Vortex pair model of Langmuir circulation
Abstract:Accumulating evidence shows spacing and length of surface convergences associated with Langmuir circulation to be random variables, their strength also variable, their lifetime limited. Analysis of several sets of quantitative observations reveals the probability distribution of windrow spacing to be lognormal. This suggests generation of surface convergences at random times and locations on the sea surface.
Moving shear stress anomalies, under wind gusts or breaking long waves, are capable of generating surface convergence in their wakes, and are the type of random event likely responsible for the stochastic properties of windrows. The generation mechanism envisaged is a “forced” version of the Craik-Leibovich “CL2” theory, Stokes drift tilting the vertical vortex lines at the edges of stress anomalies. This contrasts with the feedback mechanism of the CL2 theory operating on infinitesimal spanwise disturbances. Realistic shear stress anomalies produce vortex pairs strong enough to account for Langmuir circulation without feedback amplification.
A vortex pair just under the sea surface induces motion bringing the vortices together at first, and then causing them to dive deep into the mixed layer. This inviscid kinematic effect limits the surface presence of convergences, and accounts for the finite lifetime of windrows without even taking into account viscous decay.
Converging motion at the surface, and low eddy viscosity, combine to channel the downward momentum transfer from the air flow into descending branches of the Langmuir circulation. One result is the increase of windward surface velocity in the convergences.
Document Type: Research Article
Publication date: July 1, 1994
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