Abstract
A study of the mesoscale eddy field was conducted in oceanic regions dominated by highly energetic flows and strong coherent structures, with a focus on the properties of the eddy transport of passive tracers. Parameterization models able to reproduce the observed eddy features and particle dispersion characteristics were also explored.The research was carried out by using both observed and numerical Lagrangian data at 700 m depth in the North-West Atlantic. The trajectories were analyzed in a number of quasi-homogeneous geographical subregions of the Gulf Stream extension and recirculation area. In each region, the eddy field was computed and characterized mainly in terms of velocity autocovariance and crosscovariance functions, eddy diffusivity and single-particle dispersion. These eddy statistics were used to test possible Lagrangian stochastic (LS) models most suitable to describe the eddy properties.The main result is that, in regions dominated by the presence of coherent vortices, the mesoscale eddy field can be considered as a superposition of two different dynamical regimes associated with looping and non-looping trajectories. The loopers regime produces subdiffusive eddy characteristics due to the trapping effects of the coherent vortices. The non-loopers regime is associated to the background eddy field and produces nearly diffusive eddy statistics. Both regimes can be parameterized using a first-order LS model with a spin parameter that represents the mean rotation of the Lagrangian eddy velocity vector. The spin exhibits an approximately bi-modal probability distribution, reflecting the distribution of loopers (finite spin) and non-loopers (zero spin). It also has an Eulerian physical meaning for the looping trajectories trapped inside coherent vortices, representing a very good proxy for the vortex relative vorticity field.The LS model reproduces accurately the overall eddy statistics and dispersion properties because it takes correctly into account both the effects of the coherent vortices (enhanced spreading at short times due to the vortex high energy, followed by suppressed dispersion at longer times due to the vortex trapping mechanism) and the simply diffusive effects of the background eddy field. Appropriate values of asymptotic eddy diffusivity are also estimated. A general methodology for extending the results on a broader scale is provided.