A numerical model of internal tides with application to the Australian North West Shelf

Abstract

A nonlinear, primitive equation, finite-difference numerical model is applied to the problem of the generation, propagation, and dissipation of internal tides over a cross section of the continental slope and shelf topography of a region on the Australian North West Shelf. The model is forced through the specification of the offshore tidal elevation and as such the full tidal field is modeled for the M2 constituent. An energetic internal tide is produced in the model with results showing sensitivity to changes in both stratification and bathymetry. The ratio of the slope of the internal wave characteristics to the bathymetry is generally less than or close to one, producing subcritical and approximately critical conditions. Model results are compared to previously reported observations and show reasonable agreement in terms of wave structure, propagation direction, and regions of generation and energy dissipation. The model shows a high degree of spatial variability in the amplitude and phase of internal wave currents and vertical displacements with motion tending to propagate along characteristic paths as beams of signal. However, dissipation prevents the beams from radiating large distances from the generation regions. The energy flux of the internal tide propagates both onshore and offshore and the magnitude of the flux is strongly dependent on the slope of the bathymetry with largest values occurring for steepest topography. The internal wave amplitude and hence energy flux is also found to be dependent on the magnitude of the vertical and horizontal mixing of momentum with maximum values achieved under conditions of no mixing.