ACOUSTIC BACKSCATTER FROM TURBULENT MICROSTRUCTURE

Acoustic backscatter has produced spectacular images of internal ocean processes for nearly two decades, but interpretation of the images re mains ambiguous because several mechanisms can generate measurable bac kscatter. The authors present what is thought to be the first simultan eous measurements of calibrated acoustic returns and turbulent microst ructure, collected in a set of 20-m-tall billows. The observations are from Admiralty Inlet, a salt-stratified tidal channel near Puget Soun d. Scattering due to turbulent microstructure alone is strong enough t o explain the measured backscatter at specific sites within the billow s. Existing formulations underestimate the strength of acoustic backsc atter from turbulent microstructure. Due to a misinterpretation of the high-wavenumber temperature spectrum, some previous formulations unde restimate the differential scattering cross section (delta) when scatt ering from the viscous-convective subrange. Also, the influence of sal inity on refractive-index fluctuations tan be as large as or greater t han that of temperature when the density stratification is dominated b y salinity. Using temperature alone to estimate delta in coastal and e stuarine waters may lead to significant underestimates. A simple formu lation is derived that takes these two factors into account. Because o f high ambient scattering from zooplankton in Admiralty Inlet, the aco ustic data are conditionally sampled along modeled profiler trajectori es to avoid using bulk statistics. Scalar dissipation is greatest in t he bounding surfaces of the billows, consistent with these surfaces pr oducing the most intense scattering. Acoustic backscatter can be used to remotely sense the spatial structure of scalar dissipation in turbu lent events where delta due to turbulent microstructure exceeds the ba ckground level set by scattering from biology. In lakes and the deep o cean where scattering from zooplankton is expected to be negligible, s cattering from microstructure may be the dominant mechanism. The large st uncertainties in the comparison result from the very large differen ce in sampling volume of the acoustic system and microstructure profil er.