Wave-produced bubbles observed by side-scan sonar

When a wave breaks in deep water, forming a whitecap, many small bubbles are generated and carried below the surface by turbulence. The breaking wave forms a strong acoustic target when viewed from below by an inverted side-scan sonar. The bubbles remaining in the water after the breaking wave has passed out of the sonar beam are also good acoustic scatterers and their drift towards or away from the sonar provides a means of remotely measuring that component of the near-surface current without interference of the flow and without exposing instruments to the hazard of violent near-surface conditions. This current changes rapidly as the wind changes. The sonar display shows regions of convergence or divergence of bubble clouds, perhaps associated with Langmuir circulations, and may, in calm weather, respond to the presence of surface slicks.