Recent advances on the characterization of mesoscale vortices from altimetric maps
Jordi Isern-Fontanet(1) , Emilio Garcia-Ladona(2) , Jordi Font(2) , and Antonio Turiel(2)
IFREMER, Centre de Brest,
(2) CSIC, Passeig Maritim de la Barceloneta 37-49, 08003, Spain
Satellite altimetry has proved to be a very valuable tool for the study of the ocean mesocale. At these scales the ocean is characterized by the presence of coherent vortices, which make it resemble two-dimensional turbulence. Such similarity has been exploited to analyze altimetric data using ideas and techniques developed in the domain of turbulence with very successful results (e.g. Stammer 1997, Gille and Llewellyn Smith 2000). One of these ideas is the view of coherent vortices as the basic building blocks of the flow. Indeed, ocean vortices are known to play a key role in the ocean dynamics due to their effectiveness in moving energy and matter through the ocean and their impact on mixing.
In order to be successful in this approach to the dynamics of the ocean a robust vortex detection algorithm is necessary. A possible choice of such a criterion is one based on the sign of the Okubo-Weiss parameter (Okubo 1970, Weiss 1991), which measures the relative contribution of deformation and vorticity. Then, a coherent vortex can be defined as the simply connected region with negative values of the Okubo-Weiss parameter. Besides this definition only captures vortex cores, it has been shown that this method is adequate to detect marine eddies in altimetric maps and when the geometry of streamline contours in Sea Level Anomalies (SLA) maps is unclear it appears to work more consistently (Isern-Fontanet et al. 2002, Isern-Fontanet et al. 2004).
When a vortex, or its core, has been identified it is possible to estimate the individual properties of the vortex such as size, mean kinetic energy and amplitude and then analyze its statistical distribution. Initially, this criterion has been applied to SLA maps that combine TOPEX and ERS data (Le Traon et al. 1998, Larnicol et al. 1995) of the Mediterranean sea produced by CLS (France) for the period October-1992 to October-1999 produced by CLS (France). The distribution of such properties for the vortices in the Mediterranean, suggests an heuristic criterion to extract and select very coherent and long lived vortices from the whole set of structures identified in altimetric maps.
An algorithm to systematically locate and track such vortices provides for the first time a general picture of their preferential paths in the Mediterranean basin
(Isern-Fontanet et al. 2005a) and at global scale. Furthermore, it has been shown that these very coherent vortices are the principal responsible for the non-Gaussian behavior of the Velocity Probability Density Functions having a great impact on the dispersion and mixing processes (Isern-Fontanet et al. 2005b). Indeed, a wavelet decomposition of the vorticity field from SLA maps shows that an important fraction of the global properties of the flow dynamics as total energy and enstrophy is
retained by the contribution of a small fraction of such coherent structures (Turiel et al. 2005).