Laboratory Observations and Consequences of Parametric Subharmonic Instability in an Internal Wave Field

Diapycnal mixing in the ocean interior modulates the meridional overturning circulation. Since energy is typically injected at scales that are too large to mix effectively, mixing requires nonlinear processes to transfer energy toward small scales where 3D turbulence occurs. Here we consider internal waves generated by the flow of the barotropic tide over ocean ridges. The dynamics of a barotropic tide over an ocean are akin to a stationary fluid forced by an oscillating topographic feature. Therefore, we were able to simulate the energy transfer in a laboratory experiment by oscillating an artificial ridge in a linearly stratified tank. We observed the emergence of the iconic internal wave beams, and after a sufficiently long time, the beams engendered Parametric Subharmonic Instability (PSI) which created vertically periodic mixed regions that spread as thin intrusions. Our work shows that under appropriate conditions internal wave beams efficiently transfer energy from large forcing scales into small mixing scales through PSI, an effective agent of mixing in the ocean.