Bubbles and surf zone oceanography

Abstract

Breaking waves in the surf zone entrain high-density clouds of bubbles which are subsequently transported offshore via rip currents and turbulent diffusion. The presence of the bubbles presents a complex problem for acousticians interested in the propagation of natural and man-made sound in this shallow water regime. To investigate the acoustical properties of this area, a multi-investigator experiment was performed near the Scripps Pier in March 1997. A broadband acoustic technique, developed at the Marine Physical Laboratory, was deployed in a four-channel configuration in water of 6-m depth. Each channel of the system allows the determination of the bubble size distributions at a discrete depth via inversion of the measured sound speed and attenuation. Analysis of the data shows high concentrations of bubbles advecting offshore via rip currents, resulting in acoustic attenuations ranging from O(1)–O(100) dB/m. Vertical gradients were observed in the bubble concentrations. Simultaneous measurements of the wind, waves, and currents provide a synoptic view of the overlying oceanography during the course of the experiment. Results of the experiment will be discussed in the context of the transport of surf-generated bubbles through the measurement site.