Simulations of the three-dimensional structure of instabilities beneath shoaling internal waves

The simulation of instability and transport in the bottom boundary layer by internal solitary waves has been documented for over twenty years.  However, the challenge of shallow slopes and a disparity of scales between the large scale wave and the small scale boundary layer has proven challenging for simulations.  We present laboratory scale simulations that resolve the three-dimensionalisation in the boundary layer during the entire 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 separation bubble instability, but to the interaction of vortices shed from the separation bubble with the overlying pycnocline.