Ocean Water Masses

Most of what is known about the three-dimensional circulation of the ocean has been deduced from the study of bodies of water that are identifiable because they have particular combinations of physical and chemical properties. Such bodies of water are referred to as water masses, and the properties most often used to identify them are temperature and salinity.

Temperature and salinity can be used to identify water masses because (as mentioned in Section 4.2.4) they are conservative properties, that is, they are altered only by processes occurring at the boundaries of the ocean; within the ocean, changes occur only as a result of mixing with water masses having different characteristics. Non-conservative properties, on the other hand, are subject to alteration by physical, chemical or biological processes occurring within the oceans.

Water masses that form in semi-enclosed seas provide particularly clear examples of bodies of water with recognizable temperature and salinity characteristics. As discussed in the previous Section, deep water leaving the Mediterranean Sea through the Straits of Gibraltar is of unusually high salinity (Figure 6.13). This Mediterranean Water forms in winter in the north-western Mediterranean. Intense cooling and higher than normal evaporation, associated particularly with the cold, dry Mistral wind, increase the density of surface water to such an extent that there is vertical mixing, or convection, right to the sea-floor at more than 2000 m depth. The homogeneous water mass so formed has a salinity of more than 38.4 and a temperature of about 12.8°C. As it leaves the Straits of Gibraltar at depth.

intense mixing occurs at the interface with the incoming Atlantic water (Figure 6.13). and both its salinity and its temperature are somewhat reduced.

The least-mixed layer of Mediterranean Water in the adjacent Atlantic has a salinity of 36,5 and a temperature of i I °C. Because of its relatively high density, it sinks down to about 1000m depth where it becomes neutrally buoyant. and it spreads out at this level. Although it is being continually modified by mixing. Mediterranean Water can be recognized throughout much of the Atlantic Ocean by its distinctive signature of high temperature combined with high salinity (Figure 6.14).

Figure 6.11 Distribution of (a) temperature c=C| and (b) salinity at 1000 m depth in the North Atlantic, showing the spread ol Mediterranean Water. The broken black line is the 1000 m isohaih Nole that on leaving the Straits ol Gibraltar, Mediterranean Water first turns north under the influence of the Conolis force and spreads along the coast of Portugal. It gradually mixes mto the waters of the subtropical gyre and eventually spreads southwards and westwards

Figure 6.11 Distribution of (a) temperature c=C| and (b) salinity at 1000 m depth in the North Atlantic, showing the spread ol Mediterranean Water. The broken black line is the 1000 m isohaih Nole that on leaving the Straits ol Gibraltar, Mediterranean Water first turns north under the influence of the Conolis force and spreads along the coast of Portugal. It gradually mixes mto the waters of the subtropical gyre and eventually spreads southwards and westwards

Within the ocean there are a large number of distinct water masses, each characterized by temperature and salinity values reflecting a particular set of surface conditions, and generally considered to originate in a particular source region. You saw in Section 6.1 that the temperature of surface waters at any location in the ocean depends on the relative sizes of the components of the heat budget in that region; similarly, the salinity will depend upon the relative importance of the various factors discussed in Section 6.2. However, a water mass with particular temperature and salinity values will only result if a body of water is subject to specific meteorological influences over a significant period, during which it remains in the mixed surface layer. Furthermore, if the water is eventually to become isolated from the atmosphere, it must sink down from the sea-surface. These necessary conditions are satisfied in regions where surface waters converge.

QUESTION 6.6 From your reading of Chapters 4 and 5 in particular, suggest two regions of convergence of surface water, one in mid latitudes and the other at high southern latitudes.

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