Surface gravity wave effects in the oceanic boundary layer: large-eddy simulation with vortex force and stochastic breakers

The wind-driven stably stratified mid-latitude oceanic surface turbulent boundary layer is computationally simulated in the presence of a specified surface gravity-wave field. The gravity waves have broad wavenumber and frequency spectra typical of measured conditions in near-equilibrium with the me...

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Bibliographic Details
Published in:Journal of fluid mechanics 2007-12, Vol.593, p.405-452
Main Authors: SULLIVAN, PETER P., McWILLIAMS, JAMES C., MELVILLE, W. KENDALL
Format: Article
Language:English
Subjects:
Boundary layers
Earth, ocean, space
Exact sciences and technology
External geophysics
Fluid dynamics
Gravity
Gravity waves
Oceanography
Physics of the oceans
Simulation
Surface waves, tides and sea level. Seiches
Turbulence
Wind speed
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Summary:The wind-driven stably stratified mid-latitude oceanic surface turbulent boundary layer is computationally simulated in the presence of a specified surface gravity-wave field. The gravity waves have broad wavenumber and frequency spectra typical of measured conditions in near-equilibrium with the mean wind speed. The simulation model is based on (i) an asymptotic theory for the conservative dynamical effects of waves on the wave-averaged boundary-layer currents and (ii) a boundary-layer forcing by a stochastic representation of the impulses and energy fluxes in a field of breaking waves. The wave influences are shown to be profound on both the mean current profile and turbulent statistics compared to a simulation without these wave influences and forced by an equivalent mean surface stress. As expected from previous studies with partial combinations of these wave influences, Langmuir circulations due to the wave-averaged vortex force make vertical eddy fluxes of momentum and material concentration much more efficient and non-local (i.e. with negative eddy viscosity near the surface), and they combine with the breakers to increase the turbulent energy and dissipation rate. They also combine in an unexpected positive feedback in which breaker-generated vorticity seeds the creation of a new Langmuir circulation and instigates a deep strong intermittent downwelling jet that penetrates through the boundary layer and increases the material entrainment rate at the base of the layer. These wave effects on the boundary layer are greater for smaller wave ages and higher mean wind speeds.
ISSN:0022-1120
1469-7645
DOI:10.1017/S002211200700897X