The roles of Langmuir circulations in the dispersion of surface tracers.

Abstract

A hiearchy of theoretical and numerical models for the dispersion of discrete floating tracers on lakes and oceans is presented. Central to these models is the role of Langmuir circulations, which concentrate tracers into narrow windows thus inhibiting tracer dispersion. But time dependent Langmuir circulations cause the rows of tracers to wander and to split, by local time dependence and by downwind advection, thus promoting dispersion. Accordingly, the Langmuir circulations generally render the smaller scale background turbulence irrelevant for direct estimates of surface dispersion. Analytical models include: 1)a theory of tracers in a linear mean flow convergence plus homogeneous turbulence, this theory being applicable to the width of windows; and 2)a model with a spatially periodic mean flow and a periodic small scale eddy diffusion coefficient that allows an estimate of the Langmuir scale dispersivity for steady parallel cells. Random flight calculations for a model of complex time dependent and downwind dependent Langmuir circulations have led to the explicit prediction.