On the southerly extent of the Antarctic Circumpolar Current in the southeast Pacific

A front at 67°S in the Bellingshausen Sea at 85°W is shown to be part of the Antarctic Circumpolar Current (ACC) and the southernmost zone of concentrated eastward transport at that longitude. The front lies 6° south of the Polar Front at 88°W and 3° north of the Continental Water Boundary. The front is continuous to the east through Drake Passage where it forms a southern branch of the Polar Front, for which reason we have named it the Southern Polar Front. Data from a towed profiling CTD were able to distinguish the Polar Front from the Southern Polar Front, even though they were only 0.5° apart. Thus the width of the ACC south of the Polar Front varied considerably. About a third of the transport of the ACC also lay south of the Polar Front, with 15 Sv carried by the Southern Polar Front alone at 85°W. Distinguishing features of the Southern Polar Front were a water mass boundary associated with a zone of concentrated baroclinic flow and a surface salinity minimum. These features also have been found at the Greenwich Meridian at 53°S, so the Southern Polar Front can be traced round at least a quarter of the globe. To the west of the Bellingshausen Sea both Eltanin data and the Fine Resolution Antarctic Model show that the AAC is at its narrowest at 145°W, where its southern boundaries lie as far north as 56°S. At this longitude the ACC meets the topographic barrier of the Pacific-Antarctic Ridge. To conserve potential vorticity the current is forced to make a southward loop as it crosses the ridge and the current broadens dramatically. The flow remains broad until forced to sharpen by the constriction of Drake Passage. A similar broadening of the ACC is seen where it crosses the Southwest Indian Ridge south of Africa at 30°E. Here it remains broad until it encounters the Kerguelen Plateau. Thus the eastern boundaries of both the Weddell and Ross Gyres are determined by where the ACC crosses midocean ridges.

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