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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...

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The two principal fronts within the Antarctic Circumpolar Current (ACC), the Subantarctic Front (SAF) and the Polar Front (PF), are investigated with respect to their spatial patterns and relations to deep water masses from the northern Drake Passage to the western and southern Argentine Basin. Observations from four recent cruises in adjoining and overlapping regions are supplemented with satellite infrared imagery and trajectories from surface drifters. East of Drake Passage, the width of the Polar Frontal Zone (PFZ) increases rapidly when the SAF turns sharply northward as a part of the Falkland (Malvinas) Current, while the PF remains oriented mainly toward the northeast. Found within this region of the PFZ are mesoscale eddies, the upper layers of which consist of Subantarctic Mode Water from the Pacific. The contrasts in upper layer water properties that identify the SAF can be traced the entire length of the Falkland Current to the Brazil‐Falkland confluence zone and then southward to the southern Argentine Basin. There the SAF turns eastward and later, but perhaps not always, merges with the PF north of Ewing Bank to form a single, intense current core strongly influenced by bottom topography. At times this eastward current loops southward through a gap in the Falkland Ridge into the Georgia Basin, thereby supplying relatively warm and salty Subantarctic water to the Antarctic Zone. East of the gap, the two fronts become separated and are once again distinct features in the vicinity of the Islas Orcadas Rise. At depth, within Drake Passage and the western Scotia Sea, the SAF and PF are not lateral boundaries between distinct water masses, but are instead identified by enhanced vertical displacements of property isopleths. With the northward turn of the SAF east of Drake Passage, a thick layer of Circumpolar Deep Water (CDW) is advected over the Falkland Plateau into the Argentine Basin. There it is joined by waters entering the Argentine Basin via a deep spreading route through the Georgia Basin: denser CDW, deep water from the Weddell Sea, and episodically, deep water from the southeastern Pacific Ocean. Together, these waters form a northward flow off the Patagonian shelf that extends from the sea surface to the bottom, most of which turns back toward the south with the Falkland Current return. Also flowing south, along the seaward side of the Falkland Current return well away from the western boundary, is the main core of North Atlantic Deep Water (NADW). Detatched masses of NADW are observed within the CDW at the western boundary as well as within the ACC south of the SAF in the south central Argentine Basin.