Potential and Limitations of a Commercial Broadband Echo Sounder for Remote Observations of Turbulent Mixing

Abstract

Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally under-resolved 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 towards routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles.

Emergent Antarctic seafloor seeps: A tipping point reached?

“…Features off Cape Hallet Peninsula gave a similar re ectivity to subglacial discharge measured from the nearby Tucker Glacier (Fig. S8) and to signals obtained from strati ed turbulences in other global regions [16][17][18] . We consistently found metazoan dead zones at the centre of uid seeps, with transition zones dominated by opportunistic mega-epifaunal taxa that were relatively rare in the surrounding biologically-rich area at these sites (Fig.…”

Diapycnal Mixing Induced by Rough Small‐Scale Bathymetry

“…Muchowski et al. (2022) showed that in areas where biological scattering does not dominate the signal, turbulent microstructure is the primary source of acoustic backscatter recorded with the R/V Electra EK80 system in this region and time of year. In this study, real‐time acoustic observations enabled us to plan positions of the MSS measurements and to target local mixing hotspots.…”

“…While the MSS profiler measures small‐scale turbulent velocity fluctuations, the EK80 measures acoustic backscatter from density and sound speed fluctuations, caused by temperature and salinity fluctuations (e.g., Lavery et al., 2013). Therefore, acoustics only register turbulent structures in regions with existing background temperature and salinity gradients, where turbulent stirring induces temperature and salinity microstructure and thus increased acoustic backscatter (Muchowski et al., 2022). In contrast, MSS measurements show dissipation rates also in well‐mixed parts of the water column.…”