The characteristics of breaking waves, bubble clouds, and near-surface currents observed using side-scan sonar

An inverted 248 kHz two-component side-scan sonar mounted on the sea bed in a mean depth of 34 m has been used to detect the clouds of bubbles produced by breaking surface waves. The sonar has a range of about 150 m. The breaking waves appear on the sonograph records as short-lived intense echoes, and form patterns which can be explained in terms of the behaviour of groups of waves in which the highest are breaking. The bubble clouds are slightly elongated in the wave direction when they are first formed and, in winds of 5.1 m s−1, have lifetimes of up to about 5 min. Soon after a wave breaks, the horizontal motion of the fluid in which the bubbles are formed becomes similar to that of the surroundings, and the bubbles continue to be advected by the near-surface currents. The rate of drift of bubble clouds along the directions of the sonar beams allows the components of the currents to be measured. The sonographs show that large changes in currents can occur over horizontal distances of as little as 5 to 10 m when fronts are passing. The motion of the fronts through the water can be measured. The sonographs have been used to measure surface currents induced by internal waves. Bands of bubbles associated with Langmuir circulation can be detected in strong winds or when moderate winds are accompanied by heavy rain.