Some Dynamic Features of Langmuir Circulation

Abstract

Two sets of experiments on Langmuir Circulation (LC) were carried out about 180 nm south of Nantucket Island on July 22 and 23, 1982, on board the USNS Hayes. During the experiments, dye plumes, smoke bombs and surface drifters (computer cards) were released and photographed using two airplanes equipped with special camera packages together with meteorological, wave and hydrographic measurements. The card rows had predominent distances and angles from 2 to 23 m and between 0 and 12 degrees to the right of wind, respectively. Edge of dye plumes perpendicular to the wind showed a wavy form. The spectra of these waves had peaks at wave lengths between 4 to 60 m. The wind speed was almost constant at 11 ms⁻¹ and the current shear between the surface and 4 m was 0.1 s⁻¹. Horizontal eddy diffusivity parallel and normal to the wind was determined from card row stretching and from change of row distances, respectively, with order of 10²–10³ cm²s⁻¹. From the steady state non-linear, vertically integrated momentum equation scaled with wind stress and LC width along the wind direction is derived an equation for the along-wind component U of the surface current with a sinusoidal across-wind component. U has a peak along the convergence line with about 1 to 60 cm s⁻¹ corresponding to wind stress of 1 to 10 cm²s⁻² for LC width of 10 m. The sharpness of the peak increases with increasing inertia effect and decreasing horizontal eddy viscosity or cell bottom stress, which becomes effective along the LC divergence lines.