The frontal structure of the Antarctic Circumpolar Current (ACC) at the Greenwich Meridian is similar to that at Drake Passage even though the current is not confined to flow between two continents: There are sharp horizontal gradients in all properties throughout the water column, the fronts are narrow relative to the total width of the current, and most of the transport occurs within the frontal zones. East of Drake Passage, saline North Atlantic Deep Water (NADW) is incorporated into the Circumpolar current, and at the Greenwich Meridian it influences the water characteristics as far south as the Polar Front. Although the transport between Antarctica and Africa is close to that between Antarctica and South America, the transport within the ACC at our section is about 20% greater than at Drake Passage, probably due in part to the addition of NADW. Separating the ACC from the Weddell Gyre is a sharp front, south of which the signature of all but the densest Circumpolar Deep Water (CDW) is lost by mixing with the surface waters. The intermediate water of the central Weddell Gyre is formed from this dense CDW, which is modified by biochemical processes to become oxygen poor and nutrient rich. Warm, salty, less dense CDW from the southern edge of the ACC rounds the eastern end of the gyre and appears in the southern limb, which meanders around Maud Rise. As a result of the InternationalSouthern Ocean Studies (ISOS), the ACC at Drake Passage has been described in detail. The kinematics and thermohaline structure were described by Gordon et al. [1977]; Sievers and Nowlin [1984] characterized the water masses and stratification, and the lowfrequency flow and its variability were discussed by Hofmann and Whitworth [1985] and Inoue [1985]. With few exceptions, comparable high-quality data from sections with close station spacing are not available from other areas for detailed study of the ACC. Furthermore, because of logistic constraints, such sections across the open ocean far from continents are particularly difficult to obtain and thus will continue to be rare. We have made such a section along the Greenwich Meridian. The main purpose of this paper is to describe the structure and circulation of the Antarctic Circumpolar Current and the Weddell Gyre at the Greenwich Meridian. We also relate the observed distributions of characteristics to the reported struc-location. Such comparisons may eventually provide clues for understanding the reason why the transition from antarctic to subantarctic waters at the ACC takes the form of circumpolar, steplike fronts. As a major part of this comparison, we examine the modifications of characteristics of the Circumpolar Current in the southwest Atlantic. The high salinity characterizing the Circumpolar Deep Water (the principal water mass of the ACC by volume) is derived from the North Atlantic Deep Water (NADW). The "new" NADW, carrying relatively high salinity and oxygen and low nutrients, first encounters the ACC in the Argentine Basin [Reid et al., 1977]. The characteristic...
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 souther...
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