The Antarctic Circumpolar Current between the Falkland Islands and South Georgia

Abstract

Hydrographic and lowered acoustic Doppler current profiler data along a line from the Falkland Islands to South Georgia via the Maurice Ewing Bank are used to estimate the flow of circumpolar water into the Argentine Basin, and to study the interaction of the Antarctic Circumpolar Current with the Falkland Plateau. The estimated net transport of 129 21 Sv (Sv 10 6 m 3 s 1) across the section is shared between three major current bands. One is associated with the Subantarctic Front (SAF; 52 6 Sv), and the other two with branches of the Polar Front (PF) over the sill of the Falkland Plateau (44 9 Sv) and in the northwestern Georgia Basin (45 9 Sv). The latter includes a local reinforcement (20 Sv) by a deep anticyclonic recirculation around the Maurice Ewing Bank. While the classical hydrographic signature of the PF stands out in this eastbound branch, it is less distinguishable in the northbound branch over the plateau. Other circulation features are a southward entrainment of diluted North Atlantic Deep Water from the Argentine Basin over the eastern part of the Falkland Plateau, and an abyssal anticyclonic flow in the western Georgia Basin, opposite to what was generally assumed. The different behavior of the SAF and PF at the Falkland Plateau (no structural modification of the former and partitioning of the latter) is attributed to the PF being deeper than the sill depth on the upstream side of the plateau, unlike the SAF. It is suggested that the partitioning takes place at a location where the 2500-m and 3000-m isobaths diverge at the southern edge of the plateau. The western branch of the PF crosses the plateau at a distance of 250 km to the east of the SAF. Comparison with a section across the Falkland Current farther downstream shows that its deep part subsequently joins the SAF on the northern side of the plateau where the 2000-3000 m isobaths converge in the steep Falkland Escarpment. The result of this two-stage bathymetric effect is a net transfer of at least 10 Sv from the PF to the SAF at the crossing of the Falkland Plateau.

Mesoscale eddies in the Subantarctic Front-Southwest Atlantic

“…The vertical structure of low temperature centred at XBT 17, with a minimum of < 1.5 °C located at about 200 m depth, suggests that this cold eddy may have been shed from the PF, as it interacts with the bottom topography. This temperature distribution is similar to that found further north along 51 o S (Arhan et al, 2002).…”

Circulation and Water Mass Modification in the Brazil–Malvinas Confluence

“…This branch flows alongside the BC overshoot, staying in contact with NADW for an extended period and facilitating mixing between NADW and CDW. Peterson and Whitworth (1989), Arhan et al (1999), Wienders et al (2000), and Heywood and King (2002) also showed evidence of pronounced NADW influence in the CDW in the southern Argentine Basin.…”

“…4d) on the western side of the Maurice Ewing Bank (MEB), where evidence of mixing with saltier NADW is visible. At this location, Arhan et al (2002) identified an anticyclonic recirculation around the Maurice Ewing Bank that entrains NADW across the Falkland Plateau. Colder and fresher CDW that entered the Georgia Basin by circumnavigating South Georgia (SG) rather than overflowing the North Scotia Ridge ) is present at station 60 on the A23 section (Fig.…”

Modeling a turbulence effect in formation of the Antarctic Circumpolar Current

“…The approach stems from the momentum balance with the momentum transfer by shear from the surface to the bottom, complemented by the meridional overturning (Johnson and Bryden, 1989;Morrow et al, 1992;Gille, 1997;Tansley and Marshall, 2001, etc.). An alternative approach by Wyrtki (1960), Baker (1982), Hellerman and Rosenstein (1983) and Mestaz-Nunez et al (1992) is based on the Sverdrup balance. Both approaches are still intensively debated (Warren et al, 1996;Olbers, 1998;Gnanadesikan and Hallberg, 2000;Rintoul et al, 2001;Karsten et al, 2002).…”

“…The latter is estimated from direct velocity measurements in the Drake Passage and in the southwestern Argentine Basin, above 3000 m depth, by different authors to be about 140 Sv while the geostrophic transport predicted by the geostrophic balance relative to 3000 m is only about 100 Sv. A summary of the net transport estimates based on the direct velocity measurements and on the geostrophic balance relative to 3000 m depth can be found in Whitworth et al (1982); Arhan et al (2002). The difference is commonly explained by an eastward barotropic flow velocity constituent not predicted by the geostrophic balance (Whitworth et al, 1982).…”