Stratified turbulent mixing from the small-scale oceanic internal wave field*

In the pycnocline, where internal waves produce transient stratification and shear, the dominant mechanisms producing mixing are unknown. Identifying these mechanisms is key for determining the contribution of the wave field to diapycnal mixing in the ocean and improving parametrizations of mixing efficiency. We use direct numerical simulations (DNS) to investigate the properties of stratified turbulence representative of such small-scale ocean dynamics. Initializing the large-scale flow with an internal wave spectrum inspired by Garrett & Munk, we perform DNS of a Boussinesq fluid in a triply-periodic domain subject to a linear stratification. Forcing is applied to low-wavenumber internal waves throughout the simulation to represent energy transfer from larger scales. We then identify how key quantities such as mixing efficiency, diapycnal diffusivity and vertical buoyancy flux vary both locally within each simulation, and with different background stratifications. The local and temporal variation in these quantities also allows us to investigate the mechanisms by which turbulence is generated and sustained in the flow.