ON THE SOUTHERLY EXTENT OF THE ANTARCTIC CIRCUMPOLAR CURRENT IN THE SOUTHEAST PACIFIC

A front at 67 degrees S in the Bellingshausen Sea at 85 degrees W is s hown to be part of the Antarctic Circumpolar Current (ACC) and the sou thernmost zone of concentrated eastward transport at that longitude. T he front lies 6 degrees south of the Polar Front at 88 degrees W and 3 degrees north of the Continental Water Boundary. The front is continu ous 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 Po lar Front. Data from a towed profiling CTD were able to distinguish th e Polar Front from the Southern Polar Front, even though they were onl y 0.5 degrees apart. Thus the width of the ACC south of the Polar Fron t 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 degrees W. Distinguishing features of the Southern Polar Front were a water mass boundary associated with a zone of conce ntrated baroclinic flow and a surface salinity minimum. These features also have been found at the Greenwich Meridian at 53 degrees S, so th e Southern Polar Front can be traced round at least a quarter of the g lobe. To the west of the Bellingshausen Sea both Eltanin data and the Fine Resolution Antarctic Model show that the ACC is at its narrowest at 145 degrees W, where its southern boundaries lie as far north as 56 degrees S. At this longitude the ACC meets the topographic barrier of the Pacific-Antarctic Ridge. To conserve potential vorticity the curr ent 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 degrees 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.