Internal Wave Generation Processes at Deep-Sills in the Luzon Passage Region of the South China Sea

Abstract : We are focused on understanding small-scale processes that influence the ocean s thermodynamic and dynamic properties on the sub-mesoscale (scales less than 10 km). This includes the enhanced internal waves generation and turbulent processes occurring in the Luzon Passage at the Lan-Yu and Heng-Chun ridges. The waters of the Luzon Passage are among the most energetic environments in the global ocean. Tidal currents as large as 0.8 m/s and strong stratification over the steep ridge topography give rise to internal wave generation, driving baroclinic currents. In addition, the Kuroshio Current often drives strong flow through the area. Not surprisingly, this environment supports exceptional turbulent energy levels distributed over full depth, leading to mixing between the South China Sea and Kuroshio water masses. Here, we present the first full-depth, direct measurements of turbulence along the Lan-Yu Ridge of the Luzon Straits over a spring neap cycle of tidal forcing. Data were collected on the east and west slopes of a ridge crest suggested by model results to be the site of exceptional baroclinic conversion of the barotropic tide. Measurements at each of these locations show that the strongest turbulence levels occur during the phase of the flood-ebb cycle when flow is in the downhill direction, indicating either hydraulic or slope convection processes. Our analysis shows very large turbulence levels, with dissipation rates reaching 105 W/kg at 1000-m depth, and depth-integrated dissipation rate levels reaching 1 W/m2. This implies that at least 10% of the locally generated baroclinic energy is dissipated directly before radiating away and suggests that local turbulent processes have considerable impact on the internal wave generation occurring at this region.