Line Vortices and the Vacillation of Langmuir Circulation

Abstract

AbstractThree types of breakdown of Langmuir circulation (Lc) are observed, two of which are represented in large-eddy simulation (LES) models, but the third, vacillation, is not. The stability of Lc can be examined by representing the downwind-aligned vortices by line vortices that are subjected to perturbations. Earlier conclusions relating to stability in homogeneous water of infinite depth are found to be in error because no stationary unperturbed state exists. The motion of vortices is examined and shown to be consistent with an explanation of Lc devised by Csanady. Motion of line vortices in water of limited depth or bounded below by a thermocline is examined. The motion replicates some of the features of vacillation observed by Smith in deep water bounded by a thermocline, including its periodicity and fluctuations in the formation of bubble bands. Vortices describe closed orbits within the Langmuir cells. Particle motions in the vacillating Lc pattern exhibit trapping close to the line vortices or near the cell boundaries. Vacillation appears not to have been observed in water of limited depth. Here, the vacillation period is predicted to be longer than the deep-water equivalent and may be too long for vacillations to be detected.