OCEANOGRAPHY: DEEP CURRENTS
The supply of deep water to the Pacific Ocean is dominated by a single source, the deep western boundary current (DWBC) that flows north out of the Southern Ocean along the east side of the Campbell Plateau-Chatham Rise-Hikurangi Plateau, east of New Zealand (Figs. 1, 5). The volume transport of the DWBC is ~20 x 106 m3 s-1 (Sv), which composes ~40% of the total input of deep water to the world's oceans (Warren, 1973; 1981). A secondary, but minor, flow of ~3 Sv of deep-water flows north into the Peru-Chile basin (Lonsdale, 1976). The magnitude of DWBC flow, and the low temperature of the water involved, are major determinants of the oceanography of the Pacific Ocean and of the global heat balance. Monitoring the DWBC flow at its entry into the Pacific is a key area where the "global salt conveyor" hypothesis (Gordon, 1986; Broecker et al., 1990; Schmitz, 1995) can be tested, as the flow thereafter is believed to spread out to fill the Pacific. Some water upwells and returns at shallower depths across the Indian Ocean and on to the Atlantic, whereas other waters return south as North Pacific Deep Water (NPDW).
The supply of cold water to the deep Pacific from the main generating regions in the Weddell and Ross Seas is modulated by the ACC, which mixes these waters with North Atlantic Deep Water (NADW) in the South Atlantic to form Circumpolar Deep Water (CDW). Deep water output to the Pacific therefore carries the combined signatures of Southern Ocean processes in the region of deep water formation, chemical composition related to Southern Ocean gas exchange, and NADW. Despite its turbulent passage around Antarctica, CDW is not completely mixed, and a distinct NADW salinity maximum can be recognized at depths of 2800 m (at 55°S) deepening northward to 3400 m (at 28°S). In the Southwest Pacific, the DWBC comprises three main divisions: lower CDW, a mixture of bottom waters generated around Antarctica, in particular cold Weddell Sea Deep Water; salinity-maximum middle CDW, representing the NADW core; and strongly nutrient-enriched and oxygen-depleted upper CDW, mainly derived from Indian Ocean outflow added to Pacific outflow returning through Drake Passage (Fig. 5). The DWBC has its upper boundary at depths around 20002500 m. On the eastern side, the DWBC is overlain between 2550 and 1450 m depth by south-flowing NPDW, marked by an oxygen minimum and high silica. Regionally, both DWBC and NPDW are overlain by low salinity, AAIW (Figs. 1B, 5).
The ACC-DWBC enters the Southwest Pacific around and through gaps in the Macquarie Ridge complex before passing along the 3500 m high margin of the Campbell Plateau. Near the mouth of the Bounty Trough, the ACC uncouples and continues its eastward path, whereas the DWBC flows north around the eastern end of Chatham Rise and through Valerie Passage, where a small part of the flow diverges through gaps in the Louisville Seamount chain (McCave and Carter, 1997). Valerie Passage, the 250-km-wide gap between the Chatham Rise and the Louisville Ridge, therefore marks the gateway to the Pacific for the DWBC (Warren, 1973).
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