Deep-Reaching Wave Energy Flux in the Southwestern Tropical Pacific Ocean during El Niño Events

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Abstract The present study investigates the vertical structure of wave energy in the tropical Pacific Ocean using the output of a linear, continuously stratified ocean model driven by interannual wind forcing. We have applied an energy flux diagnosis which seamlessly connects the tropical and subtropical zones. During El Niño events, the downward energy flux is extremely strong in the equatorial and off-equatorial region of the eastern Pacific Ocean, which is associated with the vertical propagation of Kelvin and Rossby waves that are important features for El Niño events. In the western basin, the downward transfer of wave energy penetrates deep into the ocean interior and exhibits a large vertical extent (from around 20- to 1200-m depths) in the southwestern tropical Pacific Ocean (SWTP; 15°–5°S, 150°E–150°W). The authors have demonstrated that none of the local wind input, wave reflection, or wave diffraction near the eastern boundary in the Southern Hemisphere contributes to the peak of downward energy flux in the SWTP. The significant downward energy flux in the SWTP originates from the wind input in the central equatorial Pacific Ocean (CEP; 5°S–5°N, 180°–130°W). This energy input generates strong southwestward energy flux to the SWTP in austral summer during El Niño. The horizontally converged wave energy in the SWTP then transfers downward to the deeper layer below 1200-m depth forced by the local wind. The local wind forcing is closely linked to the equatorward migration of the South Pacific convergence zone during El Niño events, which causes Ekman suction with a negative wind stress curl anomaly in the SWTP.

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