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Open Access item Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin

Authors:d'Ovidio, Francesco
Isern-Fontanet, Jordi
López, Cristóbal
Hernández-García, Emilio
García-Ladona, Emilio
Keywords:Sub-mesoscale, Filaments, Altimetry, Mediterranean circulation, Lagrangian dynamics
Issue Date:Jan-2009
Publisher:Elsevier
Citation:Deep Sea Research I: Oceanographic Research Papers 56(1): 15-31 (2009)
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.
Description:17 pages, 11 figures.-- Available online Aug 14, 2008.-- ArXiv pre-print available at: http://arxiv.org/abs/0807.3848
Publisher version (URL):http://dx.doi.org/10.1016/j.dsr.2008.07.014
URI:http://hdl.handle.net/10261/14594
ISSN:0967-0637
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