COSEM - COntaminant Spread in the Eastern Mediterranean

Lagrangian diagnostics of the surface velocity field

Analysis of transport by eastern Mediterranean surface currents is based on the derivation of Lagrangian diagnostics from the time-dependent velocity field (derived from the SELIPS model and from satellite altimetry data), and more precisely transport barriers and trajectories of synthetic passive tracers. As recently shown (e.g. Lehahn et al., 2007; d'Ovidio et al., 2010) these Lagrangian tools are very effective in reconstructing the specific effect of horizontal stirring on individual patterns. Lagrangian techniques are based, in general, on the identification of the velocity field characteristics along particle trajectories. They are very well suited for diagnosing properties of tracers like chlorophyll, since they allow to quantify the dynamical properties experienced by a parcel of water during its motion.

The Lagrangian diagnostics performed here is based on analyzing the spatial structure of unstable manifolds (also referred to as attracting Lagrangian coherent structures [see Haller and Yuan, 2000]) from calculation of finite size Lyapunov exponents (FSLE) [Boffetta et al., 2001; d’Ovidio et al., 2004; Lehahn et al. 2007] (Figure 1b). These manifolds induce in advected tracer fields filament patterns with typical length in the range of ∼10–∼100 km and lifetime in the range of days/ weeks (though it can be much longer if the patterns are associated to long‐lived and energetic mesoscale features with low temporal variability). Since unstable manifolds represent transport barriers and tracer boundaries , they separate between water bodies with different physical-biogeochemical properties.