The anisotropy and structure of turbulence simulated by large-eddy simulations with and without Stokes-drift forcing are analyzed, with an emphasis on the linkage between the distinctive structure of Langmuir turbulence near the surface where cellular vortices aligned with the wind and wave propagation direction are apparent and the Langmuir-enhanced mixed layer entrainment at the base of the ocean surface boundary layer (OSBL) where turbulent structures differ. The tensor invariants of the Reynolds stresses, the variance of vertical velocity and buoyancy, and the velocity gradient statistics are used to categorize turbulence structures as a function of depth, including an extension of the barycentric map to show the direction as well as the magnitude of turbulence anisotropy and a vector-invariant extension of the Okubo-Weiss parameter. The extended anisotropic barycentric map and the velocity gradient statistics are demonstrated to be useful, providing compact information of the anisotropy, orientation, and structure of turbulent flows. It is found that the distinctive anisotropy and structures of Langmuir turbulence are quickly lost below regions where Stokes drift shear is significant and vortices are apparent, consistent with past observations and model results. As a result, the turbulent structures near the base of the OSBL are not significantly affected by the presence of Stokes drift above but are instead dominated by local Eulerian shear, except in one important manner. Langmuir turbulence does affect the mixed layer entrainment by providing extra available turbulent kinetic energy (TKE) via enhanced near-surface TKE production and higher vertical TKE transport energizing the turbulent structures near the base of the OSBL. The additional TKE is utilized by structures similar to those that exist without Stokes drift forcing in terms of anisotropy of their Reynolds stresses, but they are more energetic because of the Langmuir turbulence. Thus, parametrizing the major aspects of Langmuir turbulence on entrainment at the base of the OSBL can be incorporated through enhancing available energy without other modifications.

• Received 3 August 2019

DOI:https://doi.org/10.1103/PhysRevFluids.5.013803