Circumpolar Deep Water

Figure 13-9 Schematic views of (A) meridional and vertical transport of nitrate in the upper Atlantic basin and (B) associated horizontal transport pathways. Nitrate-rich Circumpolar Deep Water (CDW) upwells in the Southern Hemisphere and the residual mean flow transports some of this upwelled water across the polar front into the regions of intermediate and mode water formation. Nitrate-rich Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water move northward in the thermocline, ultimately outcropping in the subpolar North Atlantic. Ekman transfer provides a source of nitrate to the subtropical gyre along with lateral and vertical eddy transport processes.

Lea D. W. (1993) Constraints on the alkalinity and circulation of glacial circumpolar deep water from benthic foraminiferal barium. Global Biogechem. Cycles 7, 695 -710. 

Pacific intermediate and deep water dominantly display high values, of s d = to —4 (Piepgras and Wasserburg, 1982 Piepgras and Jacobsen, 1988 Shimizu et al., 1994). Near southern Chile, and in the west-central Pacific, where circumpolar deep water moves northward, neodymium-isotope ratios display lower values closer to those typical of circumpolar seawater. In the central Pacific the circumpolar neodymium-isotope signal can be detected as far north as —40° N (Shimizu et al., 1994). 

After isolation from the atmosphere a water mass ages, and as it reflects biological cycling its S Cdic decreases and its nutrient content increases (Kroopnick 1985). Values of deep-water mass S Cdic range from 1.2%o in the core of North Atlantic Deep Water, via 0.4%o in Circumpolar Deep Water, to — 1.0%o in northem Pacific Deep Water. 

Figure 6 Zonal section of A C in the South Pacific collected during the WOCE program. The two minima at 2000-2500 m depth are thought to be the core of southward-flowing North Pacific Deep Water. Northward-flowing Circumpolar Deep Water is identified by the relatively high values in the Kermadec Trench area at the bottom between 140°W and the Date Line.

In remote regions of the open ocean that receive little atmospheric input, the vertical transport of iron-rich deep waters is particularly important [73,156]. The primary source of iron to the Fe-limited Southern Ocean is entrainment of Fe-enriched water during deep winter mixing, augmented by the summer upwelling of circumpolar deep water at the Polar Front and the southern front of the Antarctic Circumpolar Current [119,155,157]. 

The conveyor belt is shown schematically in Fig. 10-11. Warm and salty surface currents in the western North Atlantic (e.g., the Gulf Stream) transport heat to the Norwegian-Greenland Seas where it is transferred to the atmosphere. This heat helps moderate the climate of northern Europe. The cooling increases the density resulting in formation of the now cold and salty North Atlantic Deep Water (NADW) (Worthington, 1970). The NADW travels south through the North and South Atlantic and then joins the Circumpolar Current that travels virtually unimpeded in a clockwise direction around the Antarctic Continent. 

Deep water also forms along the margins of Antartica and feeds the Circumpolar Current. The Weddell Sea, because of its very low temperature, is the main source of Antarctic Bottom Water (AABW) which flows northward at the very bottom into the South Atlantic, and then through the Verna Channel in the Rio Grande Rise into the North Atlantic. It ultimately returns southward as part of the NADW. 

The Circumpolar Current is a blend of waters of North Atlantic ( 47%) and Antarctic margin ( 53%) origin (Broecker, 1997). This current is referred to as the Pacific Common Water and is the source of deep water to the Indian and... 

The NO3 content of upweUing water is strongly influenced by the location of a system (Codispoti et al., 1982) in the global oceanic conveyer belt that transports deep water through the ocean basins (Broecker and Peng, 1982, their Fig. 1.12). From the North Atlantic to the Pacific and Indian Oceans through the Antarctic Circumpolar Current, there are increasing nutrients, from the faUout of surface productivity. 

Neodymium-isotope ratios of intermediate and deep water in the Indian Ocean are intermediate to the Atlantic and Pacific. They generally fall between s d = 1 to —9, and are consistent with domination by northward flowing circumpolar water (Bertram and Elderfield, 1993 Jeandel et al., 1998). A depth profile east of southern Africa (Figure 7) displays the same zig-zag pattern as South Atlantic intermediate and deep water, reflecting advection of NADW into the western Indian Ocean (Bertram and Elderfield, 1993). 

A useful apphcation of preformed nutrient concentrations is that they are intrinsic to different water masses and sometimes can be used as conservative tracers. For example, the main sources of deep water in the Pacific Ocean are North Atlantic Deep Water (NADW), Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW), all of which are at least partly homogenized in the Antarctic Circumpolar Water (AACW). It is not possible to determine how much of each of these sources contributes to Pacific deep water by using end member mixing of the conservative properties temperature and salinity because salinities of the end members are not sufficiently different. Since concentrations of DIP are well above detection limits in... 

As the Antarctic Bottom Water flows north, it gradually mixes with the southward flowing North Atlantic Deep Water, which lies immediately above. As the North Atlantic Deep Water flows to the south, it incorporates not only the Antarctic Bottom Water but also the Mediterranean Water and the Antarctic Intermediate Water which lie above. The North Atlantic Deep Water is eventually entrained into the Antarctic Circumpolar Current and flows unimpeded into the Indian and Pacific Oceans. 

The deep-water temperature cooling across the Eocene/Oligocene boundary (Figure 7B) was not accompanied by tropical cooling, and resulted from the first step in the thermal isolation of Antarctica. In modern ocean, the Antarctic Circumpolar Current is a vigorous surface-to-bottom current that provides... 

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