Langmuir Circulation

Langmuir circulation is a set of wind-aligned counter-rotating helical roll cells in the ocean surface mixed layer, marked at the surface by parallel windrows of foam and debris over the convergence zones. It arises from the Craik-Leibovich vortex force, the interaction of the wave Stokes drift with the vertical shear of the wind-driven current, which tilts and stretches vorticity into streamwise rolls (the CL2 instability). The Langmuir number La_t = (u* / U_s)^(1/2) measures wave forcing against wind stress; smaller values give stronger cells. Langmuir turbulence deepens and homogenizes the mixed layer beyond what shear and convection alone produce.