We analyze with the tools of lobe dynamics the velocity field from a
numerical simulation of the surface circulation in the Northwestern
Mediterranean Sea. We identify relevant hyperbolic trajectories and their
manifolds, and show that the transport mechanism known as the `turnstile',
previously identified in abstract dynamical systems and simplified model flows,
is also at work in this complex and rather realistic ocean flow. In addition
nonlinear dynamics techniques are shown to be powerful enough to identify the
key geometric structures in this part of the Mediterranean. In particular the
North Balearic Front, the westernmost part of the transition zone between
saltier and fresher waters in the Western Mediterranean is interpreted in terms
of the presence of a semipermanent ``Lagrangian barrier'' across which little
transport occurs. Our construction also reveals the routes along which this
transport happens. Topological changes in that picture, associated with the
crossing by eddies and that may be interpreted as the breakdown of the front,
are also observed during the simulation.Comment: 34 pages, 6 (multiple) figures. Version with higher quality figures
available from
http://www.imedea.uib.es/physdept/publications/showpaper_en.php?indice=1764 .
Problems with paper size fixe