Acoustic remote sensing of double-diffusive instabilities and intrusions in ocean frontal zones

Abstract

Small-wavelength sound has been observed to scatter from turbulent microstructure and plankton in the ocean. Additional scattering from salt fingers and diffusive-convective flows that result from double diffusive instabilities (DDI) is also theoretically possible in this same wavelength band, and has been measured in a laboratory setting, though with artificially high gradient parameters. If measurable, small-wavelength backscatter would enable remote sensing of water mass interleaving and mixing from intrusive flows at fronts, which spawn DDI. The factors controlling the visibility of echoes from DDI features is the strength of the features in terms of density and sound-speed anomaly, and the level of masking return from plankton, which may merely interfere or act as passive tracers of these features. Microstructure data collected in double-diffusive instability features are used to compute the expected monostatic backscatter signals. These results suggest that DDI microstructure, from both salt fingering and convection, would be acoustically observable in areas with low to average plankton. The results also suggest that density interfaces (layering structures) associated with DDI would be visible. Acoustic backscatter data show echoes from both the interfaces and DDI microstructure in a frontal zone across a range of wavelengths. [Work supported by the Office of Naval Research.]