A tale of two species: Temporal behavior of diurnal and semidiurnal internal tides

Diurnal and semidiurnal internal tides were observed at four locations near the shelf edge in the northeastern South China Sea. Baroclinic energy-flux time series over a four-week period are computed at each site for each species. These suggest weak (or nonexistent) semidiurnal internal tide generation, and strong diurnal internal tide generation. Mean diurnal internal tide energy flux at one site (water depth 200 m) was 1.7 kilowatts per meter, with mean energy 3.0 kilojoules per square meter. Over large expanses of the region the seafloor slope is less steep than diurnal internal-wave rays, and much less steep than semidiurnal internal- wave rays (subcritical geometry). Time series of diurnal baroclinic energy flux differ from simple functions describing forcing, for example the Baines force (proportional to perturbation density), suggesting that other details such as barotropic forcing velocity orbits may play a role. Three quantities potentially guiding barotropic to baroclinic tidal energy conversion are presented: particle trajectory, a depth-dependent criticality parameter, and a generation region size scaling parameter. These empirical parameters highlight that forcing occurs within volumes, and that the coherencies (throughout the volume) of the phase and the directivity of long-wavelength forcing with respect to a propagating short-wavelength response wave may greatly influence the conversion process.