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The global ocean has a variety of different water masses, parcels of ocean identifiable by their temperature, salinity, and density characteristics that determine their place within the vertical structure of the oceanic water column. It is important to keep in mind that certain properties of a water mass are determined during its sojourn at the surface, e.g. temperature and salinity. Those are conservative properties and are changed only when a water mass mixes with another. In the deep ocean, water masses can mix only when their densities are roughly equal, otherwise they remain stratified. Therefore vertical movements of water, away from or toward the surface, require a weakly stratified water column, such as is found near the poles. Figure 1.12 is a standard diagram of oceanic temperatures at depth at low, middle, and high latitudes.

Figure 1.11 T‐S diagram. Temperature–salinity plot from an oceanographic station in the Atlantic. The axes represent salinity (X) and temperature (Y). The curved lines represent isopycnals (equal density). AAIW, Antarctic Intermediate Water; NADW, North Atlantic Deep Water; AABW, Antarctic Bottom Water.

Source: Brown et al. (1989), figure 6.26 (p. 191). Reproduced with the permission of Pergamon Press.

Five generic water masses are found at temperate and tropical latitudes. Surface water extends from the surface to about 200 m depth and includes the seasonal thermocline. Central water extends from just below surface water to the bottom of the permanent thermocline, usually at about 1000 m. Intermediate water resides below central water to a depth of about 1500 m, where deep water begins. Deep water is found below intermediate water but is not in contact with the bottom; it is found between 1500 and 4000 m. Deepest of the oceanic layers is bottom water, which is in contact with the seafloor.

Each of the generic water masses has a large number of specific examples that can be identified by their temperature and salinity; they are named for their source region. Upper water masses (surface and central water), shown in Figure 1.13, are numerous and correspond fairly closely with surface currents. Intermediate waters are mapped in Figure 1.14. It is important to note the difference in areal extent between upper and intermediate waters. Antarctic Intermediate Water (AAIW), for example, is widespread at intermediate depths throughout the world ocean. The temperature and salinity characteristics that define AAIW (temperatures of 2–4 °C and salinities of about 34.2) provide a fairly uniform environment for the species that live within it. It is thus easy to understand why deeper‐living oceanic species are typically far more wide‐ranging than those inhabiting surface waters (Briggs 1974).

Figure 1.12 Standard depth profiles of temperature at low, middle, and high latitudes.

Deep and bottom water masses are even less numerous and more widespread than intermediate water masses. North Atlantic Deep Water (NADW) forms in the north Atlantic above 60 °N when cold, saline waters from the Norwegian and Greenland seas spill over the mid‐Atlantic ridge into the depths of the Atlantic (Figure 1.15). Those waters mix with overlying water as well as water flowing south out of the Labrador Sea to create a general southward flow along the west side of the mid‐Atlantic ridge. NADW is the most important deep‐water mass in the Atlantic, extending well into the southern hemisphere.

Antarctic Bottom Water (AABW) is the most widespread of the water masses, dominating the bottom water in all three ocean basins. It is formed in winter near the Antarctic continent, mainly in the Weddell and Ross Seas, the southernmost portions of the Atlantic and Pacific Ocean basins, respectively. As discussed earlier, when ice crystals form in seawater most of the dissolved salts are excluded as brine, creating very cold and saline water. AABW is the densest water mass in the world ocean; the source water mixes very little with any less dense waters. In many areas of the Antarctic, particularly in the Ross Sea, the temperature from the edge of the continental shelf to the coast is about −2.0 °C from surface to bottom.

The Antarctic is the southern end of the line for several water masses and the mixing in the Southern Ocean is complex (Figure 1.16). Warmer water from the north, including NADW, reaches the surface at the Antarctic divergence where it gives up its heat. Cooled water flows northward and sinks at the Antarctic Convergence, creating AAIW. The cycle of heat delivery, cooling, and sinking is the southern counterpart to the same basic process in the North Atlantic. The process at both poles is termed the thermohaline circulation or the “Great Ocean Conveyor” (Broecker 1992).

Figure 1.13 Surface and central water masses.

Source: Lalli and Parsons (1993), figure 2.13 (p. 37). Reproduced with the permission of Pergamon Press.

Figure 1.14 Intermediate water masses.

Source: Lalli and Parsons (1993), figure 2.15 (p. 38). Reproduced with the permission of Pergamon Press.

Figure 1.15 Deep and bottom water masses: sources and flow patterns. AAIW, Antarctic Intermediate Water; AABW, Antarctic Bottom Water; NADW, North Atlantic Deep Water.

Source: Lalli and Parsons (1993), figure 2.17 (p. 39). Reproduced with the permission of Pergamon Press.

Figure 1.16 Flow patterns and mixing of water masses in the Southern Ocean. AAIW, Antarctic Intermediate Water; NADW, North Atlantic Deep Water; AABW, Antarctic Bottom Water; ACW, Atlantic Central Water.

Source: Brown et al. (1989), figure 6.20 (p. 184). Reproduced with the permission of Pergamon Press.