1Institute for Chemistry and Biology of the Marine Environment, Theoretical Physics/Complex Systems, Carl von Ossietzky University Oldenburg, Oldenburg, Germany
2Leibniz Institute for Baltic Sea Research, Rostock-Warnemünde, Germany
3Institute of Meteorology and Climatology, Leibniz Universität Hannover, Hanover, Germany
Received: 29 Jan 2016 – Discussion started: 10 Feb 2016
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.
Revised: 19 May 2016 – Accepted: 02 Jun 2016 – Published: 05 Jul 2016
Vortmeyer-Kley, R., Gräwe, U., and Feudel, U.: Detecting and tracking eddies in oceanic flow fields: a Lagrangian descriptor based on the modulus of vorticity, Nonlin. Processes Geophys., 23, 159-173, doi:10.5194/npg-23-159-2016, 2016.