelvin waves are strongly influenced by the phase of ENSO. The waves are much more strongly damped during La Nina conditions than during El Nino, primarily as a result of increased viscous dissipation associated with enhanced vertical shear. We suggest that this mechanism represents a significant source of nonlinearity between El Nino and La Nina states. It may influence the onset, the lifetime and the magnitude of warm or cold anomalies. Copyright © 2002 Royal Meteorological Society