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The Antarctic Circumpolar Current (ACC) as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The ACC is poorly represented here because of the lack of data. The MGSVA is based on ship-drift estimates of sea surface velocities that are mostly available along major shipping routes. Click here for example plots of seasonal averages. | |
The ACC is arguably the "mightiest current in the oceans" (Pickard and Emery, 1990). Despite its relatively slow eastward flow of less than 20 cm s-1 in regions between the fronts, the ACC transports more water than any other current (Klinck and Nowlin, 2001). The ACC extends from the sea surface to depths of 2000-4000 m and can be as wide as 2000 km. This tremendous cross-sectional area allows for the current's large volume transport. The Antarctic Circumpolar Current's eastward flow is driven by strong westerly winds. The average wind speed between 40°S and 60°S is 15 to 24 knots with strongest winds typically between 45°S and 55°S. Historically, the ACC has been referred to as the 'West Wind Drift' because the prevailing westerly wind and current are both eastward.
Without the aid of continental reference point, except for the Drake Passage, where by convention, all flow through the Passage is the ACC, the current's boundaries are generally defined by zonal variations in specific water properties of the Southern Ocean (Gordon et al., 1977). Variations in these properties have been used to classify regions whose edges are defined by fronts, where there is rapid changes in water properties which occur over a short distance. North of the ACC is the Subtropical Convergence or Subtropical Front (STF), usually found between 35°S and 45°S, where the average Sea Surface Temperature (SST) changes from about 12°C to 7 to 8°C and salinity decreases from greater than 34.9 to 34.6 or less. Three fronts and three zones south of the STF and associated with the ACC are, from north to south; the Subantarctic Zone (SAZ), the Subantarctic Front (SAF), the Polar Frontal Zone (PFZ), the Polar Front (PF), the Antarctic Zone, and the Southern ACC Front. The Antarctic Convergence is approximately 200 km south of the Polar Front. In the Antarctic Convergence, summer SST varies between 3°C to 5°C, while winter SST varies between 1°C to 2°C. North of the SAF, average Sea Surface Temperature (SST) is greater than 4°C, while south of the Polar front, average SST is less than 2°C. A fourth zone, the Continental Zone, and the westward flowing Antarctic Coastal (or Polar) Current are located even further poleward, between the Southern Front and the Antarctic continent. SST poleward of 65°S is about -1.0°C (Deacon, 1984).
The eastward flow of the Subantarctic Front (SAF), found between 48°S and 58°S in the Indian and Pacific Ocean and between 42°S and 48°S in the Atlantic Ocean, defines the ACC's northern boundary. A region of upwelling, the Antarctic Divergence, occurs at the Southern Front. This area of divergence has been considered to be the ACC's southern boundary (Klinck and Nowlin, 1986) but new analysis puts the southern boundary of the ACC further poleward. Orsi et al. (1995) define the southern boundary of the ACC as the poleward edge of the Upper Circumpolar Deep Water (T > 1.8°C). The southern boundary of the ACC is approximately at 65°S in most of the Indian and Pacific Ocean, from 50°E to the dateline; moves northward to 60°S, east of the dateline to 140°W; is near 70°S by 120°W and moves northward to 60°S, east of the Drake Passage; and northward to 55°S at 10°E. Northward displacement of the southern boundary of the ACC are in the areas of gyres with clockwise surface circulation in the Weddell Sea and in the Ross Sea.
Strong, nearly zonal, westerly winds force a large, near-surface, northward Ekman transport and a northward pressure gradient. The ACC current is in approximately geostrophic equilibrium, so that inclined layers of constant density slope towards the surface poleward across the ACC to balance the current's northward sea surface height elevation. The alignment between the prevailing winds and the resulting geostrophic current intensifies the ACC. Because stronger gradients give rise to stronger flow, the majority of the ACC transport is associated with the fronts within the current. Gille (1994) analyzed GEOSAT altimeter data and found two well-defined jets in the ACC, at the PF and the SAF, with widths between 35 and 50 km and a dominant meander wavelength of 150 km. Other investigators have found meander wavelengths between 100 and 200 km.
In the vicinity of the fronts, eastward jets flow at approximately two to three times the speed of the current found between them (Klinck and Nowlin, 2001). Hoffman (1985) analyzed near-surface drifting buoys from FGGE and found average speeds of 20 cm s-1 at the STF, about 40 cm s-1 in the SAF and PF, and from 23 to 35 cm s-1 between fronts. Zambianchi et al. (1999) analyzed eleven WOCE standard drifting buoys, from 1993 and 1994, in the Eastern Pacific and found mean speeds greater than 15 cm s-1 north of the PF in the PFZ, and greater than 30 cm s-1 near the PF. They also note strong evidence of topographic steering by the Pacific-Antarctic ridge and that the floats bifurcate near 150°W, 55°S with floats either ending up in the Peru Current via the South Pacific Current or that the floats stayed in the ACC and travelled eastward toward the Drake Passage.
Meridional ridges in the bottom topography provide a force balance for the Atlantic Circumpolar Current by generating frictional form drag. As the ACC crosses these ridges, frictional drag diminishes the current's deep flow (Munk and Palmen, 1951). Bottom topography also controls the path of the ACC, since slow large-scale oceanic flows are, on the average, parallel to lines of constant planetary vorticity (approximately the Coriolis acceleration divided by the water depth). The Drake Passage, Kerguelen Plateau (and island), Campbell Plateau, Macquarie Ridge, and the Pacific-Antarctic Ridge are major topographic features that influence both the mean path of the ACC and its meandering. The degree of topographic blocking will also influence the current's eddy kinetic energy. For example, downstream of meridional ridges there will be an increase in the number of eddies present. Even relatively small eddies make up a significant percentage of the current's overall eddy kinetic energy (Knauss, 1996). Typical time and space scales of the eddies range from 2 weeks to 2 months and from 50 to 250 km.
The path of the ACC is constrained by land where it flows through the Drake Passage, between Cape Horn and the Antarctic Peninsula. To date, the majority of field studies (hydrographic surveys, mooring deployments, etc.) conducted to better understand the ACC have taken place across this 800 km wide passage (Nowlin et al., 1977). Bryden and Pillsbury (1977) found variations over a yearlong current meter study across the Drake Passage between 28-290 Sv to a depth of 2700 meters. Such a large range of transport values have been found by numerous oceanographers. Historical transport estimates are listed in Table 1 of Peterson (1988) and Table 6 of Sarukhanyan (1985) and range from -15 Sv to 262 Sv with several entries greater than 200 Sv. These earlier measurements should be viewed with suspicion since the estimates are aliased by coarse resolution sampling in an energetic eddy field, and those based on hydrography assume a reference velocity. The most dense set of current meter measurements, during the DRAKE79 experiment, yielded a mean transport of 123 Sv and a range of 87 to 148 Sv (Whitworth, 1983; Peterson, 1988). Climatological estimates by Orsi et al (1995) based on hydrographic data show that the ACC transport, relative to 3000 m, is about 100 Sv at all longitudes.
Using Drake Passage current meter measurements at 500 meters during the ISOS, Wearn and Baker (1980) found the variations with the transport correlated with the fluctuations in wind stress when considering periods of longer than 30 days. Similar results are reported for low-frequency time-scales in Peterson (1988) with strong coherence at semi-annual and annual time scales.
In recent years, other areas such as sections of the ACC south of Tasmania and New Zealand have also been examined closely. Observations have revealed a mean ACC transport of 100-150 Sverdrups (1 Sv=106 m3 s-1) that can vary by 50 Sv within time scales as short as a month or two (Knauss, 1996). Variability in the ACC is due to tides (5-10 cm s-1), mesoscale eddies (35-50 cm s-1), near-inertial motion (10 cm s-1), and those forced by changes in the large-scale wind stress (25 cm s-1) (Sarukhanyan, 1985).
The mean ACC temperature ranges from -1 to 5°C, depending on the time of year and location. The mean surface salinity decreases poleward, in general, from 34.9 at 35°S to 34.7 at 65°S. Typical salinity values are between 33.5 and 34.7, poleward of 65°S. This Temperature-Salinity signature is due to a combination of water masses that meet in the Southern Ocean and are mixed and redistributed by the Antarctic Circumpolar Current. Following the inclined isopycnals, deep waters from the North Atlantic (NADW) are upwelled at the Antarctic Divergence, the current's southern boundary. As this water rises to the surface it mixes with and becomes Antarctic Surface Water (ASW). When the water mass reaches the near surface flow it is diverted northward by Ekman transport. This newly formed Antarctic Circumpolar Water (labeled by some 'modified NADW') travels north across the ACC until it reaches the convergence of the Polar Frontal Zone. Here, near surface Sub-Antarctic Water from the north mixes with the ASW and sinks to a mid-depth becoming Antarctic Intermediate Water (AAIW). While this mixing is taking place the geostrophic component of the ACC is translating the water eastward. The AAIW will continue north but, due to the 'West Wind Drift,' will be ejected into the Atlantic, Indian, and Pacific basins, where over time, it will be upwelled to the surface. The Antarctic Circumpolar Current is a critical component of the 'Great Ocean Conveyor Belt.'
During the period July thru October 1978, Seasat radar altimeter measurements where made over the ACC. Fu and Chelton (1984) demonstrated the observational evidence of the temporal variability as well as the zonal coherence of the ACC. Satellite altimetry and sea surface height analysis have recently revealed a previously unknown feature of the Antarctic Circumpolar Current, the Antarctic Circumpolar Wave. This wave propagates westward against the current but ultimately ends up traveling eastward, due to the massive size of the ACC, at a slower rate than the mean flow. The wave circles the earth every eight to nine years (White and Peterson, 1996). It has a long wavelength (wavenumber=2) resulting in two crests and two troughs at any given time. The crests and troughs are associated with massive patches or pools of warm water and cold water respectively. The areas can be thousands of kilometers long. The warm patches are 2 to 3°C warmer than the mean sea surface temperature (SST) and the cold patches are 2 to 3°C cooler than the mean SST (White and Peterson, 1996). Though it is not yet clear how these waves are triggered or maintained, they directly influence the temperature of the overlying atmosphere. While the Wave's effects on climate are just beginning to be studied, the phase (warm pool vs. cold pool) correlates well with four to five year rainfall cycles found over areas of southern Australia and New Zealand (White and Cherry, 1998). Some scientists believe that the Antarctic Circumpolar Wave may be more important than El Niño in governing rainfall over these regions.