Water masses and baroclinic transports in the South Atlantic and Southern oceans

Abstract

We describe the World Ocean Circulation Experiment (WOCE) A23 hydrographic section from Antarctica to Brazil, nominally along 35W. The section crossed the center of the Weddell Gyre, the Antarctic Circumpolar Current (ACC) and the Subtropical Gyre in the Argentine Basin. We precisely define the locations of fronts, the changes in water mass properties across them, and their transports. The Antarctic Slope Front was crossed above the continental slope of Antarctica, with a baroclinic transport of 4 Sv, part of the cyclonic Weddell Gyre circulation of 19 Sv. We repeated a section in the Weddell Sea occupied in 1973, and saw a marked warming of the inflowing Warm Deep Water layer by some 0.2°C, but no discernible change in the outflowing northern limb of the gyre. An inflow of recently ventilated water with the same characteristics as Weddell Sea Deep Water (WSDW) was observed flowing into the Weddell Sea from the east. The Weddell Front was crossed at 61°7'S and the Southern Boundary (SB) of the ACC (often referred to as the Scotia Front) at 58°38'S. Between these lay the Weddell-Scotia Confluence, contributing 16 Sv of eastward transport. The first crossing of the Southern ACC Front (SACCF) lay south of South Georgia at ~55°30'S. It then wrapped anticyclonically around South Georgia and was encountered at 53°40'S before retroflecting and returning eastward at ~53°30'S. The baroclinic transport was ~15 Sv at each crossing. In this region the SACCF is most clearly identified by a decrease in the salinity of the temperature minimum layer. The core of the Polar Front (PF) lay at ~49S where the isotherms plunged down sharply to the north, and transported 67 Sv. The PF and Subantarctic Front (SAF) were barely distinguishable with only one station clearly in the Polar Frontal Zone. The SAF, transporting 57 Sv, was encountered at ~48°45'S where the subsurface salinity minimum of Antarctic Intermediate Water (AAIW) began to descend. The Subtropical Front (STF) marks the boundary between the waters of the subtropical gyre and the colder, fresher subantarctic waters to the south; its southernmost crossing was at 44-45S transporting ~25 Sv. This is several hundred kilometers farther south than historical locations of the STF at this longitude. The Brazil Current Front (BCF) was encountered at ~38S transporting 43 Sv. Whereas previous observations found the STF to be the primary means for eastward flow of the waters of the Brazil Current after it has separated from the coast, during A23 we find that the BCF carries the majority of this transport. A further Deep Front, possibly marking the center of the subtropical gyre, was crossed at 34°22'S associated with a transport of 2.5 Sv. In the western Vema Channel we encountered a recirculation of the Brazil Current, flowing to the northeast with a transport of 23 Sv. The section ended prematurely without crossing the Brazil Current. To close the subtropical gyre transport would require a Brazil Current significantly in excess of the historical estimates based on shallow reference levels. Both the SACCF and STF exhibited meanders probably caused by bathymetry. The STF meander may be caused by the eastern end of the circulation around the Zapiola Rise. Both the SB and PF had associated eddies. The SB had shed a cyclonic eddy to the north, which had subsequently been capped by local water. It transported 8 Sv azimuthally. The PF had shed an anticyclonic eddy to the south, which was also capped and circulated at least 3 Sv. A small anticyclonic subsurface lens of AAIW was observed in the Vema Channel with a transport of the order of I Sv. In the Vema Channel, a level of no motion between the North Atlantic Deep Water and the Lower Circumpolar Deep Water (LCDW) gives a net northward flow of 1.2 Sv WSDW and 4 Sv LCDW from the Argentine to the Brazil Basin.