The physics of the Antarctic Circumpolar Current

A region of transition of surface water characteristics from subantarctic to antarctic and an associated eastward flowing Antarctic Circumpolar Current (ACC) have long been recognized to exist as a band around Antarctica. In this review we summarize the most important observational and theoretical findings of the past decade regarding the ACC, identify gaps in our knowledge, and recommend studies to address these. The nature of the meridional zonation of the ACC is only now being revealed. The ACC seems to exist as multiple narrow jets imbedded in, or associated with, density fronts (the Subantarctic and Polar fronts) which appear to be circumpolar in extent. These fronts meander, and current rings form from them; lateral frontal shifts of as much as 100 km in 10 days have been observed. The volume transport of the ACC has been estimated many times with disparate results. Recently, yearlong direct measurements in Drake Passage have shown the mean transport to be approximately 134 × 106 m³/s, with an uncertainty of not more than 10%. The instantaneous transport can vary from the mean by as much as 20%, with most of the variation associated with changes in the reference flow at 2500 m rather than in the vertical shear. Meridional exchanges of heat across the ACC are known to be important to the heat balance of the abyssal ocean and consequently to global climate. The most likely candidate process for the required poleward heat exchange seems to be mesoscale eddies, though narrow abyssal boundary currents may also be important. Observations from ships, drifters, and satellites reveal surface mean kinetic energy to be at a maximum along the axis of the ACC and eddy kinetic energy to be large mainly in western boundary regions and off the tip of Africa. Eddy variability in the open ocean is consistent with baroclinic instability of the narrow jets. Calculations using data from Drake Passage show that the necessary conditions for baroclinic and barotropic instabilities are met in the ACC. The basic dynamical balance of the ACC is still not well known, although bottom and lateral topography and dynamic instabilities are shown to be important in balancing wind forcing. The ACC is generally conceded to be driven by the wind, but the coupling of wind and thermohaline circulations have not yet been adequately investigated. The mechanism responsible for the multiple cores of the ACC has not been identified in detail. It is suggested that future studies address: (1) the circumpolar structure and temporal behavior of the Subantarctic Front and Polar Front; (2) the general dynamical balance of the ACC and specific mechanisms for creation and maintenance of the major fronts; (3) the representativeness to the entire ACC of the existing estimates of meridional exchanges of heat and other properties, as well as kinematic and dynamic quantities, made in Drake Passage; (4) the variability of the ACC transport in several places and coherence of its variability; (5) the climatology of fields of atmosphere-ocean forcing over the southern ocean; and (6) the possibility of identifying and using simple indices as good indicators of the behavior of the ACC or parts thereof.