Acoustic Backscatter from Turbulent Microstructure

Abstract

Acoustic backscatter has produced spectacular images of internal ocean processes for nearly two decades, but interpretation of the images remains ambiguous because several mechanisms can generate measurable backscatter. The authors present what is thought to be the first simultaneous measurements of calibrated acoustic returns and turbulent microstructure, collected in a set of 20-m-tall billows. The observations are from Admiralty Inlet, a salt-stratified tidal channel near Puget Sound. Scattering due to turbulent microstructure alone is strong enough to explain the measured backscatter at specific sites within the billows. Existing formulations underestimate the strength of acoustic backscatter from turbulent microstructure. Due to a misinterpretation of the high-wavenumber temperature spectrum, some previous formulations underestimate the differential Scattering cross section (σ) when scattering from the viscous-convective subrange. Also, the influence of salinity on refractive-index fluctuations can be as large as or greater than that of temperature when the density stratification is dominated by salinity. Using temperature alone to estimate σ in coastal and estuarine waters may lead to significant underestimates. A simple formulation is derived that takes these two factors into account. Because of high ambient scattering from zooplankton in Admiralty Inlet, the acoustic data are conditionally sampled along modeled profiler trajectories to avoid using bulk statistics. Scalar dissipation is greatest in the bounding surfaces of the billows, consistent with these surfaces producing the most intense scattering. Acoustic backscatter can be used to remotely sense the spatial structure of scalar dissipation in turbulent events where σ due to turbulent microstructure exceeds the background level set by scattering from biology. In lakes and the deep ocean where scattering from zooplankton is expected to be negligible, scattering from microstructure may be the dominant mechanism. The largest uncertainties in the comparison result from the very large difference in sampling volume of the acoustic system and microstructure profiler.