Ocean acoustic backscattering: When you can ignore acoustic scatter from turbulence and when you can’t

Abstract

While models predicting measurable levels of acoustic backscattering from oceanic turbulence have been around for decades, they have proven notoriously hard to confirm. This is, in part, because potential turbulent scattering layers often coincide with zooplankton layers. Therefore, as zooplankton are known to cause measurable acoustic scatter, the source of scatter is obscured. Furthermore, estimates of zooplankton abundance from acoustic scattering measurements have been shown to agree with independent measures under a number of circumstances. This gives circumstantial evidence to fuel the belief that scatter from turbulence is negligible. In addition, even if the turbulent scattering theory is correct, it predicts that over most of the ocean the turbulent intensities and/or stratifications are too weak to give turbulent scatter of a similar magnitude as is observed from zooplankton layers. Yet, despite all this, here data are presented that show circumstances when scatter from turbulence is significant. The areas of the ocean (mostly coastal) that one might have to be concerned about turbulent scatter are discussed, as well as the intriguing idea of using technology already developed for zooplankton observation to measure turbulence. This technology could facilitate the in situ study of biophysical interactions between zooplankton and turbulence.