Shaun Johnston (Scripps Institution of Oceanography)
Internal tidal energy fluxes in the South China Sea from density and velocity measurements by gliders
Internal tidal energy fluxes were obtained from June--August 2011 using underwater gliders in the South China Sea. Spray gliders profiled every 2 hours to 500 m, which is deep enough given the shallow thermocline to compute mode-1 fluxes from vertical mode fits to tidal displacements and currents. Westward, mode-1 diurnal and semidiurnal fluxes exceeded 40 and 30 kW/m. To our knowledge, these flux observations are the first from both velocity and density measurements by gliders. Fluxes compare well with a mooring near a generation site in southern Luzon Strait and a regional model. Furthermore, the zonal-depth structure of the internal tide is obtained by binning measurements, which cover four spring-neap cycles and over 100 km along 20.5 N. Westward phase propagation is found for currents and displacements, while roughly constant phase is found along beams. Both these features of the phase suggest a narrow-banded internal tide. Semidiurnal energy density is largest along a ray path which coincides with generation sites on both the eastern and western ridges in Luzon Strait. Diurnal energy density is surface-intensified consistent with relatively shallower diurnal ray paths emanating from the eastern ridge.