DISPERSION OF TRACERS BY OCEAN GYRES

The dispersion of a tracer by a two-dimensional gyre circulation is st udied using simple numerical models. Two approaches are taken: a rando m walk model formulated in a streamline coordinate system and the nume rical solution of the advection-diffusion equation. A number of differ ent 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 characte rized in terms of a bulk Peclet number and the three length scales: L the horizontal width of the gyre, I 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 mode rately well with previous analytic models, in particular the partition ing between weak and strong diffusive regimes, even though the shear c haracteristics may be quite variable across the gyre. The analytic mod els become less valid as the length of the boundary layer increases. S imple expressions are given for the cross-streamline diffusion coeffic ient and mixing time in terms of the characteristics of the gyre. An i mportant 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 c omparing model tracer fields with observed tracer distributions.