Connecting flow–topography interactions, vorticity balance, baroclinic instability and transport in the Southern Ocean: the case of an idealized storm track

Abstract

Abstract. The dynamical balance of the Antarctic Circumpolar Current and its implications on the functioning of the world ocean are not fully understood and poorly represented in global circulation models. In this study, the sensitivities of an idealized Southern Ocean (SO) storm track are explored with a set of eddy-rich numerical simulations. The classical partition between barotropic and baroclinic modes is sensitive to current–topography interactions in the mesoscale range 10–100 km, as comparisons between simulations with rough or smooth bathymetry reveal. Configurations with a rough bottom have weak barotropic motions, ubiquitous bottom form stress/pressure torque, no wind-driven gyre in the lee of topographic ridges, less efficient baroclinic turbulence and, thus, larger circumpolar transport rates. The difference in circumpolar transport produced by topographic roughness depends on the strength with which (external) thermohaline forcings by the rest of the world ocean constrain the stratification at the northern edge of the SO. The study highlights the need for a more comprehensive treatment of the Antarctic Circumpolar Current (ACC) interactions with the ocean floor, including realistic fields of bottom form stress and pressure torque. It also sheds some light on the behavior of idealized storm tracks recently modeled: (i) the saturation mechanism, whereby the circumpolar transport does not depend on wind intensity, is a robust and generic attribute of ACC-like circumpolar flows; (ii) the adjustment toward saturation can take place over widely different timescales (from months to years) depending on the possibility (or not) for barotropic Rossby waves to propagate signals of wind change and accelerate/decelerate SO wind-driven gyres. The real SO having both gyres and ACC saturation timescales typical of our “no gyre” simulations may be in an intermediate regime in which mesoscale topography away from major ridges provides partial and localized support for bottom form stress/pressure torque.

The multi-scale response of the eddy kinetic energy and transport to strengthened westerlies in an idealized Antarctic Circumpolar Current

“…Considering the importance of the bathymetric features in modulating the real ACC flows and EKE (Holloway, 1978;Thompson, 2010;Melet et al, 2013;Howard et al, 2015;Jouanno & Capet, 2020;Zhang et al, 2023), a second pair of experiments were conducted where a topographic ridge was introduced (Abernathey & Cessi, 2014;Balwada et al, 2018). These channel experiments also doubled the domain length, following the setup from Youngs et al (2023), allowing us to distinguish more clearly between regions closer and away from the ridge.…”

Influence of the Gulf of Guinea Islands on the Atlantic Equatorial Undercurrent Circulation

“…During bifurcation, the EUC naturally generates eddies with or without islands due to different unstable modes (e.g., McCreary & Yu, 1992). Flow-topography interactions can change this instability, being responsible for the generation of mesoscale features (e.g., Jouanno & Capet, 2020) that escalates to alter larger-scale dynamics through geostrophic turbulence and the inverse energy cascade (cf. Charney, 1971;Hoskins et al, 1985).…”

The Multi‐Scale Response of the Eddy Kinetic Energy and Transport to Strengthened Westerlies in an Idealized Antarctic Circumpolar Current

“…Considering the importance of the bathymetric features in modulating the real ACC flows and EKE (Holloway, 1978;Howard et al, 2015;Jouanno & Capet, 2020;Melet et al, 2013;Thompson, 2010;Zhang et al, 2023), a second pair of experiments were conducted where a topographic ridge was introduced (Abernathey & Cessi, 2014;Balwada et al, 2018). These channel experiments also doubled the domain length, following the setup from Youngs et al (2023), allowing us to distinguish more clearly between regions closer and away from the ridge.…”