A late winter hydrographic section from Tasmania to Antarctica

A hydrographic section between Tasmania and Antarctica was occupied in late winter 1991 as part of the World Ocean Circulation Experiment (WOCE). The primary purpose of the WOCE repeat section SR3 is to measure the exchange b etween the Indian and Pacific Oceans south of Australia. This paper describ es the fronts, water masses and transport observed on the first occupation of the repeat section. The Subantarctic Front (SAF) is located between 50 d egrees S and 51 degrees S and is the most striking feature of the vertical sections. Two additional fronts at 53 degrees S and 59 degrees S are associ ated with the Polar Front (PF), part of which turns northward to flow along the section before turning back to the east near 53 degrees S. Very deep ( > 500 m) mixed layers are found north of the SAF, confirming that Subantarc tic Mode Water (SAMW) is formed in this region by deep convection in winter . Chlorofluorocarbons (CFCs) are significantly undersaturated (approximate to 90-92% of equilibrium values) in these deep mixed layers indicating that gas exchange rates are not rapid enough to bring these deep mixed layers t o equilibrium by the end of the winter period of deep convective mixing. No rthward Ekman drift of cold, fresh water across the SAF is likely to be res ponsible for the cooler, fresher mixed layers observed immediately north of the SAF. The Antarctic Intermediate Water (AAIW) on the SR3 section is rel atively low in oxygen and CFCs(approximate to 60-70% and 10-20% of saturati on values, respectively), high in potential vorticity, and high in nutrient s. These characteristics suggest that the AAIW on this section is not renew ed by direct and rapid ventilation near this location. Water mass propertie s suggest that water from the Tasman Sea spreads south and west across the northern portion of the SR3 section between 800 and 3000 m depth. A cold, f resh, CFC-rich variety of Antarctic Bottom Water is formed along the Wilkes -Adelie coast of Antarctica. The net transport across the section relative to the deepest common depth is 160 Sv. The band of eastward flow between 50 degrees S and 53 degrees S including the SAF carries 137Sv to the east and dominates the net transport. Weaker flow south of 58 degrees S contributes an additional 70Sv. The eastward flow is compensated in part by 37Sv of we stward flow between Tasmania and 48.5 degrees S and 8Sv of flow to the west over the southern flank of the mid-ocean ridge. The trajectories of six AL ACE floats deployed at about 950m confirm the sense of flow inferred from t he choice of a deep reference level. (C) 1999 Elsevier Science Ltd. All rig hts reserved.