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Abstract: 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 sub-mesoscale analysis, because of 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 instantaneous and climatological transport and mixing properties in the south-western Mediterranean. By comparing with sea-surface temperature patterns, we find that the two approaches provide similar results for slowly evolving eddies like the first Alboran gyre. However, the Lyapunov exponent is also able to predict the (sub-)mesoscale filamentary processes occurring along the Algerian current and above the Balearic Abyssal Plain. Such filaments are also observed, with some mismatch, in sea-surface temperature patterns. 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 by reprocessing available altimetric data with Lagrangian tools, giving insight into (sub-)mesoscale stirring processes relevant to tracer observations and complementing traditional Eulerian diagnostics. [Copyright &y& Elsevier]

Abstract: Thermal NOAA/AVHRR satellite images relative to the years 1997–2000 are analyzed in this study, in order to obtain a first systematic identification of the sites of highest frequency in cold filaments in the Mediterranean Sea, as well as their properties. These sites are characterised by upwelling and/or the funnelling of strong cold winds by a somewhat irregular coastal orography. Indeed, intense air–sea interaction in the coastal zone generates a particularly strong input of potential vorticity into the sea. This in turn gives origin to cold filaments and jets. In the Mediterranean Sea, the geographical zones with a higher frequency in these jets are the two lobes of the southern Sicilian coast, the sea off eastern Sardinia, that south of the island of Crete, where a particularly intense mesoscale field is evident, and the Balkan coast of the Adriatic Sea. A chosen subset of the examined images is here used to obtain an estimate of a bulk coefficient M for mass exchange between filaments and ambient water, in order to explain the along-flow warming observed in most of the satellite images, at each filament site. [Copyright &y& Elsevier]