On the three-dimensional structure of instabilities beneath shallow-shoaling internal waves

The stimulation of instability and transport in the bottom boundary layer by internal solitary waves has been documented for over twenty years. However, the large domain size required to simulate a large internal wave shoaling over a shallow slope, and the fine grid size required to resolve boundary layer phenomena have proven challenging for numericists. We present laboratory scale simulations that resolve three-dimensional dynamics induced in the boundary layer during the shoaling process. We find that the late stage, in which the incoming wave fissions into boluses, provides the most consistent source of three-dimensionalisation. In the early stage of shoaling, three-dimensionalisation occurs not so much due to a separation bubble instability, but to the interaction of vortices shed from the separation bubble with the overlying pycnocline. This interaction overturns the pycnocline, and creates bursts in kinetic energy and viscous dissipation, suggesting that the shed vortices induce turbulent motion and have the potential to induce sediment resuspension in the water column above and behind the separation bubble.