Dispersion of Tracers by Ocean Gyres

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Abstract
The dispersion of a tracer by a two-dimensional gyre circulation is studied using simple numerical models. Two approaches are taken: a random walk model formulated in a streamline coordinate system and the numerical solution of the advection-diffusion equation. A number of different gyres are considered. Attention is focused on the characteristics of the gyre that determine the spreading and mixing time of the tracer. The authors find that the dispersion by a given gyre can be characterized in terms of a bulk Péclet number and the three length scales: L the horizontal width of the gyre, l the width of the boundary current, and L the length of the boundary current. By taking into account the length of the boundary layer, gyre dispersion is found to conform moderately well with previous analytic models, in particular the partitioning between weak and strong diffusive regimes, even though the shear characteristics may be quite variable across the gyre. The analytic models become less valid as the length of the boundary layer increases. Simple expressions are given for the cross-streamline diffusion coefficient and mixing time in terms of the characteristics of the gyre. An important conclusion coming from the present study is the importance of the structure of the recirculation region in determining the shape of the tracer distribution. The results highlight the need for care in comparing model tracer fields with observed tracer distributions.

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