Antarctic Bottom Water

In their analysis of abyssal water masses, Mantyla and Reid (1983) drew attention to the often inappropriate use of the term Antarctic Bottom Water, which has tended to be used generically for any southern-sourced bottom water. They demonstrated that true AABW did not extend far from Antarctica before mixing with other waters (see also Orsi et al., 1999). This is particularly true for the deep western boundary currents in which AABW is mixed with CDW derived from the ACC. Thus, at 30°N in the NW Atlantic, Amos et al. (1971) recorded <20% AABW near the seabed.

To better characterise AABW and thereby improve assessments of its dispersal and contribution to bottom waters worldwide, Orsi et al. (1999) defined AABW by its neutral density (gn) whereby gn>28.27kgm~3. Such dense waters are confined mainly to the deep (down to ~ 6,000 m), circum-Antarctic Basins that include the Argentine and Brazil basins (SW Atlantic), Mozambique, Crozet and Australian-Antarctic basins (Indian) and the SE Pacific Basin (Figs. 4.1, 4.4-4.6). Less dense, southern bottom water with 28.18 <gn <28.27 kg m-3, is not confined to the circum-Antarctic basins, but instead spreads out from the deep levels of the northern edge of the ACC into all major oceans (Orsi et al., 1999; also see Section 4.3.3).

AABW density is determined by a combination of different sources that produce regionally distinct waters (Mantyla and Reid, 1983; Orsi et al., 1999; Jacobs, 2004). The freshest and coldest (Sr 34.64 psu; 8r-1°C) bottom water occurs in the Weddell Sea, whereas the SE Pacific Basin has the most saline and least cold (S> 34.72 psu; — 0.6<8<-0.3°C) bottom water. The Australian-Antarctic Basin contains water with properties intermediate between the two end members. Traditionally, the Weddell Sea was regarded as the prime source of AABW, but recently two other sources have come to the fore. The Weddell Sea's contribution is now regarded as ~50%, with the Wilkes Land margin including the Adelie coast (Rintoul, 1998) contributing ~ 30% mainly to the Indian Ocean, and the Ross Sea producing ~20% AABW that is destined primarily for the SE Pacific Basin (Jacobs, 2004).

Equally varied are the modes of bottom water formation (Jacobs, 2004). Foster and Carmack (1976) invoked the formation of highly saline shelf water (HSSW) by brine rejection from sea ice. However, salt-driven increases in density may also be influenced by intrusions of NADW-bearing LCDW onto the upper slope and shelf (Toggweiler and Samuels, 1995). Super cold, Ice Shelf Water (ISW), formed by freezing and melting below ice shelves, can mix with HSSW and reach the outer shelf before flowing down slope (Baines and Condie, 1998). Alternatively, the simple mixing of cold AASW and high-salinity LCDW, with or without ISW, may produce negatively buoyant waters at the shelf edge (e.g. Jacobs, 2004). Finally, dense waters may sink via convection chimneys and polynyas such as the well-documented but short-lived Weddell Sea polynya (Gordon, 1982).

Rates of AABW formation, as estimated from hydrographic data, usually fall within a range of 5-15 Sv (1 Sv — 106m3s—1). Anthropogenic tracers, in particular chlorofluorocarbons, record a flux of 8.1 Sv for AABW descending at the 2,500 m isobath off Antarctica (Orsi et al., 2001). This compares to 7.6 Sv of lower NADW flowing out of the Nordic and Labrador seas at the N Atlantic source. Because AABW is colder than its northern counterpart, Antarctic overturning probably plays the dominant role in cooling the deep ocean. The time at which deep water circulates through the ocean is often quoted to be a millennium or more, for example 1,000 years between the N Atlantic and Southern Oceans and a further 1,000 years from the Southern

Figure 4.6: Extent of dense Antarctic Bottom Water as identified by the neutral density field at 3500 m (Orsi et al., 1999; Orsi and Whitworth, 2005). With the exception of the SE Atlantic, where AABW extends well north via the deep Argentine and Brazil basins, the remaining AABW is captured in circum-Antarctic basins where further northwards dispersal is inhibited by oceanic ridge systems shown in white. Basins annotated as Arg. B., Argentine Basin, which extends north into the Brazil basin (not shown on chart); W.B., Weddell Basin; E.B., Enderby Basin; C.B., Crozet Basin; Aust. A.B., Australian-Antarctic Basin, and SE P.B., SE Pacific Basin. Chart is generated from the WOCE Southern Ocean Atlas at http://woceatlas.tamu.edu/.

Figure 4.6: Extent of dense Antarctic Bottom Water as identified by the neutral density field at 3500 m (Orsi et al., 1999; Orsi and Whitworth, 2005). With the exception of the SE Atlantic, where AABW extends well north via the deep Argentine and Brazil basins, the remaining AABW is captured in circum-Antarctic basins where further northwards dispersal is inhibited by oceanic ridge systems shown in white. Basins annotated as Arg. B., Argentine Basin, which extends north into the Brazil basin (not shown on chart); W.B., Weddell Basin; E.B., Enderby Basin; C.B., Crozet Basin; Aust. A.B., Australian-Antarctic Basin, and SE P.B., SE Pacific Basin. Chart is generated from the WOCE Southern Ocean Atlas at http://woceatlas.tamu.edu/.

Ocean to the North Pacific. However, in a re-analysis of radiocarbon dated ocean waters, Matsumoto (2007) indicates much shorter circulation ages thereby supporting but refining earlier radiocarbon-based studies (Stuiver et al., 1983). Thus, the circulation age for the Southern Ocean below 1,500 m is — 300 14C years with a similar age for the Atlantic. For the Pacific, the basin circulation age is — 900 14C years.

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