Recent progress in direct numerical simulations (DNSs) of stratified turbulent flows has led to increasing attention to the validity of the constancy of the dissipation flux coefficient Γ in the Osborn’s eddy diffusivity model. Motivated by lack of observational estimates of Γ, particularly under weakly stratified deep-ocean conditions, this study estimates Γ using deep microstructure profiles collected in various regions of the North Pacific and Southern Oceans. It is shown that Γ is not constant but varies significantly with the Ozmidov/Thorpe scale ratio ROT in a fashion similar to that obtained by previous DNS studies. Efficient mixing events with Γ ~ O(1) and ROT ~ O(0.1) tend to be frequently observed in the deep ocean (i.e., weak stratification), while moderate mixing events with Γ ~ O(0.1) and ROT ~ O(1) tend to be observed in the upper ocean (i.e., strong stratification). The observed negative relationship between Γ and ROT is consistent with a simple scaling that can be derived from classical turbulence theories. In contrast, the observed results exhibit no definite relationships between Γ and the buoyancy Reynolds number Reb, although Reb has long been thought to be another key parameter that controls Γ.

Among the existing finescale parameterizations of turbulent dissipation rates, the Gregg-Henyey-Polzin (GHP) parameterization is thought to produce the most accurate estimates of turbulent dissipation rates since it takes into account distortions from the Garrett-Munk (GM) spectrum using the shear/strain ratio R ω . The GHP parameterization, however, applies the single wave approximation to infer turbulent dissipation rates in broadband internal wave spectra with a multiplication factor up to 3 so as to adjust the predicted value at R ω = 3 to the theoretical value for the GM. Because of this multiplication, the GHP parameterization overestimates the dissipation rates for R ω 3. In this study, we explore the possibility of further improvements of the GHP parameterization and re-formulate the parameterization to make it applicable to both (i) a narrowband frequency spectrum characterized by a prominent near-inertial peak (R ω 3) and (ii) a broadband frequency spectrum like the GM (R ω ∼ 3). Furthermore, we assess the performance of the modified parameterization in comparison with the GHP parameterization using the available microstructure data obtained near prominent topographic features in the North Pacific. Jayne, S. R., 2009: The impact of abyssal mixing parameterizations in an ocean general circulation 340 model. , 2013: The latitudinal de-342 pendence of shear and mixing in the Pacific transiting the critical latitude for PSI. J. Phys.

Existing parameterizations of vertical mixing over a rough ocean bottom neglect the transformation of linear internal waves into quasi‐steady internal lee waves, which occurs when tide‐topography interactions strengthen. In the present study, we perform a series of eikonal calculations to investigate the energy transfers from upward propagating quasi‐steady internal lee waves to dissipation scales through nonlinear interactions with the background Garrett‐Munk internal waves. For a fixed density stratification, the vertical group velocity of the quasi‐steady internal lee wave increases as either the horizontal wave number of the bottom topography or the tidal flow amplitude increases, whereas the life time of the quasi‐steady internal lee wave decreases as the horizontal wave number of the bottom topography increases but is relatively independent of the tidal flow amplitude. Consequently, the resulting bottom‐enhanced vertical mixing extends further upward as the tidal flow amplitude increases, nearly independent of the bottom roughness. We also find a tradeoff between the fraction of energy dissipated at the ocean bottom and the vertical extent of the energy dissipation region above the ocean bottom.

Upper-ocean turbulence is central to the exchanges of heat, momentum, and gasses across the air/sea interface, and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed-layer depths and sea surface temperature. In part, progress has been limited due to the difficulty of measuring turbulence from fixed moorings which can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring Turbulent Kinetic Energy (TKE) dissipation rates, ϵ, from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-ocean Regional Study (SPURS) to collect two year-long data sets. We find the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate two-week missions for (10−8) ≤ ϵ ≤ (10−5) m2 s−3. Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1-10 km. We also find that dissipation estimates from two different moorings at 12.5 m, and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory.

Closing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.