The generation of Langmuir circulations by an instability mechanism

Equa,tions governing the current system in the upper layers of oceans and lakes were derived by Craik & Leibovich (1976). These incorporate the dominant effects of both wind a,nd waves. Solutions comprising the mean wind-driven current and a system of ' Langmuir ' cells aligned parallel to the wind were found for cases in which the wave field consisted of just a pair of plane waves. However, it was not clear that such cellular motions would persist, for the more realistic case of a continuous wave spectrum. The present paper shows that, in the latter case, infinitesimal spanwise periodic perturbat,ions will grow on account of an instability mechanism. Mathematically, the inst,ability is closely similar to the onset of thermal convection in horizontal fluid layers. Physically, the mechanism is governed by kinematical processes involving the mean (Eulerian) wind-driven current and the (Lagrangian) Stokes drift associated wit,h the waves. The relationship of this mechanism to instability models of Garrett and Gammelsrrad is clarified.