Water Masses and Circulation Pathways

By 1990 the basic elements of the subpolar circulation and the identities of the main water masses were established. First, consider the near-surface ocean, nominally the upper 1,000 m. At these depths, relatively warm (roughly, 8-15 °C), saline (35-36 psu) thermocline waters enter the Newfoundland and then the West European Basins in the North Atlantic Current (NAC). On the southern (warmer, T > 10 °C) side of the NAC some fluid separates to circulate south and leave the

Water), or another property such as stratification (for example, Sub-Polar Mode Water). Finally, the water mass definitions are not intended to be exclusive of each other. Table 1 should be read with these caveats in mind. In ambiguous cases, the reader should refer to the primary papers involved (see also McCartney (1992)).

subpolar region, for example, in the Portugal Current. On the cooler northwestern side of the NAC some fluid detrains and circulates along the Reykjanes Ridge to form the Irminger Current. The remaining NAC water flows past Ireland, the Faroes, and into the Norwegian Sea. Cold, fresh (roughly, T < 4 °C, S < 34.60 psu) water enters the area from the north mainly through Denmark Strait (the East Greenland Current) and Davis Strait (the Baffin Island Current). The East Greenland Current partly merges with the Irminger Current by Cape Farewell forming a relatively strong cyclonic boundary current system around the Irminger and Labrador Basins. Past Cape Farewell the outer part of the jet is called the Irminger Current, the inner part is called the West Greenland Current, and off Canada the system is collectively called the Labrador Current. As water moves around this circuit it is progressively cooled and freshened by air/sea exchange - especially in winter -and mixing with the northern source waters. Starting from cooler varieties of North Atlantic Central Water (NACW; T < 10 °C, S < 35.5 psu) the transformation in the T/S plane is to progressively denser types of Sub-Polar Mode Water (SPMW) and finally, in the southeastern Labrador Sea, Labrador Sea Water (LSW) at, or colder than, 4 °C (McCartney and Talley 1982). The mode waters (including LSW) are associated with 200-2,000 m deep well-mixed convective layers in late winter (Clarke and Gascard 1983) and so are weakly stratified and form a voluminous "mode" in the T/S plane. Near-surface flow out of the Labrador Sea in the Labrador Current follows the shelf-break and upper continental slope. Some of the Labrador Current detaches from the bathymetry and joins the NAC northwest wall near Flemish Cap, some flows as far as the Grand Banks before recirculating, and some passes south of Newfoundland into the Gulf of St. Lawrence through Cabot Strait.

At mid-depths (nominally 1,000-3,000 m) the subpolar gyre is dominated by the circulation of LSW. LSW is the densest of the subpolar mode waters formed by deep convection in the subpolar basins and exhibits local minima in both salinity and stratification. It is often taken to have a temperature between 3 °C and 4 °C and a salinity less than 34.94 psu (for example, see Worthington 1976). LSW is considered to be the lightest constituent of NADW and penetrates as far as the equatorial Atlantic along the western boundary (Talley and McCartney 1982; Weiss et al. 1985). The other influential mid-depth subpolar water mass is Mediterranean Water (MW). Pure MW overflows into the eastern North Atlantic through the Strait of Gibraltar at a temperature near 11.9 °C and salinity 36.50 psu (Wüst 1935). It is diluted with ambient water during descent into the deep sea then spreads out as a high salinity water mass with temperature above about 3 °C.

Talley and McCartney (1982) identify the main mid-depth transport pathways for LSW. Some LSW flows in the lower part of the Labrador Current described above. It splits near Flemish Cap where some is entrained into the deep NAC, while the remainder flows west round Grand Banks into the subtropics. The eastward flowing LSW in the deep NAC subsequently splits into a part that passes into the Irminger Sea (some LSW also seems to flow directly into the Irminger Sea from the convection area) and a part that crosses the mid-Atlantic Ridge into the West European Basin. There, some of the LSW is lost south to the subtropics while progressively mixing with MW. The rest circulates past Rockall and then back across the mid-Atlantic Ridge into the Irminger Basin. There is no evidence for LSW crossing the Iceland-Scotland Ridge into the Norwegian Sea (McCartney 1992; Dickson and Brown 1994). Finally, the LSW subpolar circuit is completed by recirculation into the Labrador Sea following the Irminger Current.

Three main water masses enter the subpolar region to occupy the deep and abyssal basins (nominally, deeper than 2,500 m). They are Denmark Strait Overflow Water (DSOW), Iceland-Scotland Overflow Water (ISOW) - both derived from the upper 1,000 m in the Nordic Seas - and Antarctic Bottom Water (AABW) from the south. (The Greenland-Scotland Overflows are also discussed in detail in Chapters 18-22 of this volume.) DSOW passes across the 600 m deep saddle between Greenland and Iceland with temperature and salinity in the range 0-2 °C and 34.88-34.93 psu, respectively (Warren 1981). ISOW crosses the 850 m deep saddle in the Faroe Bank Channel at a slightly higher temperature and salinity, around 1.8-3 °C and 34.9835.03 psu. Some water also crosses the Iceland-Faroe Ridge (at 450 m) and the Wyville-Thomson Ridge (at 500-600 m), but is less important than ISOW for the formation of NADW (see also Chapter 18). Both DSOW and ISOW descend into the abyssal ocean as bathymetry-following turbulent boundary currents. They both entrain substantial amounts of warmer, mid-depth, ambient water - mainly SPMW at temperatures > 8 °C for ISOW and > 6 °C for DSOW and LSW near 4 °C (McCartney 1992) - and approximately double their volume flux by the time they reach the abyssal floor. ISOW flows along the eastern flank of the Reykjanes Ridge and then passes through the 3,600 m deep Charlie-Gibbs Fracture Zone (near 52°N) into the Irminger Basin. It then circulates along the bathymetry to join the slightly denser DSOW although some water from the Charlie-Gibbs Fracture Zone may also enter the Labrador Sea directly (Clarke 1984).

AABW that has penetrated the North Atlantic is also an important component of the abyssal water mass structure. McCartney (1992) has shown how AABW enters the subpolar domain along the eastern slopes of both the Newfoundland and West European Basins. He describes how AABW is the precursor to the deep northern boundary current and is entrained into DSOW and ISOW to form the boundary jet in the Iceland, Labrador, Newfoundland, and, possibly, the Irminger Basins. Together, these water masses circulate around Cape Farewell, the Labrador Basin, the Flemish Cap, and finally the Grand Banks of Newfoundland. In this view, the abyssal flow recirculates in gyres that actually carry heat (weakly) equatorward. Consequently, the West European and Iceland Basins exhibit increasing bottom potential temperatures to the North while the Irminger, Labrador, and Newfoundland Basins have the opposite gradient. West of the mid-Atlantic Ridge, bottom potential temperatures and salinities are lower than on the eastern side, and densities are greater (Dietrich 1969). At the Grand Banks, Swift (1984) estimates that the NADW consists of, roughly, 37% ISOW (comprising 15% entrained SPMW and 22% eastern overflow), 32% LSW, and 31% western overflow - a more or less even split between overflow waters and waters formed in the subpolar region - although he was not aware of the AABW contribution highlighted by McCartney (1992) and so included no AABW component. Once past Grand Banks the southward flow of NADW is conventionally called the Deep Western Boundary Current and is lost from the subpolar system.

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