Abstract
Abstract. Since eddies play a major role in the dynamics of oceanic flows, it is of great interest to detect them and gain information about their tracks, their lifetimes and their shapes. We present a Lagrangian descriptor based on the modulus of vorticity to construct an eddy tracking tool. In our approach we denote an eddy as a rotating region in the flow possessing an eddy core corresponding to a local maximum of the Lagrangian descriptor and enclosed by pieces of manifolds of distinguished hyperbolic trajectories (eddy boundary). We test the performance of the eddy tracking tool based on this Lagrangian descriptor using an convection flow of four eddies, a synthetic vortex street and a velocity field of the western Baltic Sea. The results for eddy lifetime and eddy shape are compared to the results obtained with the Okubo–Weiss parameter, the modulus of vorticity and an eddy tracking tool used in oceanography. We show that the vorticity-based Lagrangian descriptor estimates lifetimes closer to the analytical results than any other method. Furthermore we demonstrate that eddy tracking based on this descriptor is robust with respect to certain types of noise, which makes it a suitable method for eddy detection in velocity fields obtained from observation.
Highlights
Transport of particles and chemical substances mediated by hydrodynamic flows are important components in the dynamics of ocean and atmosphere
We have shown that the Lagrangian descriptor MV based on the modulus of vorticity provides good insight into the structure of a hydrodynamic flow
Eddy cores can be found as local maxima of MV, while distinguished hyperbolic trajectories (DHTs) correspond to minima of MV
Summary
Transport of particles and chemical substances mediated by hydrodynamic flows are important components in the dynamics of ocean and atmosphere. One of the most popular methods with which to identify eddies is based on the Okubo–Weiss parameter (Okubo, 1970; Weiss, 1991) This method relies on the strain and vorticity of the velocity field and has been applied to both numerical ocean model output and satellite data (Isern-Fontanet et al, 2006; Chelton et al, 2011). We compare our method to four others, namely the original Lagrangian descriptor using the arc length (Madrid and Mancho, 2009; Mendoza et al, 2010), an oceanographic method based on geometric properties of the flow field (Nencioli et al, 2010) and detection tools which employ the Okubo–Weiss parameter (Okubo, 1970; Weiss, 1991) and the modulus of vorticity itself.
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