Variability of the Antarctic Circumpolar Current South of Australia

Stephen R. Rintoul, Antarctic CRC & CSIRO Marine Laboratories, Hobart, Australia
Nathan B. Bindoff, Antarctic Cooperative Research Centre, Hobart, Australia
Douglas S. Luther, Univ. of Hawaii at Manoa, Honolulu, Hawaii
James G. Richman, Oregon State Univ., Corvallis, Oregon
D. Randolph Watts, Univ. of Rhode Island, Kingston, Rhode Island
Alan D. Chave, Woods Hole Oceanographic Institution, Woods Hole, Mass.

Abstract:

The Antarctic Circumpolar Current (ACC) provides the primary means by which water, heat and salt are exchanged between the ocean basins. As a result, variations in the transport of the ACC may influence the earth's climate on a range of time-scales. The variability of the ACC south of Australia has been measured for the first time as part of the World Ocean Circulation Experiment (WOCE). Six occupations of the repeat hydrographic section between Tasmania and Antarctica, including both winter and summer sections, have been completed between 1991 and 1996. Two additional partial sections (44-54S) with high quality ADCP data were collected in 1993 and 1995. Twenty high density summer XBT sections obtained between 1992 and 1996 are used to supplement the CTD observations. The net baroclinic volume transport across the section ranges from 145 to 167 Sverdrups to the east, relative to the deepest common depth. Direct velocity measurements from ALACE floats and shipboard ADCP are used to assess the validity of the choice of a deep reference level.

The net transport is dominated by the Subantarctic and Polar Fronts, which form a broad zone of eastward flow between 50S and 53S with a transport of 120 Sv. An additional 70 Sv of eastward flow occurs south of 58S. Several bands of westward flow are found: a zone of westward flow from 500-3000 m depth between Tasmania and the South Tasman Rise of water whose water mass properties indicate has originated in the Tasman Sea; an anticyclonic, deep-reaching gyre between the SAF and the South Tasman Rise, whose westward branch varies in magnitude from 20 to 40 Sv; and a weaker westward flow over the southern flank of the mid-ocean ridge. Most of the transport variability is associated with the SAF and the anticyclonic gyre north of the SAF; the current cores south of 58S are particularly stable with time.