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

摘要

The dynamical balance of the Antarctic CircumpolarCurrent and its implications on the functioning of the world ocean are notfully understood and poorly represented in global circulation models. Inthis study, the sensitivities of an idealized Southern Ocean (SO) stormtrack are explored with a set of eddy-rich numerical simulations. Theclassical partition between barotropic and baroclinic modes is sensitive tocurrent–topography interactions in the mesoscale range 10–100 km, ascomparisons between simulations with rough or smooth bathymetry reveal.Configurations with a rough bottom have weak barotropic motions, ubiquitousbottom form stress/pressure torque, no wind-driven gyre in the lee oftopographic ridges, less efficient baroclinic turbulence and, thus, largercircumpolar transport rates. The difference in circumpolar transportproduced by topographic roughness depends on the strength with which(external) thermohaline forcings by the rest of the world ocean constrainthe stratification at the northern edge of the SO. The study highlights theneed for a more comprehensive treatment of the Antarctic Circumpolar Current(ACC) interactions with the ocean floor, including realistic fields ofbottom form stress and pressure torque. It also sheds some light on thebehavior of idealized storm tracks recently modeled: (i) the saturationmechanism, whereby the circumpolar transport does not depend on windintensity, is a robust and generic attribute of ACC-like circumpolar flows;(ii) the adjustment toward saturation can take place over widely differenttimescales (from months to years) depending on the possibility (or not) forbarotropic Rossby waves to propagate signals of wind change andaccelerate/decelerate SO wind-driven gyres. The real SO having both gyresand ACC saturation timescales typical of our “no gyre” simulations may bein an intermediate regime in which mesoscale topography away from majorridges provides partial and localized support for bottom formstress/pressure torque.