Parametric Subharmonic Instability in the Ocean Bottom Boundary Layer
Internal waves with frequency larger than twice the local minimum allowable wave frequency can be susceptible to parametric subharmonic instability (PSI). This instability draws energy from the wave and provides a mechanism for generating small-scale turbulence and mixing. In the ocean, strongly baroclinic flows at the submesoscale adjust the minimum frequency of internal waves such that it is possible for PSI to occur for locally near-inertial waves. One setting where this may occur is in baroclinic bottom boundary layers along sloping topography, where near-bottom interior flows in the sense of Kelvin-wave propagation lead to a reduction of bottom boundary layer Ertel potential vorticity, and consequently lower the minimum frequency sufficiently to allow PSI. Linear stability analysis, and nonlinear simulations, show that PSI grows at a rate determined by the vertical stratification of the bottom boundary layer, and the slope Burger number. Wave shear production is the primary energy source for the instability, with additional contributions from buoyancy production that depend on the slope parameters. A partially compensating loss of energy to geostrophic shear production becomes increasingly important as the flow approaches the marginally stable state. These results suggest PSI as a potential mechanism for generating near-bottom mixing in the ocean.