Solutions are analysed from large-eddy simulations of the phase-averaged
equations for oceanic currents in the surface planetary boundary layer
(PBL),
where the averaging is over high-frequency surface gravity waves. These
equations
have additional
terms proportional to the Lagrangian Stokes drift of the waves, including
vortex
and Coriolis forces and tracer advection. For the wind-driven PBL, the
turbulent
Langmuir number, Latur =
(U∗/Us)1/2,
measures
the relative influences of directly
wind-driven shear (with friction velocity U∗) and the Stokes
drift
Us. We focus on
equilibrium solutions with steady, aligned wind and waves and a realistic
Latur = 0.3.
The mean current has an Eulerian volume transport to the right of the wind
and
against the Stokes drift. The turbulent vertical fluxes of momentum and
tracers are
enhanced by the presence of the Stokes drift, as are the turbulent kinetic
energy and its dissipation and the skewness of vertical velocity. The dominant
coherent structure in the turbulence is a Langmuir cell, which has its
strongest vorticity aligned longitudinally (with the wind and waves) and
intensified near the surface on the scale of the
Stokes drift profile. Associated with this are down-wind surface convergence
zones
connected to interior circulations whose horizontal divergence axis is
rotated about
45° to the right of the wind. The horizontal scale of the Langmuir
cells
expands with depth, and there are also intense motions on a scale finer
than
the dominant cells very near the surface. In a turbulent PBL, Langmuir
cells
have irregular patterns with finite correlation scales in space and time,
and
they undergo occasional mergers in the vicinity of Y-junctions between
convergence zones.
Vertical fluxes conditioned on vorticity and strain reveal submesoscale ventilation
