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Circulation and Water Masses of the Southern Ocean: a Review Developments in Earth & Environmental Sciences, 8 F. Florindo and M. Siegert (Editors) r 2009 Elsevier B.V. All rights reserved DOI 10.1016/S1571-9197(08)00004-9 Chapter 4 Circulation and Water Masses of the Southern Ocean: A Review Lionel Carter1,Ã, I. N. McCave2 and Michael J. M. Williams3 1Antarctic Research Centre, Victoria University, P.O. Box 600, Wellington, New Zealand 2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK 3National Institute of Water and Atmospheric Research, P.O. Box 14901, Wellington, New Zealand ABSTRACT The Southern Ocean is a major component of Earth’s ocean and climate. Its circulation is complex, with a zonal Antarctic Circumpolar Current (ACC) interacting with a meridional thermohaline circulation. The ACC is a highly variable, deep-reaching eastward flow driven mainly by the westerly winds. It is the longest (24,000 km), largest (transport 137–147.106 m3 sÀ1) and only current to connect the major oceans. The Ekman component of the westerly winds also drives surface waters north. Near the ACC’s northern limit, these waters sink to form Subantarctic Mode and Antarctic Intermediate waters, which continue north at depths oB1,400 m. Interacting with the ACC is the density-forced thermohaline circulation. Super cooling and increased salinity of shelf waters off the Weddell, Wilkes Land and Ross coasts cause these waters to sink and flow equatorwards. The densest component, Antarctic Bottom Water, is captured in deep basins around Antarctica. Less dense water is entrained by the ACC and mixed with deep water moving south from the Atlantic, Indian and Pacific oceans. The resultant Lower Circumpolar Deep Water is tapped off by deep western boundary currents that enter the three oceans at depths WB2,000 m. These northward inflows, with a total volume transport of B55.106 m3 sÀ1, disperse Antarctic and ÃCorresponding author. Tel.: þ64 4 463 6475; Fax: þ64 4 463 5186; E-mail: [email protected] (L. Carter). 86 L. Carter et al. northern-sourced waters throughout the world ocean. Other circulation elements are the deep-reaching, cyclonic Weddell, Ross and unnamed gyres located south of the ACC. Further south again are the westward Antarctic Slope and Coastal currents that pass along the Antarctic continental margin under easterly polar winds. 4.1. 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),‘y 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 surround- ing 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 Circulation and Water Masses of Southern Ocean 87 180° 150°E 150°W New Zealand Chatham Rise Campbell Plateau S. Tasman Rise ° 120°E 120 W Pacific Antarctic R. Ross G. E. Pacific Rise . G d e m a n - ° n 90°W 90 E U Drake Passage 70˚S 60°S F. Pl. ell G. 50°S Wedd 40°S ° 60°W 60 E SW Indian R. SB Mid Atlantic R. SAF Legend Depths <3500 m 30°E 30°W ACC 0° Main DWBC inflow 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). 88 L. Carter et al. ACC ASF SB SF PF SAF STF 0 SAMW AAIW 1000 UCDW 2000 3000 NADW AABW DEPTH(m) Antarctic Mid-Ocean 4000 Margin Ridge LCDW 5000 70°S 60°S 50°S 40°S 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 (gnW28.27 kg mÀ3). 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. 4.2. Water Mass Formation and Dispersal 4.2.1. Surface Ocean A series of ocean fronts – narrow, variable bands defined by abrupt changes in water properties, in particular, temperature and salinity – divide the surface waters of the Southern Ocean into several zones (e.g. Gordon, 1975; Deacon, 1982; Whitworth, 1988). Early studies identified (from south to north) the Polar, Subantarctic and Subtropical fronts (Fig. 4.2). More recent hydrographic transects, especially those carried out during the World Ocean Circulation Experiment (WOCE), have revealed additional boundaries located south of the Polar Front, and termed the ‘southern’ and ‘southern Circulation and Water Masses of Southern Ocean 89 boundary’ fronts (Figs. 4.2 and 4.3; Orsi et al., 1995; Orsi and Whitworth, 2005). Furthermore, these detailed and sometimes repeated transects, along with satellite-borne observations of ocean height, temperature and drifter tracks, reveal the complex and dynamic character of the frontal systems 180° 150°E 150°W 40°S Tasman STF Basin ° SW Pacific 50 S Basin ° 120°E SAF 120 W P14 60°S in s a ° e B 70 S SF Ris c cific ti . Pa c E r ta n -A n a SE Pacific li a Basin tr s u A PF 90°E 90°W n ele gu er au K ate Pl Enderby l Basin el Crozet dd e n Basin W si Ba 60°E SB 60°W Madg. Basin Argentine Mozb. Basin Basin S2 Legend 30°E Cape Basin 30°W Depths <3500 m 0° Figure 4.3: Location of the principal ocean frontal systems in the Southern Ocean (based on Orsi et al., 1995, but modified for the New Zealand region according to Carter et al., 1998 and Morris et al., 2001). Repeated hydrographic transects, satellite observations and drifting floats reveal the frontal systems as dynamic features with marked temporal and spatial variability but generally within the constraints imposed by the ocean floor topography (Moore et al., 1999). P14 and S2 are locations of WOCE hydrographic transects portrayed in Figs. 4.4 and 4.5. Madg., Madagascar Basin; Mozb., Mozambique Basin. Names of fronts are given in Fig. 4.2. The base chart is modified from Orsi and Whitworth (2005). 90 L. Carter et al. (Hofmann, 1985; Davis, 1998; Moore et al., 1999; Rintoul et al., 2001; Kostianoy et al., 2004; Sokolov and Rintoul, 2007). While cognizant of these complexities, the main fronts can still be used to define the distribution of three major surface waters characterised mainly by their potential temperature (y), salinity (S) and oxygen content (see hydrographic charts in Orsi and Whitworth, 2005). (1) Near-freezing and relatively fresh Antarctic Surface Water (AASW) forms a layer about 100 m thick that extends from the Antarctic continental shelf to the Polar Front, commonly defined as the northernmost extent of the subsurface temperature minimum (Belkin and Gordon, 1996; Figs.
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