4.5 Surface water masses
The surface circulation patterns (Figure 15) and the resulting Ekman drift can be compared with the SST (Figure 9) and the sea-surface salinity (Figure 10b). There are large volumes of water in the mixed layers where both the temperature and salinity are relatively constant. Such a water volume with relatively constant temperature and salinity is called a water mass. Section 3 noted that the surface temperature and salinity are determined by the regional climate and so water masses are regionally named (Figure 20).

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Would all of the surface water masses shown in Figure 20 have the same density anomaly? Hint - you may have to look at Figures 9, 10b and 12.
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No, they would not. The density of water is a function of temperature and salinity, and each water mass has a different temperature and salinity value. You can see from Figure 12 that different temperatures and salinities lead to different densities.