Water Mass Formation and Dispersal 421 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 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

Tasman Basin

SW Pacific

500S Basin

P14 600S

900E

SE Pacific Basin

900W

Crozet

Basin i

600E

Madg. Basin

Mozb. Basin

300E

Argentine

Cape Basin _

300W

Legend

U Depths <3500 m

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).

(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 (8), 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. 4.4 and 4.5). AASW temperatures are typically <0°C, but may rise to 2.5°C near the Front or where warm, deep water approaches the surface (Gordon, 1975; Deacon, 1982). Salinity (S) varies regionally with highest values of S> 34.3 psu found in the Ross and Weddell seas, whereas AASW elsewhere around Antarctica commonly has S< 34.0 psu. (2) Between the Polar and Subantarctic fronts resides surface water that is transitional between AASW and Subantarctic Surface Water (SASW). The structure is complex in response to mixing and interleaving of AASW as it sinks near the Polar Front (e.g. Gordon, 1975; Rintoul et al., 2001). Thus, properties are variable, but generally S is ~ 34.0-34.4 psu and 8 is 3-8°C (Fig. 4.5). (3) SASW occurs north of the Subantarctic Front and encompasses water that usually warms northwards from ~6°C to 12°C. Salinity is typically >34.3 psu except in the SE Pacific and Drake Passage where values decline to <34.16 psu. Like its more southern counterpart, SASW may be affected by vertical mixing as surface waters subduct and mix (e.g. Morris et al., 2001). The northern limit of subantarctic waters is the Subtropical Front where temperatures increase sharply by 4-5°C and salinity by 0.5 psu (Fig. 4.3; Deacon, 1982). Subtropical surface water prevails north of the Subtropical Front.

4.2.2. Subantarctic Mode Water and Antarctic Intermediate Water

Isopycnals - surfaces of constant density - of near-surface to deep waters in the Southern Ocean rise up in a step-like profile towards Antarctica. Close to ocean fronts, isopycnals may outcrop indicating either the rise of deep waters to the ocean surface or the descent of surface waters (Figs. 4.2, 4.4 and 4.5). In the case of the latter, winter cooling and mixing of the surface waters on the northern side of the Subantarctic Front forms Subantarctic Mode Water (SAMW) (McCartney, 1977; Morris et al., 2001; Rintoul et al., 2001). This well mixed, ventilated water descends northwards to ~500m depth along much of the front (Fig. 4.2). AAIW also descends from the surface, passing

70.0°S 65.0°S 60.0°S 55.0°S

density (C) from WOCE Line P14 across a major constriction in the ACC between the Ross Sea and New Zealand (see Fig. 4.3 for location). Isolines at the Antarctic margin indicate descent of dense shelf waters, which may be mixed with NADW-influenced, LCDW as suggested by the salinity field. The resultant AABW (cf. Fig. 4.5) is contained within the SW Pacific Basin. At the surface, north of the polar front, low salinity AAIW descends northwards (Fig. 4.4B). Hydrographic profiles were derived from the WOCE Southern Ocean Atlas at http://woceatlas.tamu.edu/. Names of fronts are given in Fig. 4.2.

Figure 4.4: (Continued).

under SAMW to reach a maximum depth of -1,400 m (Figs. 4.2 and 4.4). AAIW is identified by a salinity minimum (34.3-34.5 psu) and temperatures of — 3-7°C. However, the processes driving AAIW formation are unclear. Formation may be related to wind-forced or density-driven sinking of cold AASW and indeed there appears to be continuity between the AASW and AAIW salinity fields (Figs. 4.4 and 4.5). However, McCartney (1977) suggested AAIW may evolve at least in part from dense SAMW. Whatever the origin, compared to the widespread formation of SAMW, new AAIW presently appears to form mainly in the SW Atlantic and SE Pacific. From these sites AAIW circulates the oceans in anticyclonic subtropical gyres that extend north towards and locally beyond the equator before returning south as 'old' AAIW, which is transported within western boundary currents (Rintoul et al., 2001; Ridgway and Dunn, 2007).

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