A Model of the Wind-Driven Seasonal Variability in the Tropical North Pacific, with Validation through Altimeter Data

Abstract

The wind-driven seasonal variability in the tropical North Pacific Ocean is analyzed by means of a two-layer primitive equation ocean model implemented in an idealized Pacific extending from 30°S to 52°N and forced by an analytic seasonally varying wind stress field derived from ECMWF winds. The oceanic response in the tropical region is found to be mainly in the form of annual beta-refracted baroclinic Rossby waves radiating from the eastern boundary. Validation of these results with TOPEX/Poseidon (T/P) altimeter data is carried out. Zonal longitude?time diagrams of sea surface height anomalies and zonally integrated meridional transports computed from T/P data and from the winds through the Sverdrup relation by Stammer are compared with the corresponding quantities obtained from the model results. Such comparison shows that the model does capture the essential features of the wind-driven seasonal variability in the tropical North Pacific. The analysis of the model results, based also on sensitivity experiments, shows that the waves radiating from the eastern boundary are generated by the passage of northward-propagating coastal Kelvin waves, which are in turn produced remotely by seasonally varying winds in the equatorial wave guide, through a dynamical mechanism known to play a major role in the dynamics of El Niño events. Therefore, these model results suggest that the annual Rossby waves observed in the eastern tropical North Pacific may be mainly generated remotely in the equatorial band rather than by varying winds along the eastern coast, as considered in other studies.