Subpolar Circulation

4.3.2.1. Major gyres

A key element of the circulation, south of the ACC, is three large, deep-reaching cyclonic gyres that extend from the ACC to the Antarctic continental margin (Fig. 4.1). The better known are the Weddell and Ross gyres that occupy ridge-bounded sectors of the Weddell and Ross seas (e.g. Orsi et al., 1993; Jacobs et al., 2002). A third and as yet unnamed gyre was suggested to occur south of Kerguelen Plateau (Bindoff et al., 2000), and has now been confirmed by McCartney and Donohue (2007). As documented for the Weddell Gyre (Orsi et al., 1993), it is likely that all three gyres transport salt and heat from the ACC to the Antarctic continental margin where deep and bottom waters are produced (e.g. Jacobs, 2004). Furthermore, McCartney and Donohue (2007) suggest a strong connectivity between the three gyres with a westward flow along their southern limbs and an eastward flow joining their northern limbs. This latter flow is just south of another eastward flow, this time associated with an anticyclonic supergyre covering most of the S Pacific and S Indian oceans (Ridgway and Dunn, 2007). This eastward limb of the supergyre appears to reside between the Subtropical and Subantarctic fronts. As a result, it may entrain SAMW, formed in the vicinity of the Subantarctic Front and help distribute it through the ocean basins as suggested by Rintoul et al. (2001).

Of these cyclonic circulations, the Weddell Gyre is the largest, extending from ~50°W to between 20°-30°E (Fig. 4.1; Orsi et al., 1993). At the surface it has the form of a NE-SW aligned, elongated gyre whereas at depth it comprises two cyclonic cells located to the east and west of 15°W. Basically, the Weddell Gyre occupies the >4,000 m deep re-entrant formed by Antarctica and the ridge systems that extend eastward from near the tip of the Antarctic Peninsula (Fig. 4.1). While such a location implies containment, the northern limb of the gyre overlaps the ridges to interact with the ACC. CDW entrained from the ACC is moved within the gyre and can eventually mix with cold shelf waters to form Weddell Sea Bottom Water, the local variety of AABW and the densest water in the Southern Ocean (Foster and Carmack, 1976).

The Ross Gyre extends from 160°E to 140°W, and is largely confined to the > 4,000 m deep western reach of the SE Pacific Basin (Fig. 4.1). Like its Weddell Sea counterpart, the Ross Gyre is a deep-reaching feature that entrains CDW to make it available for mixing with shelf and slope waters.

Situated to the south and southwest of Australia, the unnamed gyre appears to favour the southern part of the Australian-Antarctic Basin, most of which is >4,000m deep (Fig. 4.1). Indeed, the volume transport field is compressed against the Antarctic continental margin and the associated westward slope current (Bindoff et al., 2000; McCartney and Donohue, 2007). Transport along the eastward-flowing northern limb of the gyre is estimated at 35 Sv. However, the amount of transport along the west-moving, southern limb is unclear due to merging with the slope current, the combined flows reaching 76 Sv (McCartney and Donohue, 2007).

4.3.2.2. Antarctic slope and coastal currents

As summarised by Heywood et al. (2004) much of the Antarctic margin is bathed by two westward currents. One is associated with the Antarctic Slope Front that constitutes the boundary between fresh, cold Antarctic shelf waters and less cold, saline CDW (Jacobs, 1991; Whitworth et al., 1998). Deacon (1937) regarded the frontal flow as a consequence of the prevailing easterly winds and coined the name, East Wind Drift. On the basis of classical theory, he reasoned that polar easterly winds produced an onshore Ekman transport with the resultant generation of a westward geostrophic current below the wind-mixed layer (Whitworth et al., 1998; Bindoff et al.. 2000). The second significant feature is the Antarctic Coastal Current, which forms a narrow, rapid flow across broad sections of the continental shelf, for example, in the SW Weddell Sea. However, where the shelf is narrow the coastal current is difficult to distinguish from flows associated with the Antarctic Slope Front and the southern limbs of the subpolar gyres where they approach the continental margin.

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