  A new mechanism has been identified that explains the generation of Langmuir
circulations. A wind-driven current in the presence of surface waves gives rise
to an instability where the emerging circulations redistribute the turbulence
in the cross-wind direction. The non-uniform eddy-viscosity locally changes the
rate of momentum transfer from the wind to the shear current, producing a
non-uniform velocity field. The interaction of this non-uniform velocity field
with the surface waves, due to the Craik-Leibovich vortex force, amplifies the
circulations and creates a feedback mechanism. The currently accepted CL2 model
of instability assumes a constant eddy-viscosity. This paper presents a model
which explains the generation of Langmuir circulations and its predictions of
both spatial and time scales are in good agreement with experimental results.
The modeling approach combines a perturbation method with a RANS turbulence
model. Through parametric variation of the perturbation, the growth rate and
spatial scales of the circulations are extracted from the simulations.
