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.
Authors: Read, J.F., Pollard, R.T., Morrison, A.I., Symon, C.
1 January, 1995
Deep Sea Research Part II: Topical Studies in Oceanography / 42