High‐frequency internal waves near the Luzon Strait observed by underwater gliders

Abstract

AbstractTidal flow through the Luzon Strait produces large internal waves that propagate westward into the South China Sea and eastward into the Pacific. Underwater gliders gathered sustained observations of internal waves during seven overlapping missions from April 2007 through July 2008. A particular focus is the high‐frequency internal waves, where the operational definition of high involves periods shorter than a glider profile taking 3–6 h. Internal wave vertical velocity is estimated from measurements of pressure and glider orientation through two methods: (1) use of a model of glider flight balancing buoyancy and drag along the glider path and (2) high‐pass filtering of the observed glider vertical velocity. By combining high‐frequency vertical velocities from glider flight with low‐frequency estimates from isopycnal depth variations between dives, a spectrum covering five decades of frequency is constructed. A map of the standard deviation of vertical velocity over the survey area shows a decay from the Luzon Strait into the Pacific. The growth of high‐frequency vertical velocity with propagation into the South China Sea is observed through two 2‐week time series stations. The largest observed vertical velocities are greater than 0.2 m s−1 and are associated with displacements approaching 200 m. The high‐frequency waves are observed at regular intervals of 1 day as they ride on diurnal tidal internal waves generated in the Strait.