If you compare Figure 6.15 with Figure 3.1, you will see that the geographical distribution of the world's upper water masses is strongly influenced by the pattern of surface currents. Upper water masses are generally considered to include both the mixed surface layer and the water corresponding to the upper part of the permanent thermocline, and they are therefore of varying thickness. If, as is the case in the region of the Equator, salinity is kept low by high precipitation and the temperature is high, the density of surface water will be low; the upper water column will therefore be stable, and only a very shallow water mass can form.
By contrast, the water masses that form in the subtropical gyres - also known as 'central waters' (cf. Figure 6.15) - are upper water masses of considerable thickness. As discussed in Section 3.4, in regions of convergence like the subtropical gyres (cf. Question 6.6) the sea-surface is raised and the thermocline depressed, leading to a thickening of the mixed surface layer (Figure 3.24(c) and (d)). Water sinks from the surface continually, but in winter, cooling of surface water leads to instability and vigorous vertical mixing occurs. As a result, water is alternately brought into contact with the surface and then carried deep down, so that a thick and fairly homogeneous water mass is formed.
The central water mass formed in the Sargasso Sea in winter (labelled Western North Atlantic Central Water on Figure 6.15) has temperatures ranging from 20.0 °C down to 7.0 °C.
What is the approximate lower depth limit of this water mass according to the temperature section across the Sargasso Sea show n in Figure 4.22''
It is about 1000-1100 m. Figure 4.22 also demonstrates two other relevant points. The first is that a large volume of water in the North Atlantic subtropical gyre has a temperature close to 18 °C. This '18 °C water' is an example of a mode water, that is. a volume of water within which temperature varies very little. The concept of a mode water is intimately related to the second point shown by Figure 4.22, which is that within the main body of Western North Atlantic Central Water, the isotherms are widely spaced; in other words, the waters are characterized by a thermostad or pycnostad (Section 5.1.1).
East N Pacific îiansiiion Waier
Bengal Bay Water
- Atlantic Central Waier
Subamarrf^ ^¡ajdK Surfo^
Figure 6.15 Thp global distribution of upper water masses. The boundaries between different water masses are not as sharp as (he lines on this map might suggest (You need not remember the details of this map i
1 lie temperature-salinity characteristics of Western and Eastern North Atlantic Central Waters are very similar. In common with central water masses in the other oceans. North Atlantic Central Waters have moderately high temperatures and above-average salinities.
Bearing in mind that the central water masses form below the antics clonic subtropical wind systems, can you explain why their salinities are above average?
The subtropical anticyclones are regions where dry air subsides (cf. Figure 2 20). and net evaporation (¿' - P) is high, leading to high salinities in: the mixed surface layer (cf. Figure h. M ) and hence in the water mass as a whole. However, because of the differing environ mental/climatic conditions on the two sides of the ocean, the salinities of Eastern North Atlantic Central Water are on average about 0.1 to 0.2 higher than those of its western counterpart. One reason for (his is the stronger influence of Mediterranean Water nil the eastern side of the ocean: another possibility is that because surface mixing penetrates to deeper levels on the western side of ihe ocean, the uppei water mass is brought into close contact with the low-salinity Western Atlantic Sub-Arctic Water that underlies it.
Western Atlantic Sub-Arctic Water and Mediterranean Water are examples of inicnneiUatc water masses, w hich How between the tipper water masses and ihe deep and bottom water masses. Of the two. Western Atlantic SubArctic Water is the more typical because, like most intermediate water masses, it forms in subpolar regions (cf. Figure 6.16) where precipitation exceeds evaporation, and Us salinity is therefore low.
However, Western Atlantic Sub-Arctic Water consists largely of Labrador Sea Water, which - as you will see - may be found ai great depths in (he ocean, as well as at the intermediate depths covered by Figure 6.1b. Most Labrador Sea Water forms in a cyclonic gyre on the offshore side of the Labrador Current (cf. Figure 5.26). In summer, the density of surface water in the Labrador Sea is lowered by the addition of Ireshwaier from melting sea-ice and icebergs (Section 5.5.1). In winter, however, the surface water is cooled by ihe pack ice and by cold, dry Arctic air masses that have passed over northern Canada
int Watet ff
__ Pacifc Subarctic
Antarctic intermediate Water
Arctic Intermediate Water
E Atlantic Subarctic (nt Water
Mediterranean frr , Water
Antarctic Intermediate Water
Antarctic Intermediate Water
Easi S Pacific intermeaiate Wale'
Figure 6.16 The global distribution of intermediate wafer masses (between about 550 and 1500 m depth). The source regions of the water masses are indicated by blue toned areas. Note that Antarctic Intermediate Water is by far the most widespread intermediate water mass. (You need not remember the details of this map.)
Which heal-budget loss terms are increased by the passage ol these air masses'!
Heat will be lost to the cold air masses by conduction and convection and, because they are also dry. by evaporation. <2h and Qc are therefore both increased. The density of surface water in the Labrador Sea is therefore increased through a fall in temperature and a rise in salinity. The increased salinity is still fairly low in absolute terms (-34.9) and the decreased temperature is relatively high (~3 °C) but their combined effect is sufficient to increase the density of surface water above that of the underlying water. The upper water column is therefore destabilized and vertical convection occurs, with denser surface water sinking and displacing less dense subsurface water, which rises to the surface. Surface water circulating in the gyre is thus repeatedly subjected to vertical mixing, so that eventually a water mass 1500 m thick or more may be formed. There is great variability in the depth of mixing and in the amount of Labrador Sea Water formed in any one year. In some years, there is no deep convection and no Labrador Sea Water is formed at all; Figure 6.17 shows data for a year when an enormous volume of the water mass was produced.
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