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Comparison between Eulerian diagnostics and Finite-Size Lyapunov Exponents computed from Altimetry in the Algerian basin

d'Ovidio, F; Isern-Fontanet, J;López, C; Hernández-García, E; García-Ladona, E
Deep-Sea Research I 56, 15-31 (2009)

Transport and mixing properties of surface currents can be detected from altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have the advantage of exploiting both spatial and temporal variability of the velocity field and are in principle able to unveil subgrid filaments generated by chaotic stirring. However, one may wonder whether this theoretical advantage is of practical interest in real-data mesoscale and submesoscale analysis, due to the uncertainties and resolution of altimetric products, and the non-passive nature of biogeochemical tracers. Here we compare the ability of standard Eulerian diagnostics and the finite-size Lyapunov exponent in detecting instantaneaous and climatological transport and mixing properties. By comparing with Sea Surface Teperature patterns, we find that the two diagnostics provide similar results for slow evolving eddies like the first Alboran gyre. However, the Lyapunov exponent is also able to predict the (sub-)mesoscale filamentary process occuring along the Algerian current and above the balearic Abyssal plane. Such filaments are also observed, with some mismatch, in SST patterns. Contrary to what was found in a recent work for the Tasman sea (Waugh et al. 2006), in the Algerian basin climatologies of Lyapunov exponents do not show any compact relation with other Eulerian diagnostics, unveiling a different structure even at the basin scale. We conclude that filamentation dynamics can be detected from available altimetric data giving insight into stirring processes relevant to tracer observations and complementing traditional Eulerian diagnostics.