Reflected and locally wind‐forced interannual equatorial Kelvin waves in the western Pacific Ocean

Abstract

Evaluating long equatorial wave reflections at the Pacific western boundary has been a concern for many past studies in order to discuss whether the delayed action oscillator mechanism may indeed be at work to terminate El Niño events. In the present study, we investigate the respective contributions of Rossby wave reflection and wind forcing in the generation of interannual long equatorial Kelvin waves in the western Pacific Ocean using satellite data over the 1993–2001 period. The reflection efficiency of the western Pacific boundary (east of 165°E) is found to be about 80–90% of a perfect wall at interannual timescales. This result would suggest that the delayed action oscillator mechanism is indeed at work during that period. However, the wind contribution increases significantly from 145°E to 175°E and actually represents more than 80% of the Kelvin wave variance at 175°E. As a conclusion, although Rossby wave reflection is efficient and sufficient to theoretically terminate an El Niño event, easterly wind anomalies in the western Pacific during the mature phase of the 1997–1998 event mostly contribute to the upwelling Kelvin signal involved in the termination of the event confirming previous studies mainly based on other methods. If such a mechanism were found to be at work during other El Niño events, then the negative feedback mechanism of the delayed action oscillator mechanism would be a combination of easterly wind anomalies and wave reflection with a significant role played by easterly wind anomalies. Our result may support some aspects of the western Pacific oscillator paradigm.