Langmuir Turbulence In Coastal Zones: Structure and Length Scales

Langmuir turbulence is a boundary layer oceanographic phenomenon of the upper layer that is relevant to mixing and vertical transport capacity. It is a manifestation of imposed aerodynamic stresses and the aggregate horizontal velocity profile due to orbital wave motion (the so-called Stokes profile), resulting in streamwise-elongated, counterrotating cells. The majority of previous research on Langmuir turbulence has focused on the open ocean. Here, we investigate the characteristics of coastal Langmuir turbulence by solving the grid-filtered Craik-Leibovich equations where the distinction between open and coastal conditions is a product of additional bottom boundary layer shear. Studies are elucidated by visualizing Langmuir cell vortices using isosurfaces of Q. We show that different environmental forcing conditions control the length scales of coastal Langmuir cells. We have identified regimes where increasing the Stokes drift velocity and decreasing surface wind stress both act to change the horizontal size of coastal Langmuir cells. Furthermore, wavenumber is also responsible in setting the horizontal extent Ls of Langmuir cells. Along with that, wavenumber that is linked to the Stokes depth δ_s controls the vertical extent L_h ^{SSV} of small-scale vortices embedded within the upwelling limb, while the downwelling limb occupies the depth of the water column H for any coastal surface wave forcing (i.e., L{_h ^d} =H and L{_h^{ssv} ~δ_s). Additional simulations are included to demonstrate insensitivity to the grid resolution and aspect ratio.