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.  

Diapycnal mixing in the oceans is crucial for local ecosystems as well as the large-scale circulation because it impacts vertical transport rates of heat, salt, oxygen, and other dissolved substances. We investigate the effect of extremely rough bathymetry on mixing and energy dissipation in a coastal region characterized by small-scale seafloor features penetrating a strongly-stratified density interface. While most studies of this type focus on tidal flow, we present shear microstructure measurements and co-located acoustic observations from the non-tidal Baltic Sea. Acoustic observations indicate temperature and salinity microstructure variance and therefore regions of diapycnal mixing. Due to their high resolution, acoustics enable us to resolve the variability and intermittency of stratified turbulence in the vicinity of the obstacles. Scale analysis and acoustic imaging suggest that the underlying mixing mechanisms are topographic wake eddies and, to a smaller extent, breaking internal waves. Depth averaged dissipation rates (1.1∙10^-7W kg^-1) and turbulent vertical diffusivities (7∙10^-4m²s^-1) in the halocline exceed those at a nearby reference station with smooth bathymetry by up to two orders of magnitude. Our study emphasizes the importance of rough small-scale (<1km) bathymetric features for energy dissipation and vertical transport of e.g. salt, heat and oxygen in coastal areas.

We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the Southern Quark located in the Åland Sea, connecting the Northern Baltic Proper with the Bothnian Sea. Our data were acquired during two one-week cruises in February-March 2019 and 2020 and include high resolution broadband acoustic observations of turbulent mixing, in situ microstructure profiler measurements and current velocities from Acoustic Doppler Current Profilers. A temporally reversing non-tidal stratified flow is observed over the steep bathymetric sill, creating a dynamic and extremely energetic environment. Saltier, warmer, and less oxygenated deep water south of the sill is blocked, the flow is at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill depending on the flow direction, and sub-mesoscale processes in the surface layer leading to high spatial variability at small scales. Mixing and vertical salt flux rates are increased by 3-4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow. We suggest that underlying mechanisms causing the highly increased mixing across the halocline are a combination of shear and topographic lee waves breaking at the halocline interface. We discuss the resulting deep-water and surface-water modification in the Southern Quark, which in turn impacts exchange processes between the Bothnian Sea and the Northern Baltic Proper. The here observed mixing is hypothesized to be important for the development of oxygen and nutrient conditions in the Bothnian Sea.