Finite-scale dispersion in the southwestern Atlantic Ocean: analysis of Lagrangian drifters data

In the framework of Monitoring Ocean by Drifters (MONDO) Project, a set of Lagrangian drifters were released in proximity of the Brazil Current, the western branch of the Subtropical Gyre in the South Atlantic Ocean. The experimental strategy of deploying part of the buoys in clusters offers the opportunity to examine relative dispersion on a wide range of scales. Adopting a dynamical systems approach, we focus our attention on scale-dependent indicators, like the finite-scale Lyapunov exponent (FSLE) and the finite-scale (mean square) relative velocity (FSRV) between two drifters as function of their separation, and compare them with classic time-dependent statistical quantities. The relevance of mechanisms like two-dimensional turbulence for the dispersion process is addressed. The results of our data analysis are found to be overall compatible with a Quasi-Geostrophic turbulence scenario. Nevertheless, some discrepancies from the expected behavior of 2D turbulent trajectories are found, which can likely be ascribed to the non stationary and non homogeneous characteristics of the flow, as well as to possible ageostrophic effects. Submesoscale features are considered to play a role, to some extent, in determining the properties of relative dispersion as well as the shape of the energy spectrum. We present, also, numerical simulations of an OGCM of the South Atlantic, and discuss the comparison between experimental and model data about mesoscale dispersion.