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Communication Dans Un Congrès Année : 2015

Lyapunov exponents and oceanic fronts

Résumé

Lyapunov exponents and Lyapunov vectors are precious tools to study dynamical systems: they provide a mathematical framework characterizing sensitive dependence on initial conditions, as well as the stretching and the contraction occurring along a trajectory. Their extension to finite size and finite time calculation has been shown to lead to the location of Coherent Lagrangian Structures, which correspond in geophysical flows to frontal regions. In this case, the Lyapunov exponent and the Lyapunov vector provide respectively the cross front gradient amplification and the front orientation. Here we present global maps of Lyapunov exponents/vectors computed from satellite-derived surface currents of the oceans and we quantify their capability of predicting fronts by comparing with Sea Surface Temperature images. We find that in high energetic regions like boundary currents, large relative separations are achieved in short times (few days) and Lyapunov vector mostly align with the direction of jets; in contrast, in lower energetic regions (like the boundaries of subtropical gyres) the Lyapunov calculation allows to predict tracer lobes and filaments generated by the chaotic advection occurring here. These results may be useful for a global calibration and validation of the Lagrangian technique for multidisciplinary oceanographic applications like co-localization of marine animal behaviours to frontal systems and adaptive strategies for biogeochemical field studies. The ocean is a turbulent system where its physical and biogeochemical trac-ers (like heat, salinity, phytoplankton) present strong inhomogeneities that are structured over a large range of spatiotemporal scales by features like vortices (eddies) and fronts. Several methods have been proposed to analyze the surface currents and track the physical features that constrain tracer distributions through the horizontal transport. In particular, Lagrangian methods allow to mimic the transport dynamics by creating synthetic particle trajectories which are obtained by integrating the velocity field and then analyzed. One powerful diagnostic which has been used to identify frontal structures, i.e. lines where discontinuities or strong gradients are expected to occur in the ocean, is the calculation of the local Lyapunov exponent. In general the Lyapunov exponents are used in a dynamical system approach in order to detect chaotic behaviour for an invariant system by measuring the growth of the perturbations occurring along particle trajectories. For geophysical systems, the calculation of the Lyapunov exponent is usually performed at finite time and finite space.
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Dates et versions

hal-01291093 , version 1 (20-03-2016)

Identifiants

  • HAL Id : hal-01291093 , version 1

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Francesco Toselli, Francesco d'Ovidio, Marina Lévy, Francesco Nencioli, Olivier Titaud. Lyapunov exponents and oceanic fronts. CS-DC’15 World e-conference, Sep 2015, Tempe, United States. ⟨hal-01291093⟩
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