Numerical investigation of variable reflection of the mode-1 internal tide from the Tasmania continental slope.

Semidiurnal internal tides are radiated from the Macquarie Ridge into the Tasman Sea in the form of an internal tidal beam that crosses 1000km of the deep ocean with little energy loss. The energy deposition is thought to happen on the continental slope of Tasmania where the beam reflects and scatters from the supercritical topography, the subject of a recent field program, The Tasmania TIdal Dissipation Experiment (TTIDE). Here the results of realistic ocean simulations are presented and discussed to unfold dynamics of the mode-1 reflection from Tasmania under varying ocean conditions. The bulk reflection coefficient is found to be 0.7 with a time variability of $\pm 0.13$. The higher values are associated in part with irregular generation of a leaky superinertial trapped wave that converts a portion of the local surface tide and adds a source term to the reflected wave budget. However, changes in reflectivity due to time variable stratification resulting from changes to the location of the East Australian Current has a stronger effect and explains the lower bound of variability of the reflection. This dependence is diagnosed using an extension to the coupled-mode framework of Kelly et al. (2016) that explicitly expresses forward and backward traveling components and reveals energy transfers as mode-1 undergoes reflection. Further, the coupled-mode equations are rewritten to address some aspects of the leaky wave dynamics and its irregularity. Particularly, it is obtained that the wave generation and existence are determined by the gradient of the coupling term describing interaction of mode-1 with barotropic mode. In conclusion, the numerical experiments highlight the variable dynamics of the reflection. It alters between a simple, mainly two-dimensional problem to three-dimensional, energetically nonconservative system. Within the constraints of the simulation numerics, most of the dissipation occurs not on the slope, but offshore of Tasmania.