Surface Waves and Spatially Coherent Structures in the Near-surface Layer of the Ocean

Spatially coherent organized structures have been recognized as an important part of turbulent boundary layer processes. Langmuir cells and ramp-like structures are believed to transfer an appreciable portion of the momentum, heat and other substances in the upper layer of the ocean. Models of Langmuir circulation are now very sophisticated but still pose some questions (Soloviev and Lukas, 2014). In particular, the traditional models of Langmuir circulation do not account for ramp-like structures, which are widespread features in the upper ocean turbulent boundary layer, with axes perpendicular to the axes of Langmuir circulation. From the basic principles of nonlinear dissipative systems, the process of self-organization reduces chaos and dissipation in the system and increases the effectiveness of property transport. However, existing LES models of Langmuir circulation demonstrate significant increase of turbulent dissipation. We have developed a new concept, which links the Langmuir circulation and ramp-like structures to wave stirring and Tollmien-Schlichting instability. Using computational fluid dynamics tools, we have been able to reproduce both Langmuir cells and ramp-like structures coexisting in space though intermittent in time (see Figure). This mechanism resembles convective initiation of longitudinal rolls in the atmospheric boundary layer (Brown, 1991). However, in the upper ocean turbulent boundary layer, the vorticity transport in the vertical direction is by breaking waves stirring the near-surface layer of the ocean; while, in the atmospheric boundary layer, by penetrative convection. The proposed mechanism for generation of Langmuir circulation may compete with that of Craik and Leibovich (1976) within a certain range of wind/wave conditions (in particular, under high wind speed conditions when the vortex force term including Stokes drift can be suppressed due to strong near-surface turbulence). In conclusion, we discuss observational data that can help with verification of models of spatially coherent structures in the upper ocean turbulent boundary layer. References: Brown, R.A. , 1991: Fluid Mechanics of the Atmosphere. International Geophysics Series 47, Academic Press, San Diego. Craik, A.D.D., and S. Leibovich, 1976: A rational model for Langmuir circulations. J. Fluid Mech. 73, 401–426. Soloviev, A.V. and R. Lukas, 2014: The Near-Surface Layer of the Ocean: Structure, Dynamics, and Applications (Second edition), Springer, NY, 552 pp.