Estimating turbulent dissipation rates from acoustic backscatter

Recent work has suggested the possibility of using high-frequency (10 kHz to 1 MHz) acoustics to remotely sense turbulent microstructure in the ocean. Once developed, this technique will provide a powerful tool for examining the spatial and temporal distributions of ocean mixing in a way not possible using traditional microstructure methods. In this paper turbulent dissipation rates are estimated through an inversion of 307 kHz acoustic scattering data collected in the lee of a sill in Knight Inlet, British Columbia. These data have been shown previously to be strongly correlated with temperature and shear microstructure measurements. Here we show that inversion methods can be used to get turbulent dissipation rates from acoustic backscatter, provided that independent measurements of temperature and salinity stratification are available. The temperature–salinity characteristics of the environment, however, can place limitations on the inversion technique. The strong negative salinity gradient in Knight Inlet decreases the slope of the functional relationship between dissipation rate and scattering cross-section for high dissipation rates and increases the uncertainty of the inversion. This limitation on the inversion technique is not an issue throughout most of the world's oceans (where dT/dS>0) and, in places where it could be a problem, the limitation can be overcome by using multi-frequency techniques.