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Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles. [ABSTRACT FROM AUTHOR]

Based on high-resolution turbulence microstructure and near-surface velocity data, frontal instability and its relation to turbulence are investigated inside a transient upwelling filament in the Benguela upwelling system (southeast Atlantic). The focus of our study is a sharp submesoscale front located at the edge of the filament, characterized by persistent downfront winds, a strong frontal jet, and vigorous turbulence. Our analysis reveals three distinct frontal stability regimes. (i) On the light side of the front, a 30-40-m-deep turbulent surface layer with low potential vorticity (PV) was identified. This low-PV region exhibited a welldefined two-layer structure with a convective (Ekman-forced) upper layer and a stably stratified lower layer, where turbulence was driven by forced symmetric instability (FSI). Dissipation rates in this region scaled with the Ekman buoyancy flux, in excellent quantitative agreement with recent numerical simulations of FSI. (ii) Inside the cyclonic flank of the frontal jet, near the maximum of the cross-front density gradient, the cyclonic vorticity was sufficiently strong to suppress FSI. Turbulence in this region was driven by marginal shear instability. (iii) Inside the anticyclonic flank of the frontal jet, conditions for mixed inertial/symmetric instability were satisfied. Our data provide direct evidence for the relevance of FSI, inertial instability, and marginal shear instability for overall kinetic energy dissipation in submesoscale fronts and filaments. [ABSTRACT FROM AUTHOR]

A 19-h time series of dissipation, stratification, and horizontal velocities has been obtained for a dense gravity current flowing into the Arkona Basin in the western Baltic Sea. The observations are compared with one-dimensional, quasi-steady theory, in which the gravity component in the flow direction is balanced by bottom friction, while that in the cross-flow direction is balanced by the Coriolis force. The observations deviate from the theory in that the bottom shear stress is more than 2 times as large as that required to balance the gravity. Several reasons for this discrepancy are discussed. A 1D turbulence model is also compared with the observations. Profiles of velocity, stratification, and dissipation rates generally show similar variations with depth as the observations, although the observed dissipation rates are somewhat larger than the modeled and the modeled transverse velocities are much larger than the observed. Subsequently, the model is used to investigate the variation of the entrainment parameter for a large range of Ekman and Froude numbers. Within the modeled parameter space, the entrainment parameter can be collapsed onto a curve that is an increasing function of both the Froude and the Ekman numbers. There is one puzzling result of the observations that differs from the model results and earlier observations: the observed entrainment rate increases dramatically during the observation period, where the Froude number decreases slightly. Some reasons for this increase are discussed. [ABSTRACT FROM AUTHOR]

In frontal regions of the ocean, submesoscale motions are frequently observed features in the surface layer. Due to their important role for surface layer restratification, lateral dispersion, and the turbulent energy cascade, this type of motion has been extensively studied over the past decade. Various mixed-layer frontal instabilities, providing potential routes to energy dissipation, have been identified. However, direct turbulence measurements in submesoscale fronts, required to test the relevance of these new theoretical concepts, are so far largely missing. Here, data from a submesoscale upwelling filament (Benguela upwelling system) are presented that combine densely-spaced cross-front turbulence microstructure measurements with simultaneously conducted high-resolution velocity measurements from a towed research catamaran. These data reveal a narrow (width: 2-3 km) submesoscale front associated with an energetic frontal jet reaching speeds up to 1.2 m/s. Our data show a strongly turbulent surface mixing layer down to 30-40 m depth in the frontal region, driven by at least two distinct processes: (i) atmospheric forcing due to down-front winds and surface buoyancy loss on the warm side of the front; (ii) symmetric and inertial instabilities induced mixing in the cyclonic side of the frontal jet, consistent with the strong negative Ekman buoyancy flux in the frontal region. The two fronts bounding the submesoscale upwelling filament exhibited a marked asymmetry in mixing-layer depth and turbulence levels, likely resulting from the stabilizing or destabilizing Ekman transports on the two sides of the filament. Breaking internal waves associated with the collapse of the front were ubiquitous, suggesting an alternative pathway to energy dissipation in frontal regions. [ABSTRACT FROM AUTHOR]

Previous theoretical and numerical studies have shown that density fronts in the ocean surface layer are often amenable to inertial and symmetric instabilities. Such instabilities may induce a forward energy cascade, which is, according to numerical simulations, reflected in enhanced energy dissipation rates inside fronts. Field observations supporting this picture are, however, virtually lacking at the moment, mainly due to difficulties in obtaining combined stratification, shear, and mixing parameters at sufficient resolution across quickly evolving submesoscale features. Here, a recent data from a frontal region in the Baltic Sea will be presented, providing first direct evidence for enhanced energy dissipation inside a submesoscale mixed-layer front. Our measurements include repeated, densely-spaced cross-front turbulence microstructure transects and simultaneous high-resolution velocity measurements. These data reveal a narrow (width: 1-2 km) frontal region inside a 55 m thick surface layer, where the titled isopycnal structure of the front delineates a 20 m thick layer with strongly enhanced dissipation rates. Based on the observed background parameters, the possibility of different instability mechanisms (inertial, symmetric, and shear instability) will be discussed. It is found that the trapping of near-inertial waves inside the front provides an important energy source for frontal turbulence. This study supports the important role played by submesoscale motions as a route to energy dissipation of balanced larger-scale motions. [ABSTRACT FROM AUTHOR]

A high resolution k – ω two-equation turbulence closure model, including surface wave forcing was employed to fully resolve turbulence dissipation rate profiles close to the ocean surface. Model results were compared with observations from Surface Wave Instrument Floats with Tracking (SWIFTs) in the nearshore region at New River Inlet, North Carolina USA, in June 2012. A sensitivity analysis for different physical parameters and wave and turbulence formulations was performed. The flux of turbulent kinetic energy (TKE) prescribed by wave dissipation from a numerical wave model was compared with the conventional prescription using the wind friction velocity. A surface roughness length of 0.6 times the significant wave height was proposed, and the flux of TKE was applied at a distance below the mean sea surface that is half of this roughness length. The wave enhanced layer had a total depth that is almost three times the significant wave height. In this layer the non-dimensionalized Terray scaling with power of − 1.8 (instead of − 2 ) was applicable. [ABSTRACT FROM AUTHOR]