Antarctic Circumpolar Current

The Southern Ocean circulation is dominated by the ACC, a current system that is rightly described by superlatives. It is the only current to connect the major ocean basins and hence plays a prominent role in the global distribution of heat, salt and gases (Fig. 4.1). It is the longest current with an estimated pathway of -24,000 km (Whitworth, 1988). Finally, the ACC is the largest major current in terms of volume transport with a mean of 136.777.8 Sv as measured across Drake Passage (Cunningham et al., 2003).

The ACC was originally termed the West Wind Drift (Deacon, 1937) in recognition of its forcing by middle-to-high latitude westerly winds (Orsi et al., 1995; Whitworth et al., 1998). However, use of the term wind drift masks the role played by the buoyancy-driven component of the circulation (see Rintoul et al., 2001). Complexities aside, the net result is an eastward current system that extends from the ocean surface to bottom, its path guided by submarine topography (Fig. 4.1; Gordon et al., 1978; Orsi et al., 1995). For much of its passage, the ACC flows along the flanks of mid-oceanic ridges except within major gaps in the Pacific and Indian ridge systems where the current shifts poleward (Fig. 4.1). Large submarine plateaux also exert an influence. The ACC widens to the north and south as it passes around the Kerguelen Plateau, whereas the Campbell and Falkland plateaux form constrictions (Fig. 4.1; Whitworth and Peterson, 1985; Morris et al., 2001). Interestingly, this interaction with the ocean floor was inferred as early as the 1950s. Estimates for a purely wind-driven ACC yielded transports that were excessive compared to observations. Thus, it was concluded that the wind stress was partly balanced by bottom stress (see Whitworth, 1988; Rintoul et al., 2001). The passage of westerly winds over the ACC also induces an Ekman drift to the north - a process that probably plays a role in the subduction and transport of mode and intermediate waters. This equatorward flow is compensated at depth by the southward transport and eventual upwelling of CDW thus contributing to the THC (Wyrtki, 1961; Toggweiler and Samuels, 1995).

Rather than a uniform flow, the ACC is a system of deep-reaching zonal jets that separates zones of relatively quiet water. The jets are marked by the circumpolar fronts (see Section 4.2 Surface Ocean) with the northern and southern boundaries of the ACC defined, respectively, by the Subantarctic and Southern Boundary fronts (Figs. 4.1-4.3). Both eddy-resolving models and satellite observations highlight the complex flow of the frontal jets (Morrow et al., 1992; Gille, 1994; Carter and Wilkin, 1999). Meanders, eddies and intricate branches are well shown especially where the flow is constricted as off Campbell Plateau and within Drake Passage (Nowlin and Klinck, 1986; Morris et al., 2001; Cunningham et al., 2003).

Most ACC transport takes place within the fronts, but their complex flow patterns and different criteria for estimating transport have led to a wide range of values for the entire ACC. Nevertheless, closely spaced and long-term monitoring sites have improved estimates of transport. In Drake Passage the mean transport is 136.777.8 Sv (Cunningham et al., 2003) compared to 147 7 10 Sv between Australia and Antarctica (Rintoul and Sokolov, 2001). Much of the transport in the Australasian reach of the ACC occurs within the Subantarctic Front, which has a mean of 10577 Sv off Australia (Rintoul and Sokolov, 2001) and ~90Sv off southern New Zealand (Morris et al., 2001). However, ACC transport can be highly variable. Time series from Drake Passage record variations at several time scales (Whitworth and Peterson, 1985). Short-term fluctuations, related to 14-day solar and lunar tides, are superimposed on longer-period fluctuations of ~ 1 year and longer that can lead to changes in transport of ~ 30-40 Sv within weeks.

The interaction of the ACC with the topography and southerly extensions of western boundary currents generates eddies that play important roles in the transfer of heat and momentum (Bryden and Heath, 1985; Morrow et al.. 1992; Rintoul et al., 2001). Off SE New Zealand, for example interception of the ACC by the South Tasman Rise and Macquarie Ridge spawns bottom-reaching eddies that migrate NE along the steep margin of Campbell Plateau (Boyer and Guala, 1972; Gordon, 1972). Both cyclonic and anticyclonic features have been observed from current meter and satellite data, which suggest a frequency of occurrence of ~9 eddies annually (Stanton and Morris, 2004). Modelled eddy kinetic energy, verified by current meter data and ocean floor sedimentary evidence, attest to the power of these perturbations, which are likely to be the cause of abyssal benthic storms (Hollister and McCave, 1984; Carter and Wilkin, 1999). Seabed topography also encourages intense mixing within the ACC. The Scotia Sea and potentially other areas of marked seabed relief below the ACC, are zones of the highest turbulent mixing in the ocean and result in rapid upwelling that may locally short-circuit the classic meridional overturning as portrayed in Fig. 4.2 (Garabato et al., 2004, 2007).

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