The use of the ratio between the acceleration and velocity time scales y = Ta/Tv to separate Lagrangian trajectories in homogeneous classes is proposed. In fact, when analyzing subsurface floats data in the Atlantic Ocean and surface drifters data in the world's ocean basins, it is observed that trajectories having different values of y are characterized by different shapes, correlation, and dispersal properties. In particular, trajectories having similar values of the acceleration and velocity time scales clearly show the influence of eddies and are characterized by an oscillating velocity correlation function. It is shown here that this trajectory screening is a useful procedure to rationalize the analysis of real Lagrangian trajectories and to avoid a mixture of different regimes, when averaging quantities. The mean statistical quantities computed averaging on quasi-homogeneous datasets put in evidence the role of the coherent structures in the dispersion properties, both in time and in the main oceanic current systems. These results are discussed in the context of the parameterization of eddy diffusivity in general circulation models. © 2007 American Meteorological Society.
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