Relative Dispersion in the Antarctic Circumpolar Current

Abstract

Stirring in the subsurface Southern Ocean is examined using RAFOS float trajectories, collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), along with particle trajectories from a regional eddy permitting model. A central question is the extent to which the stirring is local, by eddies comparable in size to the pair separation, or nonlocal, by eddies at larger scales. To test this, we examine metrics based on averaging in time and in space. The model particles exhibit nonlocal dispersion, as expected for a limited resolution numerical model that does not resolve flows at scales smaller than ~10 days or ~20–30 km. The different metrics are less consistent for the RAFOS floats; relative dispersion, kurtosis, and relative diffusivity suggest nonlocal dispersion as they are consistent with the model within error, while finite-size Lyapunov exponents (FSLE) suggests local dispersion. This occurs for two reasons: (i) limited sampling of the inertial length scales and a relatively small number of pairs hinder statistical robustness in time-based metrics, and (ii) some space-based metrics (FSLE, second-order structure functions), which do not average over wave motions and are reflective of the kinetic energy distribution, are probably unsuitable to infer dispersion characteristics if the flow field includes energetic wave motions that do not disperse particles. The relative diffusivity, which is also a space-based metric, allows averaging over waves to infer the dispersion characteristics. Hence, given the error characteristics of the metrics and data used here, the stirring in the DIMES region is likely to be nonlocal at scales of 5–100 km.

Observational Evidence of Ventilation Hotspots in the Southern Ocean

“…surface chlorophyll, occurs over horizontal scales of O(10-100km). These eddies typically have a vertical extent of O(2000 m) and stir tracers below the mixed layer in the interior (Balwada et al, 2016(Balwada et al, , 2020. Near the surface, mesoscale eddies usually have an inlay of strong submesoscale O(1-10 km, 1 hour-10 days) currents, which are often visi-mixed layer baroclinic instabilities (Boccaletti et al, 2007) or through wind-front interactions (Thomas et al, 2008).…”

“…Stirring by mesoscale eddies is known to produce filaments and anomalies of spice, AOU, and biogeochemical tracers at smaller submesoscales, particularly in regions where water masses with differing formation processes and community compositions are adjacent (Balwada et al, 2018(Balwada et al, , 2020Smith & Ferrari, 2009). In the Southern Ocean, the PF supports large gradients in temperature and salinity that, via stirring by geostrophic turbulence, can produce a direct cascade of tracer variance to small scales, especially below the mixed layer.…”

“…The imprint of coherent eddies on tracer structure, for example, surface chlorophyll, occurs over horizontal scales of O(10-100 km). These eddies typically have a vertical extent of O(2,000 m) and stir tracers below the mixed layer in the interior (Balwada et al, 2016(Balwada et al, , 2020. Near the surface, mesoscale eddies usually have an inlay of strong submesoscale O(1-10 km, 1 h-10 days) currents, which are often visible as cusps and filaments in surface tracers.…”

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

“…Until now, only a few studies have focused on the dispersion in the intermediate and deep layers of the ocean [53][54][55][56]. In this study, a powerful tool such as a high-resolution numerical model can be employed to investigate the Lagrangian dispersion along the water column.…”

Relative dispersion with finite inertial ranges

“…These have a long history in Lagrangian data analysis (e.g. Richardson 1926;Richardson & Stommel 1948;Okubo 1971;Lin 1972;Morel & Larcheveque 1974;Er-El & Peskin 1981;LaCasce & Bower 2000;Lacorata, Aurell & Vulpiani 2001;LaCasce & Ohlmann 2003;Ollitrault, Gabillet & de Verdiere 2005;Koszalka, LaCasce & Orvik 2009;Lumpkin & Ellipot 2010;Mantovanelli et al 2012;Ohlmann et al 2012;Schroeder et al 2012;Ohlmann et al 2017;Sansón, Pérez-Brunius & Sheinbaum 2017;Dräger-Dietel et al 2018;Balwada et al 2021;Spydell, Feddersen & MacMahan 2021). Reviews have been given by Babiano et al (1990), Sawford (2001), Bennett (2006), LaCasce (2008) and Salazar & Collins (2009), among others.…”