Oblique internal tide generation

Abstract

An important pathway for the dissipation of barotropic tidal energy in the ocean results from the generation of baroclinic tides at seamounts and ridges. A nonlinear hydrostatic model is used to study how barotropic to baroclinic energy conversion is affected by the Coriolis force and the orientation of undersea ridges with respect to the tidal propagation direction. Results show nearly symmetric lobes of depth-integrated baroclinic energy flux on both sides of a ridge oriented normal to the tidal velocity when the Coriolis parameter is zero and the model is forced with a rectilinear tide. This energy flux is distributed between narrow beams that emanate upward from the ridge top and much wider structures fanning downward and outward from the ridge. Rotating the ridge with respect to the tidal direction distorts the shape of the beams and reduces the overall energy flux gradually at first, then much more rapidly as the relative incidence angle (RIA) approaches zero. The energy flux is a simple function of the RIA and the aspect ratio of the ridge, which follows from the assumption that the energy flux is proportional to the maximum slope of the ridge and the effective area the ridge presents to the oncoming tide. When the model is forced by elliptical tidal currents, the directivity of the resulting internal tide beams is reduced but the energy flux is increased relative to the case of rectilinear tidal forcing for the same tidal current magnitude. The resulting energy flux is the sum of the effects of treating the tidal ellipse as two rectilinear tides along the principal axes, 90 out of phase.