FY 1999

A late winter hydrographic section from Tasmania to Antarctica

Rintoul, S.R., and J.L. Bullister

Deep-Sea Res. I, 46(8), 1417–1454, doi: 10.1016/S0967-0637(99)00013-8 (1999)

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 between the Indian and Pacific Oceans south of Australia. This paper describes the fronts, water masses and transport observed on the first occupation of the repeat section. The Subantarctic Front (SAF) is located between 50°S and 51°S and is the most striking feature of the vertical sections. Two additional fronts at 53°S and 59°S are associated with the Polar Front (PF), part of which turns northward to flow along the section before turning back to the east near 53°S. Very deep (>500 m) mixed layers are found north of the SAF, confirming that Subantarctic Mode Water (SAMW) is formed in this region by deep convection in winter. Chlorofluorocarbons (CFCs) are significantly undersaturated (≈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 to equilibrium by the end of the winter period of deep convective mixing. Northward Ekman drift of cold, fresh water across the SAF is likely to be responsible for the cooler, fresher mixed layers observed immediately north of the SAF. The Antarctic Intermediate Water (AAIW) on the SR3 section is relatively low in oxygen and CFCs (≈60-70% and 10-20% of saturation values, respectively), high in potential vorticity, and high in nutrients. These characteristics suggest that the AAIW on this section is not renewed by direct and rapid ventilation near this location. Water mass properties 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, fresh, 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°S and 53°S including the SAF carries 137 Sv to the east and dominates the net transport. Weaker flow south of 58°S contributes an additional 70 Sv. The eastward flow is compensated in part by 37 Sv of westward flow between Tasmania and 48.5°S and 8 Sv of flow to the west over the southern flank of the mid-ocean ridge. The trajectories of six ALACE floats deployed at about 950 m confirm the sense of flow inferred from the choice of a deep reference level.