Lagrangian Drifter Dispersion in the Southwestern Atlantic Ocean

Abstract

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, the authors focus their 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 a 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. The authors find that, dependently on the given observable, the quasigeostrophic turbulence scenario is overall compatible with their data analysis, with discrepancies from the expected behavior of 2D turbulent trajectories likely to be ascribed to the nonstationary and nonhomogeneous characteristics of the flow, as well as to possible ageostrophic effects. Submesoscale features of similar to 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. The authors also present numerical simulations of an ocean general circulation model (OGCM) of the South Atlantic and discuss the comparison between experimental and model data about mesoscale dispersion

Effects of vertical shear in modelling horizontal oceanic dispersion

“…Furthermore, the finite-time Lyapunov exponent (FTLE) is also used to detect Lagrangian coherent structures in ocean dynamics applications (Haller, 2000;Sulman et al, 2013). A discussion of the use of scale-dependent indicators in Lagrangian dispersion problems can be found in Berti et al (2011), while a direct comparison of FSLE and FTLE for the identification of transport barriers can be found in Boffetta et al (2001).…”

“…LaCasce, 2010;Okubo, 1971;Morel and Larchevêque, 1974;Er-el and Peskin, 1981;Berti et al, 2011). This is only partly due to the inherent difficulties of performing float or dye concentration experiments in the ocean.…”

Clustering of Floating Tracer Due to Mesoscale Vortex and Submesoscale Fields

“…Ocean circulation at the scales smaller than the mesoscale is dominated by the broad range of submesoscale processes, which have been intensively studied (Berti et al, 2011(Berti et al, , 2016Haza et al, 2016;Huntley et al, 2015;Jacobs et al, 2016;McWilliams, 2016;Ohlmann et al, 2019;Schroeder et al, 2012;Zhong & Bracco, 2013). Interactions between submesoscale and mesoscale motions are essential in the formation and breakdown of coherent mesoscale vortices, but the theoretical understanding is hindered by overwhelming computational costs due to the spatial resolution requirements (Dauhajre et al, 2019).…”

Eddy characteristics in the South Indian Ocean as inferred from surface drifters

“…Due to the long distance between satellite tracks, altimetry can hardly detect eddies with radius r < 40 km (e.g., Chelton et al, 2011). Whereas surface drifters can detect submesoscale eddies with radius r < 10 km (Berti et al, 2011;Li et al, 2011;Schroeder et al, 2012). Surface drifters have a 6 h temporal resolution, much more than altimetry data which, so far, usually have 7-or 1-day intervals available to the public.…”