Tropical ocean decadal variability and resonance of planetary wave basin modes. Part I: Theory

Oceanic response to decadal wind forcing is studied in a tropical-extratropical basin using two classical theoretical shallow water wave models: the equatorial wave model and the planetary wave model. Extratropical winds are found to generate significant thermocline variability in the tropical ocean with a spectral peak at decadal timescale; the preferred decadal time of the response is due to the resonance of the gravest planetary wave basin modes. The resonant response, however, is unimportant in the interior extratropical ocean, where it is distorted by the forced response to local Ekman pumping. It is proposed that the resonance of planetary wave basin modes may provide a mechanism for the generation of decadal variability in the tropical ocean and, potentially, in the coupled ocean - atmosphere system.