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Figure 2.4 Annual mean Atlantic Ocean zonal average (by 1° squares) of salinity (ppt) as a function of depth. Note the break in the depth scale at 1,000 m (Levitus, 1982).

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Figure 2.4 Annual mean Atlantic Ocean zonal average (by 1° squares) of salinity (ppt) as a function of depth. Note the break in the depth scale at 1,000 m (Levitus, 1982).

identified by characteristic combinations of air temperature and moisture content (humidity). These characteristics allow meteorologists to identify the history (or source regions) of the various air masses. Examples of common air masses include continental polar (cold, dry air formed over high-latitude land areas) and maritime tropical (warm, moist air formed over equatorial ocean areas).

Oceanographers (e.g. Sverdrup et al, 1942) have convincingly demonstrated that certain characteristic combinations of water temperature and salinity are associated with water masses formed in particular regions of the world's oceans. After formation, these water masses spread both vertically and laterally to occupy depth ranges of the water column consistent with their density. Moreover, these water masses are distinguishable from one another when plotted on a graph of temperature-versus-salinity, referred to as a T-S diagram. The T-S relations of the principal water masses of the Atlantic Ocean are presented in Figure 2.5 (Naval Oceanographic Office, 1972). The names of these water masses suggest their relative positions in the water column (e.g. central, intermediate, deep and bottom). From Figure 2.5, for example, one can conclude that the waters of the South Atlantic Ocean are fresher (i.e. less saline) than those of the north Atlantic.

Figure 2.5 Temperature-salinity (T-S) diagrams of the major water masses of the Atlantic Ocean (Naval Oceanographic Office, 1972).

Notes

AAIW Antarctic Intermediate Water. MIW Mediterranean Intermediate Water.

Emery and Meincke (1986) provided an updated review and summary of the global water masses.

The movement of Antarctic intermediate water (AAIW), for example, can now be identified in the distribution of salinity illustrated previously in Figure 2.4. Specifically, the movement of AAIW is evidenced by a low-salinity tongue (~34.0-34.6ppt) extending downward from the surface in the latitude belt 60-70° S, then northward at a depth of about 700-800 m and finally upward near the equator.

The distribution of sound speed in the ocean can be related to the local water-mass structure. Knowledge of the water-mass structure, then, can greatly enhance understanding of the large-scale spatial and temporal variability of the sound-speed field in the ocean.

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