Buoyancy flux and mixing efficiency from direct, near-bottom turbulence measurements in a submarine canyon

Turbulent kinetic energy and thermal variance dissipation rates ϵ and χ , buoyancy flux J b , diffusivity κ ρ , and mixing coefficient , which is simply related to the mixing efficiency , are estimated from highly resolved microstructure measurements collected in a submarine canyon that has been previously shown to be experiencing near-bottom diapycnal upwelling. It is demonstrated that turbulence arises primarily from the convective instability of the internal tide. Twelve tidally resolving stations (12–48 h long) were conducted, wherein profiles were collected from between 5–15 m and 400 m above the bottom every 13–15 min using a custom turbulence vehicle. Turbulent buoyancy flux is estimated using the Osborn and Winters and D’Asaro methods, allowing direct estimation of the mixing coefficient as a function of time, temperature, and height above bottom. Turbulent dissipation and buoyancy flux generally increase toward the seafloor. The associated turbulent diapycnal diffusivity is 10 −4 –10 −2 m 2 s −1 . Observed is ∼0.2 ± 0.05 near the top of our measurement range, as expected in the ocean interior, and increases to 0.3–0.7 approaching the bottom, consistent with turbulence generated by convective instability.