Sonar Observations of Langmuir Circulation and Estimation of Dispersion of Floating Particles

Abstract

Observations are described of Langmuir circulation obtained using upward-pointing bottom-mounted sonars, and a methodology to use the data to estimate the dispersion of floating particles is suggested. Observations of linear bands of acoustic scatterers separated by 2?20 m and detected using side-scan sonar in Loch Ness, Scotland, and in the southern North Sea are ascribed to subsurface bubbles in the convergence zones produced by Langmuir circulation. Data from the two observation sites are compared. The sonar is able to monitor the variability of the patterns over many hours. When the currents are sufficiently small, as in Loch Ness, individual bubble clouds produced by breaking waves remain in the beam long enough for their speed to be resolved, and the rate of convergence into the bands can be estimated. It increases linearly with wind speed. The acoustic data and direct measurements using current meters are used to derive estimates of the response time of bubble bands to changes in wind, and their mean separation, length, and persistence time. The bands in Loch Ness are shorter, but persist longer, than those in similar wind conditions in the relatively shallow and well-mixed North Sea. It is suggested that these differences may be ascribed to the presence of turbulence generated by the shear stress of the strong tidal currents on the seabed in the North Sea, a factor absent in Loch Ness. Models are devised to simulate the dispersion of plumes of floating particles released from a fixed position in a field of Langmuir circulation advected by tidal currents, using the sonar data. The estimates of diffusivities show an increase with wind speed, but are sensitive to the choice of some underdetermined parameters. The resulting estimates of lateral dispersion of floating particles overlap the range of those of Faller and Auer.