The Spatial Energy Transfer Through Parametric Subharmonic Instability of Internal Wave Beams at High Reynold's Number

The energy pathways within the ocean cover a vast range of scales and underlying dynamics. Energy injected via the barotropic tide at large scales ultimately ends up at small dissipative scales, which maintains the abyssal stratification and thermohaline circulations. The scale separation between injection and dissipation means the energy moves through regions dominated by wave turbulence of large internal wave beams down to eddy turbulence, making it challenging to describe the full picture of the energetics. To gain insight into the energy transfer via the wavefield, we study experimentally the flow characteristics produced by a large scale tidal forcing. This is done by oscillating a localized topography within a linear stratification in a large wavetank over three hour intervals. We collect data using a combination of background oriented schlieren and particle image velocimetry. A single conductivity probe gives fast measurements of density fluctuations within the wave beam, and an additional probe gives the overall change in the stratification. Finally, strain gauges mounted to the topography allow for completing the energy budget by quantifying the energy input to the system. Our results show that the energy is transfered from the main internal wave beam to multiple subharmonic modes leading to an energy cascade across a large range of spatial internal wave scales. In particular, we track down triads of parametric subharmonic instabilites and investigate their energy transfer across spatial scales.