Mixing Properties of Stratified Turbulence Forced by Breaking Internal Gravity Waves

Turbulent mixing has an important influence on many physical processes in the ocean. As well as being necessary for closing the global overturning circulation, this mixing governs the vertical transport of tracers such as heat, carbon and nutrients. Its spatial and temporal intermittency therefore determines the regional distribution of these important tracers throughout the ocean. Simple parameterizations are often used to infer diapycnal diffusivity estimates from observations across a wide range of oceanic flows. With this in mind, we perform direct numerical simulations of stratified turbulence arising from various types of large-scale forcing. Note that this forcing scale is large relative to turbulent length scales, but very small in the context of the ocean. By comparing flows forced by internal gravity waves with those forced by vertically uniform vortical modes, we identify a change of 20% in the mixing efficiency 𝜂≔𝜒/(𝜒+𝜀) of each flow, where 𝜀 and 𝜒 are the volume-averaged dissipation rates of turbulent kinetic energy and density variance. This change coincides with a reversal in the sign of the mean buoyancy flux, suggestive of a more convective mechanism for mixing in the internal wave-driven flow. Despite the significant difference in 𝜂 between the flows, further analysis reveals that an appropriately defined local mixing efficiency is independent of the local turbulent Froude number 𝐹𝑟≔𝜀/𝑁𝑘 in the wave-forced simulation, consistent with recently proposed scalings. We also use wavelets to produce localised energy spectra for our flows, and curiously find that an $m^{-3}$ vertical spectrum can be associated with the regions of lowest turbulent activity. Our results highlight the sensitivity of mixing to details of a small-scale oceanic flow, and the importance of understanding which mechanisms are dominant for constraining diapycnal transport budgets.