Introduction

The Southern Ocean has a profound influence on the world's ocean and climate. Cold, dense water sinks to abyssal depths around the margins of Antarctica and migrates northwards into the Atlantic, Indian and Pacific oceans via deep western boundary currents (Fig. 4.1; Stommel, 1958; Warren, 1981). As succinctly noted by Warren (1971), '... water from the Antarctic is largely responsible for keeping the rest of the deep sea cold'. Through a process of slow upwelling, these deep cold waters rise to the upper ocean. There, they contribute to the warm surface circulation that extends west from the Pacific and Indian Oceans into the Atlantic where the warm, saline water moves north. Approaching high northern latitudes, the water cools and sinks to form North Atlantic Deep Water (NADW), which migrates south, sandwiched between northbound Antarctic Intermediate Water (AAIW) above and Antarctic Bottom Water (AABW)/Lower Circumpolar Deep Water (LCDW) below (Fig. 4.2). En route, NADW mixes with other waters and eventually rises at the Antarctic continental margin. Thus, one cycle of the global thermohaline circulation (THC) - a major regulator of Earth's ocean and climate - is completed and another cycle begins (e.g. Broecker, 1991; Schmitz, 1995; Rahmstorf, 2002).

This powerful and far-reaching influence of Antarctica and the surrounding Southern Ocean largely reflects; (i) the strong buoyancy-driven and meteorologically forced circulations, and (ii) their direct access to the major ocean basins via the Antarctic Circumpolar Current (ACC) and its offshoots, the deep western boundary currents (Fig. 4.1; Moore et al., 1999; Orsi et al., 1999; Rintoul et al., 2001). In this brief synopsis we can only provide a flavour of over 70 years of oceanographic research in the Southern Ocean. Thus, we refer the reader to the reference list for a more detailed insight into the workings of this region. We present the basic elements under two sections: (1) Section 4.2 examines the main water masses, focusing on their properties and the mechanisms that control their distribution, and (2) Section 4.3 reviews the structure and dynamics of the world's largest ocean current, the ACC, together with that of the subpolar gyres and

Figure 4.1: The main Oceanographic elements of the Southern Ocean including: (i) the ACC contained by the Subantarctic Front (SAF) and southern limit of UCDW or southern boundary (SB); (ii) the Ross, Weddell and unnamed subpolar gyres; and (iii) the main exit points of deep western boundary currents from the Southern Ocean (blue arrows). The general path for the ACC is from Orsi et al. (1995) with modifications based on Heath (1985) and Morris et al. (2001). Bathymetric elevations are Annotated as R., ridge; K. Pl., Kerguelen Plateau; and F. Pl., Falkland Plateau. The base chart is Modified from Orsi and Whitworth (2005).

Figure 4.1: The main Oceanographic elements of the Southern Ocean including: (i) the ACC contained by the Subantarctic Front (SAF) and southern limit of UCDW or southern boundary (SB); (ii) the Ross, Weddell and unnamed subpolar gyres; and (iii) the main exit points of deep western boundary currents from the Southern Ocean (blue arrows). The general path for the ACC is from Orsi et al. (1995) with modifications based on Heath (1985) and Morris et al. (2001). Bathymetric elevations are Annotated as R., ridge; K. Pl., Kerguelen Plateau; and F. Pl., Falkland Plateau. The base chart is Modified from Orsi and Whitworth (2005).

currents residing south of the ACC, and the deep THC. The chapter ends with a discussion in Section 4.4 of the present debate regarding the Southern Ocean's response to a rapidly warming climate (e.g. Gille, 2002; Curry et al., 2003; Jacobs, 2004; IPCC, 2007).

ASF SB SF PF SAF STF

ASF SB SF PF SAF STF

70S 60S 50S 40S

LATITUDE

Figure 4.2: Schematic section of the main water masses and their meridional transport as compiled from Whitworth (1988); Orsi et al. (1995); Speer et al. (2000) and Rintoul et al. (2001). Water masses are SAMW, Subantarctic Mode Water; AAIW, Antarctic Intermediate Water; UCDW, Upper Circumpolar Deep Water; LCDW, Lower Circumpolar Deep Water; NADW, North Atlantic Deep Water; AABW, 'true' Antarctic Bottom Water (yn>28.27kgm~3). Frontal systems are ASF, Antarctic Slope Front; SB, Southern Boundary of the ACC; SF, Southern Front; PF, Polar Front (formerly the Antarctic Convergence), SAF, Subantarctic Front; STF, Subtropical Front (formerly Subtropical Convergence). The flow of ACC is directed towards the reader.

70S 60S 50S 40S

LATITUDE

Figure 4.2: Schematic section of the main water masses and their meridional transport as compiled from Whitworth (1988); Orsi et al. (1995); Speer et al. (2000) and Rintoul et al. (2001). Water masses are SAMW, Subantarctic Mode Water; AAIW, Antarctic Intermediate Water; UCDW, Upper Circumpolar Deep Water; LCDW, Lower Circumpolar Deep Water; NADW, North Atlantic Deep Water; AABW, 'true' Antarctic Bottom Water (yn>28.27kgm~3). Frontal systems are ASF, Antarctic Slope Front; SB, Southern Boundary of the ACC; SF, Southern Front; PF, Polar Front (formerly the Antarctic Convergence), SAF, Subantarctic Front; STF, Subtropical Front (formerly Subtropical Convergence). The flow of ACC is directed towards the reader.

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