Lagrangian Dispersion Characteristics in The Western Mediterranean

Abstract

Dispersion characteristics in the Western Mediterranean are analyzed using data from Coastal Ocean Dynamics Experiment (CODE) and Surface Velocity Program (SVP) surface drifters deployed in the period 1986–2017. Results are presented in terms of absolute dispersion A2 (mean-squared displacement of drifter individuals) and of relative dispersion (D2; mean square separation distance of drifter pairs). Moreover, the dispersion characteristics are estimated for different initial separation distances (D0) between particles: smaller, larger, or comparable with the internal Rossby radius of deformation. Results show the presence of a quasiballistic regime for absolute dispersion at small time scales and the nonlocal relative dispersion regime related to the submesoscale activities for scales smaller than the internal Rossby radius. At intermediate times, two anomalous absolute dispersion regimes (elliptic and hyperbolic regimes) related with the flow topology are observed, although the relative dispersion involves the Richardson and shear/ballistic regimes only for D0 smaller than the Rossby radius. During the subsequent 20–30 days, absolute dispersion shows quasirandom walk regime and relative dispersion follows the diffusive regime for scales larger than 100 km for which pair velocities are uncorrelated.

Spreading of Lagrangian Particles in the Black Sea: A Comparison between Drifters and a High-Resolution Ocean Model

“…There are also regions where the strain and the vorticity are balanced (s 2 ∼ w 2 , then Q * ∼ 0) [47][48][49][50]. The absolute dispersion regimes are summarized in Figure 2b, as suggested in [23,24,47,50].…”

“…The initial drifter pair tracks were considered every 5 days along the trajectories. This time interval is larger than the integral time scale estimated in the BS (T L~3 days) (see [2,23] for details).…”

“…Filtered positions were subsampled at 6-h intervals [2]. In this study, the drifters were handled in pairs (original and chance pairs) for the combination of the three drifter types (see for details, [23,24]). The pairs were selected for the initial separation distance D 0~3 .5-5 km, which is much smaller than R d (internal Rossby radius) ranging from 10 to 30 km for the BS [2,28,29].…”

“…The classical absolute dispersion approach answers the basic question of how the passive tracers spread in a turbulent field. It has been used in previous studies to distinguish the difference between the elliptic (strong rotation) and hyperbolic (dominant stretching) regions [23,24,47].…”

“…Previous studies have been focused on the dispersion analysis in the world ocean in order to demonstrate the dispersion properties and to help to describe/predict the spreading of pollutants. Examples of this focused on the Gulf of Mexico [19], the Gulf of Stream [20] and the Mediterranean Sea [21][22][23][24].…”