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Langmuir Turbulence and deeply penetrating jets in an unstratified mixed layer

Polton, Jeff A.; Belcher, Stephen E.. 2007 Langmuir Turbulence and deeply penetrating jets in an unstratified mixed layer. Journal of Geophysical Research, 112 (C09020). 10.1029/2007JC004205

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Abstract/Summary

The influence of surface waves and an applied wind stress are studied in an ensemble of large eddy simulations to investigate the nature of deeply penetrating jets into an unstratified mixed layer. The influence of a steady monochromatic surface wave propagating parallel to the wind direction is parameterized using the wave filtered Craik-Leibovich equations. Tracer trajectories and instantaneous downwelling velocities reveal classic counter-rotating Langmuir rolls. The associated downwelling jets penetrate to depths in excess of the wave's Stokes depth scale, $\delta_s$. Qualitative evidence suggests the depth of the jets is controlled by the Ekman depth scale. Analysis of turbulent kinetic energy (tke) budgets reveals a dynamical distinction between Langmuir turbulence and shear-driven turbulence. In the former tke production is dominated by Stokes shear and a vertical flux term transports tke to a depth where it is dissipated. In the latter tke production is from the mean shear and is locally balanced by dissipation. We define the turbulent Langmuir number $La_t=(v_*/U_s)^{0.5}$ ($v_*$ is the ocean's friction velocity and $U_s$ is the surface Stokes drift velocity) and a turbulent anisotropy coefficient $R_t=\overline{w'^2} / ( \overline{u'^2} + \overline{v'^2} )$. The transition between shear-driven and Langmuir turbulence is investigated varying external wave parameters $\delta_s$ and $La_t$ and by diagnosing $R_t$ and the Eulerian mean and Stokes shears. When either $La_t$ or $\delta_s$ are sufficiently small the Stokes shear dominates the mean shear and the flow is preconditioned to Langmuir turbulence and the associated deeply penetrating jets.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1029/2007JC004205
Programmes: Other/Admin
ISSN: 0148-0227
Additional Keywords: MIXING; LANGMUIR
NORA Subject Terms: Marine Sciences
Date made live: 12 Dec 2011 16:10
URI: http://nora.nerc.ac.uk/id/eprint/16036

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