Using Background Oriented Schlieren to Diagnose Changes in the Density Profile Produced by Self-Interacting Internal Wave Beams

The meridional overturning circulation is regulated by diapycnal mixing in the abyssal ocean. This mixing is produced by the nonlinear interactions of internal waves (IWs) which are created when the barotropic tide flows over topography. Mathematically, an oscillating fluid over a stationary ridge is equivalent to a stationary fluid beneath an oscillating ridge. This allows for a laboratory experiment where we studied the evolution of the density field by oscillating an artificial ridge in a linearly stratified tank. Using a novel Background Oriented Schlieren (BOS) software package, as well as a unique BOS setup, we obtained the evolution of the density and velocity fields, showing evidence of mixing in the tank. We measured the evolution of the tank locally using a conductivity probe to validate the results of BOS. Our results showed that an oscillating topography caused not only a local change in the stratification but a global change to the density field in the tank. This experiment showed that IWs can efficiently mix the deep ocean far from their origin, which is required in order to sustain the Meridional Overturning Circulation.