  In the framework of Monitoring by Ocean 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 like the mean square relative displacement between two drifters and
the effective diffusivity as functions of the time lag from the release. We
find that, dependently on the given observable, the quasigeostrophic turbulence
scenario is overall compatible with our data analysis, with discrepancies from
the expected behavior of 2D turbulent trajectories likely to be ascribed to the
non stationary and non homogeneous characteristics of the flow, as well as to
possible ageostrophic effects. Submesoscale features of O(1) km 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.
