Observations of the Antarctic Circumpolar Current and deep boundary current in the southwest Atlantic

Abstract

Fourteen‐month velocity and temperature records from an array of 14 moorings north and west of the Falkland Plateau and supporting hydrographic and tracer data reveal a narrow boundary current that carries dense Antarctic waters. The current flows west along the northern flank of the Falkland Plateau with mean speeds of more than 10 cm s−1 at 5000 m and more than 30 cm s−1 at 2500 m. The westward flow extends from the bottom to at least 1000 m, but the upper portion of the current is a branch of the Antarctic Circumpolar Current (ACC) following the only deepwater route between the Scotia Sea and the Argentine Basin. Waters colder than 0.2°C are too cold to be associated with the ACC at Drake Passage and must ultimately derive from the Weddell Sea as part of the deep thermohaline circulation. The westward transport of water colder than 0.2°C is 8.2 × 106 m3 s−1. In the mean the bottom boundary current is similar to that predicted by the Stommel‐Arons model, but considerable variability is introduced by the meandering of the overlying ACC. Chlorofluorocarbon data suggest that new Antarctic water from the Georgia Basin enters the Argentine Basin via the deep boundary current, which passes beneath the ACC; some new water is also advected east after being entrained in the ACC. Most of the water in the deep boundary current is recirculated water that has been in residence in the Argentine Basin for some time. Water colder than −0.2°C is relatively new to the basin and comprises about 2.5 × 106 m3 s−1 of the westward flow of the boundary current.

Modification and pathways of Southern Ocean Deep Waters in the Scotia Sea

“…2c and d is the occurrence of a number of relatively warm and saline features with considerable fine structure in the proximity of the salinity maximum. These are attributable to entrainment of NADW, reported in the past at mid depths above the Falkland Escarpment (Peterson and Whitworth, 1989;Whitworth et al, 1991;Arhan et al, 1999). The spatial distribution of WSDW over the South Scotia Ridge is shown in Fig.…”

“…At the entry of the Malvinas Chasm, the salinity difference between the two WSDW types has been attenuated by diapycnal mixing, but a dominance of the fresher Scotia Sea variety is apparent by reference to Arhan et al (1999, their The outlined spatial distribution of the two WSDW types highlights their likely different fates. WSDW from the South Sandwich Trench occupies the region just south of the Falkland Ridge and can thus spill into the Argentine Basin to feed the deep western boundary current (Whitworth et al, 1991). In contrast, the Scotia Sea variety constitutes the bulk of the WSDW spreading westward into the Malvinas Chasm (Cunningham and Barker, 1996) and may upwell there into the CDW layer, thereby following a more diffuse path out of the basin.…”

“…It may then escape the northern rim of the gyre through topographic clefts (Nowlin and Zenk, 1988;Locarnini et al, 1993), entering the Scotia Sea, or spread north over the South Sandwich Trench and the abyssal plains to the east (Orsi et al, 1993). Although subsequent northward flow is severely restricted by bathymetry and the plunging isopycnals of the eastward-flowing ACC (Orsi et al, 1999), some WSDW intrudes into the Argentine Basin through a deep breach in the Falkland Ridge (Whitworth et al, 1991) and becomes incorporated into the deep western boundary current of the South Atlantic (Arhan et al, 1999).…”

“…1). Discontinuities in these ridges are of extreme importance in determining flow patterns (Zenk, 1981;Nowlin and Zenk, 1988;Whitworth et al, 1991;Locarnini et al, 1993;Arhan et al, 1999) though, occasionally, they can be overshadowed by the relief of isopycnals. As isopycnic surfaces slope most abruptly at fronts, the frontal anatomy of the Scotia Sea is displayed schematically in Fig.…”

Strong export of Antarctic Bottom Water east of the Kerguelen plateau

“…These include the presence of recirculation gyres, interaction between the DWBC and other circulation regimes (e.g., the ACC and subpolar gyres) 8 , the fact that mixing and entrainment change the volume and properties of AABW along the export pathway 13 , and the lack of coherent long-term observations in other DWBCs. The only previous coherent current meter measurements of AABW export by a DWBC south of 45°S were obtained north of the Falkland Plateau, where a net transport of 1.9 Sv of Weddell Sea Deep Water (θ < 0.2°C) was found to enter the Argentine Basin (the difference between 8.2 Sv westward and 6.3 Sv recirculating to the east) 8 . In comparison, the net transport of water with θ < 0.2°C by the Kerguelen DWBC is 8.0 Sv (16.4 Sv to the northwest and an 8.4 Sv recirculation to the southeast).…”