Breaking Topographic Lee Waves in a Tidal Channel in Luzon Strait

Barotropic tides generate energetic internal tides, smaller-scale waves, and turbulence as they flow through Luzon Strait, between Taiwan and the Philippines. Three-dimensional numerical simulations of this process suggest that small-scale lee waves will form and break preferentially in "outflow channels," trough-like depressions that descend the strait's flanks. In the simulations, these sites are the locations of the most intense dissipation in the eastern strait. To investigate this numerical prediction, an 11-day cruise on R/V Roger Revelle was devoted to exploring an outflow channel on the eastern slope of the strait, north of Batan Island. Using a rapidly profiling conductivity-temperature-depth sensor and shipboard Doppler sonars, observations of velocity and density fields were made at four sites in the channel. At Site III, approximately 4 km offshore the crest, the generated lee wave was found to occupy much of the water column. It expanded upward from the seafloor as an irregular disturbance with a dominant vertical scale of 250 m. Sea-surface horizontal currents exceeded 1.5 m s–1 and were sufficient to cause surface waves to break at 1,300 m above the local topography. Widespread internal wave breaking appeared initially at the seafloor and spread to much of the water column during the outflow phase of the tide. Breaking was also seen to a lesser extent on the inflow phase, as Pacific waters were advected westward toward the crest. The average dissipation rate at Site III, 8 W m–2, exceeds typical wind energy input rates by four orders of magnitude.