The horizontal distribution of bubbles in a shallow sea

Abstract

The distribution of microbubble clouds that populate the tidal and well-mixed North Sea is analysed, using observations in water 18 m deep from side-scan sonars placed on the seabed. The images are parameterised, to allow a statistical examination of the nature and scales of the distribution. Bubbles are generated in clouds by breaking waves, and the clouds, oscillating with the surface waves, then converge toward and gradually into long bands aligned close to the wind in the downwelling zones of Langmuir circulation. The measured band lifetimes are short, characteristically about a minute, in marked contrast to the stability of windrows and bands in lakes and the open ocean. This is ascribed to turbulence generated from tidal flow over the seabed. Marked bends and twists are observed in the band pattern, correlated over a scale of up to about 20 m laterally to the bands, some time, order 1 h, after the (5 m depth) current exceeds about 0.4 m s −1. These features are also attributed to the turbulence. The coverage of the sea surface outside bands by clouds is a significant fraction of the total coverage, leading to a mean spacing between clouds in the horizontal that is not sensitive to direction. The length scale of coherence of the scattering strength along the band is much smaller than the band length. The length scale of coherence laterally along the band is not substantially smaller than the band breadth. The length, breadth and lifetime of the bands increase with wind speed. Bands on occasion converge into one another, and clouds sporadically split away from bands. The scattering strength in a band following splitting is, on average, reduced. The mean speed of convergence exceeds that of splitting, both increasing with wind speed.