Thermohaline Circulation

Wunsch (2002) drew attention to the imprecise meaning of the term, THC. Following Jacobs (2004), we prefer the broad definition of Schmitz (1995) whereby the THC is the '... buoyancy-driven flow field associated with water cooled (or heated) by contact with cold (or warm) air, or modified by sources and sinks of cold water. May also include flows whose characteristics are significantly altered by upwelling and/or mixing. Water sinking at high latitudes tends to return equatorwards in relatively strong, narrow currents called DWBC. The high latitude sinks noted are primarily the North Atlantic and Antarctica (Stommel, 1958; Warren, 1981). As described earlier, dense water formed mainly over shelf areas of the Weddell and Ross seas, and along the Wilkes Land coast, sinks and descends down the continental slope. Descent may take place in several ways according to Baines and Condie (1998). For prominent deep-water formation areas, dense water may descend as broad sheets or plumes. For weaker sources, the resultant outflows may geostrophically descend the slope to a level where the layer thins and viscous drainage prevails. Descending flows may be disrupted by eddies into discrete parcels that overall move down slope. Further complexities are introduced by the slope topography, for example, submarine canyons and channels can contain and steer flows whose density may increase through entrainment of sediment. Whatever the mode of descent, the initial dispersal of AABW is westward as outlined by the distribution of chlorofluorocarbons (Orsi et al., 1999). Those tracers show that the densest AABW usually follows a cyclonic path presumably under the influence of the subpolar gyres and basin topography (Fig. 4.1). However, the WeddellEnderby Basin experiences limited outflow at its northern rim where least dense AABW (yn~28.28kgm~3) passes north into adjoining basins of the Atlantic and Indian oceans (Fig. 4.6; Orsi et al., 1999). In contrast, the main inflows to the adjoining oceans are via deep western boundary currents, which carry mainly LCDW from the northern boundaries of the ACC (Mantyla and Reid, 1983; Schmitz, 1995).

4.3.3.1. Atlantic DWBC inflow

Weddell Sea Bottom Water, a local variety of AABW, together with Weddell Sea Deep Water and CDW from the Drake Passage, are carried by a northbound DWBC into the Atlantic Ocean below southward-moving NADW (Warren, 1981; Schmitz, 1995). The DWBC is typically found in water depths exceeding 3,500-4,000 m along the western boundary presented by the continental margin off South America. However, the current pathway is interrupted by a succession of deep basins including (from south to north) Georgia, Argentine and Brazil basins. As noted earlier, densest AABW is captured within the deep basins leaving less dense waters to move north via gaps and channels through the inter-basin ridges. Even so, not all the deep water escapes and some recirculates within the basins themselves (Hogg and Johns, 1995; Hogg, 2001). For example, around 6.9 Sv flows from the

Argentine Basin into the Brazil Basin through Vema and Hunter channels, but only 3.2 Sv leaves the Brazil Basin, leaving 3.7 Sv to recirculate.

4.3.3.2. Indian DWBC inflow

The inflow of AABW into the western Indian Ocean is via Crozet and Mozambique basins (the latter being a dead end), which largely constrain AABW to south of 30°S and 34°S, respectively (Orsi et al., 1999). LCDW from the Crozet Basin leaks northwards through fractures zones in the SW Indian Ridge (Warren, 1974; Johnson et al., 1991; Mantyla and Reid, 1995). Deep transport is via a northbound DWBC, which like its Atlantic counterpart, has a complex pathway dictated by multiple basins and ridges (Warren, 1981; McCave et al., 2005). Basins encourage recirculation of deep waters and together with mixing, account for a northward dissipation of the DWBC. For instance, LCDW below ~ 3,800 m has a northwards transport of 3.8 Sv through Madagascar Basin, but only 1.7 Sv exits north into the Somali Basin (Johnson et al., 1998).

Crozet Basin, in the western Indian Ocean, is not the only gateway for DWBCs into the Indian Ocean. The eastern Indian Ocean is also connected to the Southern Ocean thereby allowing the northward intrusion of two other boundary currents; one along the eastern side of SE Indian Ridge and the other along the eastern flank of Ninetyeast Ridge (Warren, 1981; Toole and Warren 1993; Reid, 2005). Again, the deep water carried north is LCDW, with the denser AABW retained in Australian-Antarctic Basin (Orsi et al., 1999). When extended to the 2,000 dbar reference level, the combined northward transport of the three DWBCs into the Indian Ocean is ~ 27 Sv (Toole and Warren, 1993).

4.3.3.3. Pacific DWBC inflow

The general pathway of the DWBC into the Pacific was first outlined in the classic model of the global abyssal circulation by Stommel (1958) and Stommel and Arons (1960), and was later confirmed by the hydrographic sections of Warren (1971, 1973). Because the Tasman Basin is essentially closed at its northern end, the main inflow is off southernmost New Zealand where the DWBC enters in concert with the ACC (Fig. 4.7; Carter and McCave, 1997; Carter and Wilkin, 1999). Initially, the combined inflow intercepts Macquarie Ridge to form meanders and eddies although some current filaments pass through narrow gaps in the ridge (Boyer and Guala,

Circulation Thermohaline

Figure 4.7: A schematic portrayal of the THC, which is a series of loosely linked, recirculation systems that transport, heat, salt, nutrients and ventilating gases through the world ocean. Only the main elements of the THC are shown, for example the Indian Ocean has three, deep northward inflows that include from W to E, the margin off eastern Africa (shown), and the eastern sides of the SE Indian Ridge and Ninetyeast Ridge (not shown) (image modified from Manighetti, 2001).

Figure 4.7: A schematic portrayal of the THC, which is a series of loosely linked, recirculation systems that transport, heat, salt, nutrients and ventilating gases through the world ocean. Only the main elements of the THC are shown, for example the Indian Ocean has three, deep northward inflows that include from W to E, the margin off eastern Africa (shown), and the eastern sides of the SE Indian Ridge and Ninetyeast Ridge (not shown) (image modified from Manighetti, 2001).

1972; Gordon, 1972). This perturbed combined flow continues northeast along the 3,000-3,500 m high flanks of Campbell Plateau to around 49°S where the ACC veers east leaving the DWBC to continue northwards into the Pacific Ocean (Fig. 4.7). It eventually departs the Southern Ocean off Chatham Rise between 44°S and 42°S (McCave and Carter, 1997). There, Warren (1973) observed a volume transport of ~20Sv, the largest for a single DWBC (Schmitz, 1995).

0 0

Post a comment