Chaotic Transport of Mass and Potential Vorticity for an Island Recirculation
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The method of lobe analysis is used and extended to analyze a time-dependent, boundary-trapped recirculation. The recirculation gyre occurs in a numerical model of wind-driven flow around an island, but the underlying geometry of the gyre is similar to persistent eddies such as the Alboran Gyre and the Great Whirl. Even in the steady (weak forcing) limit, the gyre leaks fluid due to the fact that the surface Ekman pumping above it is directed downward. The authors show that this leakage is rapidly superseded by chaotic transport into and out of the gyre when time dependence sets in. Variations of the traditional “turnstile” approach to transport are used to study the dynamics of the gyre. A Lagrangian recirculation boundary, consisting of pieces of stable and unstable manifolds joined by a gate, allows straightforward calculation and visualization of potential vorticity flux.