Large-scale features of the Antarctic Circumpolar Current (ACC) are described using all historical hydrographic data available from the Southern Ocean. The geopotential anomaly of the sea surface relative to 1000 db reveals the highly-sheared eastward flow of the ACC and the strong steering of the current by the ridge system around Antarctica. The near-surface property distributions differentiate the ACC waters from the warmer and saltier waters of the subtropical regimes. The Subtropical Front (STF), interrupted only by South America, marks the northern most extent of subantarctic waters. Distributions of properties on isopycnal surfaces show an abrupt end to the characteristic signal of the Upper Circumpolar Deep Water (UCDW), as this water mass shoals southward and is entrained into the surface mixed layer. This sharp water mass boundary nearly coincides with the southernmost circumpolar streamline passing through Drake Passage. To its south are the weakly-sheared circulations of the subpolar regime. Inspection of many hydrographic crossings of this transition reveals that the poleward edge of the UCD W signal is a reasonable definition of the southern boundary of the ACC. At Drake Passage, three deep-reaching fronts account for most of the ACC transport. Well-established indicators of the Subantarctic Front and Polar Front are traced unbroken around Antarctica. The third deep-reaching front observed to the south of the Polar Front at Drake Passage also continues with similar characteristics as a circumpolar feature. It is called here the southern ACC front. Stations from multiple synoptic transects of these circumpolar fronts are used to describe the average property structure within each ACC zone. Between the STF and the southern boundary of the ACC, the shear transport of the circumpolar current above 3000 m is at all longitudes about 100 Sv (1 Sv = 106 m3 s−) eastward.

On the meridional extent and fronts of the Antarctic Circumpolar Current

Circulation, mixing, and production of Antarctic Bottom Water

Scotia Sea regional tectonic evolution: implications for mantle flow and palaeocirculation

The availability of new, high-quality, hydrographic data has prompted a re-examination of the circulation in the Atlantic sector of the Southern Ocean. Dynamic topography maps and tracer distributions on selected isopycnal surfaces show that the Weddell Gyre is a large, elongated cyclone located south of the Antarctic Circumpolar Current (ACC), extending northeastward from the Antarctic Peninsula. Patterns of geostrophic shear and a southward turn of the ACC mark its northeastern end near 30°E.

On the circulation and stratification of the Weddell Gyre

Water Masses and Mixing Near the Antarctic Slope Front

Existing information is synthesized and new data presented to describe the flow of near-bottom water from the Weddell Sea into the Scotia Sea and westward through Drake Passage along the continental slope. The water characteristics and currents along the northern margins of the South Sandwich Island Arc are examined. Long-term current measurements in the bottom waters at locations over the outer shelf and slope and along the continental rise show persistent flow from Atlantic to Pacific along isobaths at speeds of 10–20 cm s−1. Three sources for the waters in these currents are identified and discussed. At the deepest levels, Weddell Sea Deep Water enters the Scotia Sea near 40°W through a depression in the South Scotia Arc and then flows westward, constrained by the bottom topography. This cold, fresh, oxygenated bottom water then flows west to enter Drake Passage via a gap in the Shackleton Fracture Zone at the base of the continental slope northwest of Elephant Island. Mid-depth water may flow from the Weddell Sea to the Scotia Sea through the Powell Basin (sill depth approximately 2000 m) located west of South Orkney Island near 48°W. The westward flowing waters along the shelf and upper continental slope, which are denser than those immediately offshore, may be a continuation of the Polar Slope Current from the Weddell Sea or may be derived principally by convection from the shelves of the South Sandwich Island Arc. A vertical section north of Elephant Island shows downslope convection off the shelf, analogous to the observed at many locations around Antartica.

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W. D. Nowlin

Papers

Westward bottom currents along the margin of the South Shetland Island Arc