Internal and surface wave effects on inverted echo sounder measurements

Abstract

Inverted echo-sounders operating at 10 kHz, long used to observe mesoscale synoptic variability from the sea-floor through changes in acoustic travel time and pressure, were recently deployed in the South China Sea to explore their potential for detecting nonlinear internal waves. The observations, based on a 6-s transmission rate, reveal both the internal tide and high-frequency nonlinear internal wave packets and are consistent with remotely sensed images and nearby temperature time series. The results motivate exploration of a refinement in instrument operation in which the raw signal is digitized and stored at the full transducer bandwidth, rather than the present scheme of narrow band filtering and threshold detection. If the raw signal is available, it will be related to the surface wave field, leading to the prospect of ocean surface environmental measurements from the sea-floor. An intriguing possibility is that internal wave modulation of the surface wave field, so evident in remotely sensed images, including the consequence of surface wave breaking in convergence zones, might be detectable in this way from the ocean floor, thus linking internal wave observations directly with surface wave modulation. Model calculations of surface scatter are used to gain insight on this approach.