Extraction of coherent vortex features from Lagrangian data

A novel method is presented for the treatment of coherent eddies in Lagrangian data, and used to examine the detailed structure of some Mediterranean outflow eddies ("meddies") as well as the properties of vortex-mediated particle transport away from an unstable jet in an idealized numerical model. The method, which is based on time-frequency theory, permits the direct and unambiguous extraction of orbital motion around a possibly elliptical vortex, treating random variability in the measurements as a source of uncertainty. Applied to an eddy-rich dataset from the Mediterranean outflow region, enhanced energy at subinertial frequencies in the clockwise rotary spectra as well as the long tails of velocity probability density functions are seen to be precisely explained by the extracted signals. It is shown that much of scatter in radius / velocity plots is actually a result of the apparent eccentricity of the particle motion. Regimes of interior versus exterior trapping may be distinguished from one another by examining the velocity slope. A second application to a barotropic westward unstable jet shows that eddy properties are correctly diagnosed and also separated from lower-frequency, more linearly polarized motions associated with jet meanders. This permits an explicit assessment of the enhanced long-distance particle propagation due to beta-drift of coherent eddies away from the jet axis. These initial applications illustrate the wealth of information which can be extracted from oceanic and modeled Lagrangian data using the proposed method.