Mixing structures in the Mediterranean Sea from Finite-Size Lyapunov Exponents

We characterize horizontal mixing and transport structures in the surface
circulation of the Mediterranean Sea, as obtained from a primitive equation
circulation model. We calculate the Finite Size Lyapunov Exponents (FSLEs) of the
velocity data set, which gives a direct measure of the local stirring. By proper
election of the FSLE parameters, we focus on the mesoscale structures, locating a
number of vortices embedded in an intricate network of high-stretching lines.
These lines control transport in the system. At the edge of the vortices, a dense
tangle of line intersections appears, identifying strong mixing. The spatial
distribution of FSLEs, averaged over one year, allows to classify areas in the
Mediterranean basin according by their mixing activity. The space average of
FSLEs on selected geographical regions gives a measure for quantifying and
comparing the mixing seasonal variability.