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

The location of the Antarctic Polar Front (PF) was mapped over a 7-year period (1987–1993) within images of satellite-derived sea surface temperature. The mean path of the PF is strongly steered by the topographic features of the Southern Ocean. The topography places vorticity constraints on the dynamics of the PF that strongly affect spatial and temporal variability. Over the deep ocean basins the surface expression of the PF is weakened, and the PF meanders over a wide latitudinal range. Near large topographic features, width and temperature change across the front increase, and large-scale meandering is inhibited. Elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the PF encounters large topographic features. The strong correlations between topography and PF dynamics can be understood in the context of the planetary potential vorticity (PPV or f/H) field. Mean PPV at the PF varies by more than a factor of 2 along its circumpolar path. However, at the mesoscale the PF remains within a relatively narrow range of PPV values around the local mean. Away from large topographic features, the PF returns to a preferred PPV value of ∼25 × 10−9 m−1 s−1 despite large latitudinal shifts. The mean paths of the surface and subsurface expressions of the PF are closely coupled over much of the Southern Ocean.

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Location and dynamics of the Antarctic Polar Front from satellite sea surface temperature data

http://www.vliz.be/imisdocs/publications/59014.pdf

Structure of Southern Ocean fronts at 140°E

The primary control on the N–S zonation of the Southern Ocean is the wind-induced transport of the Antarctic Circumpolar Current (ACC). The ACC divides the Southern Ocean into three major zones: the Subantarctic Zone (SAZ) north of the ACC; the ACC transport zone; and the zone south of the ACC (SACCZ). The zone of ACC transport is most often subdivided into two zones, the Polar Frontal Zone (PFZ) and the Antarctic Zone (AAZ), but it may be appropriate to define more subzones or indeed only one at some longitudes. To maintain geostrophic balance, isopycnals must slope upwards to the south across the ACC, thus raising nutrient-rich deep water closer to the surface as one goes polewards. In addition, silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate-rich bottom water. Surface silicate concentrations therefore decrease northwards from high levels in the SACCZ to low levels in the SAZ. Within the SAZ and PFZ and even in the northern part of the AAZ, silicate levels may drop to limiting levels for siliceous phytoplankton production during summer. Nitrate concentrations also decrease northwards, but only become limiting in the Subtropical Zone north of the SAZ. The second circumpolar control is the changing balance of stratification, with temperature dominating near-surface stratification in the SAZ and salinity dominating further south because of fresh water input to the surface from melting ice. This results in circumpolar features such as the subsurface 2°C temperature minimum and the subduction of the salinity minimum of Antarctic Intermediate Water, which are often but not always associated with frontal jets and large transports. The transport of the ACC is dynamically constrained into narrow bands, the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude. Thus it is not the fronts that are circumpolar, but the total ACC transport and scalar properties of the salinity and temperature fields. Evidence of summer silicate and nitrate uptake in all zones (SAZ, PFZ and AAZ) shows that there is productivity despite their high-nutrient low-chlorophyll status. Blooms covering large areas (say 400 km across) in the PFZ and AAZ are found in the vicinity of submarine plateaux, which suggest benthic iron fertilizatio

Physical controls on biogeochemical zonation in the Southern Ocean

Satellite data reveal a 20% decline in sea ice extent in the Amundsen and Bellingshausen Seas in the two decades following 1973 This change is negatively correlated with surface air temperatures on the west side of the Antarctic Peninsula, which have increased ;058C decade21 since the mid-1940s The recession was strongest during summer, when monthly average minima in 1991‐92 removed much of the incipient multiyear ice over the continental shelf This would have lowered the regional-mean ice thickness, impacting snow ice formation, brine production, and vertical heat flux The northern ice edge contracted by ;18 of latitude in all seasons from 1973‐79 to 1987‐93, returning toward mean conditions in 1993‐95 The decline included multiyear cycles of several years in length, superimposed on high interannual variability A review of atmospheric forcing shows winds consistent with mean and extreme ice extents, and suggests links to larger-scale circulation changes in the South Pacific Historical ocean measurements are sparse in this sector, but mixed-layer depths and upper pycnoclines beneath the sea ice resemble those in the Weddell Sea Weaker surface currents or changes in the upwelling of Circumpolar Deep Water on the continental shelf could have contributed to the anomaly persistence

Climate variability in the Amundsen and Bellingshausen Seas

A front at 67°S in the Bellingshausen Sea at 85°W is shown to be part of the Antarctic Circumpolar Current (ACC) and the southernmost zone of concentrated eastward transport at that longitude. The front lies 6° south of the Polar Front at 88°W and 3° north of the Continental Water Boundary. The front is continuous to the east through Drake Passage where it forms a southern branch of the Polar Front, for which reason we have named it the Southern Polar Front. Data from a towed profiling CTD were able to distinguish the Polar Front from the Southern Polar Front, even though they were only 0.5° apart. Thus the width of the ACC south of the Polar Front varied considerably. About a third of the transport of the ACC also lay south of the Polar Front, with 15 Sv carried by the Southern Polar Front alone at 85°W. Distinguishing features of the Southern Polar Front were a water mass boundary associated with a zone of concentrated baroclinic flow and a surface salinity minimum. These features also have been found at the Greenwich Meridian at 53°S, so the Southern Polar Front can be traced round at least a quarter of the globe. To the west of the Bellingshausen Sea both Eltanin data and the Fine Resolution Antarctic Model show that the AAC is at its narrowest at 145°W, where its southern boundaries lie as far north as 56°S. At this longitude the ACC meets the topographic barrier of the Pacific-Antarctic Ridge. To conserve potential vorticity the current is forced to make a southward loop as it crosses the ridge and the current broadens dramatically. The flow remains broad until forced to sharpen by the constriction of Drake Passage. A similar broadening of the ACC is seen where it crosses the Southwest Indian Ridge south of Africa at 30°E. Here it remains broad until it encounters the Kerguelen Plateau. Thus the eastern boundaries of both the Weddell and Ross Gyres are determined by where the ACC crosses midocean ridges.

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On the southerly extent of the Antarctic Circumpolar Current in the southeast Pacific

The structure of a front observed at 67°S in Austral spring in the Bellingshausen Sea is described. The front extended to full depth, and has been identified by Read et al. (1995) as the southernmost major front of the Antarctic Circumpolar Current where Circumpolar Deep Water (CDW) outcrops. In this paper the structure of the front is examined on scales down to 10 km using a closely spaced CTD section and down to 3 km from underway surveys with SeaSoar and an Accustic Doppler Current Profiler (ADCP) which resolved the density and velocity structure, respectively, in the top 400 m. A seasonal halocline was present throughout the survey area on both sides of the front, at the base of a mixed layer 50–70 m deep. The signature of the previous winter's mixed layer depth could be seen in the weak stratification between 70 and 150 m. The subsurface temperature minimum within the winter mixed layer was significantly colder south of the front (−1.7°C) than north of it (−1.1°C). The front, defined by eastward velocities greater than 10 cm s−1, was 70–80 km wide with flow at speeds of up to 50 cm s 1 in the surface layer. The zone within which water mass changes occurred was narrower, about 40 km, contained within the zone of eastward flow and extended down to 1000 m. The strong velocities in the frontal jet caused differential advection of surface properties from upstream (west) of the survey area, resulting in sharp cross-frontal gradients of salinity and chlorophyll a. Chlorophyll was largest (over 5 mg m−3) in a band barely 10 km wide that extended along-front for well over 100 km and was tightly constrained along the southern flank of the front where isopycnals outcropped from the winter mixed layer into the surface layer across the seasonal halocline. The large values of chlorophyll could only be maintained by advection into the survey area from an unknown source region upstream (Boyd et al., 1995). Patches of high chlorophyll were found only south of the front, and it is hypothesised that productivity south of the front is a consequence either of some property of the upwelling CDW or of water that had been under ice, identified by the −1.7°C subsurface temperature minimum. Temperature anomalies on the 27.4 kg m−3 isopycnal indicate that patches of water south of the front had broken off from the southern flank of the front, so it is probable that the bloom was spreading southwards from the front within the survey area rather than northwards from the ice edge.

A.I. Morrison

Papers

On the southerly extent of the Antarctic Circumpolar Current in the southeast Pacific

On the physical structure of a front in the Bellingshausen Sea

SeaSoar data collected on RRS "Discovery" Cruise 198 (Sterna) across Drake Passage and in the Bellingshausen Sea