Internal wave boundary layer interaction: A novel instability over broad topography

Abstract

It has been known for some time that internal wave-induced currents can drive near bed instabilities in the bottom boundary layer over a flat bottom. When the bottom is not flat, the situation can become quite complicated, with a diverse set of mechanisms responsible for instability and the subsequent transition to turbulence. Using numerical simulations, we demonstrate the existence of a mode of instability due to internal solitary wave propagation over broad topography that is fundamentally different from the two dominant paradigms of flow separation over sharp topography and global instability in the wave footprint that occurs over a flat bottom observed at high Reynolds number. We discuss both the two and three-dimensional evolution of the instability on experimental scales. The instability takes the form of a roll up of vorticity near the crest of the topography. As this region is unstratified in our simulations, little three-dimensionalization is observed. However, the instability-induced currents provide an efficient means to modulate across boundary layer transport. We subsequently extend the results to the field scale and discuss both the aspects of the instability that are consistent across scales and those that are different.