Dynamic topography
5.2 MONSOONAL CIRCULATION
6.2.1 PRACTICAL APPLICATION OF THE PRINCIPLES OF CONSERVATION AND CONTINUITY
203
Figure 6.11 (a) The mean annual distribution of surface salinity. Note that although they are effectively parts per thousand by weight, salinity values have no units because the salinity of a water sample is determined as the ratio of the electrical conductivity of the sample to the electrical conductivity of a standard. These salinity values are sometimes quoted as 'p.s.u.' or practical salinity units.
(b) Average values of salinity, S (black line), and the difference between average annual evaporation and precipitation ( E - P) (blue line), plotted against latitude.
The difference in surface salinities between the Pacific and the Atlantic is reflected in the marked difference between the average salinities of the two oceans as a whole: about 34.9 for the Atlantic and about 34.6 for the Pacific.
Awareness of the global variations of such factors as sea-surface salinity, local evaporation-precipitation b a l a n c e s - and indeed of the various heat- budget terms - is essential if we are to quantify fluxes of water (and heat) across the ocean-atmosphere boundary. The redistribution of salt and heat within the ocean is studied by monitoring the movement of bodies of water with characteristic combinations of temperature and salinity. These
identifiable bodies of water are the subject of Section 6.3.
First, however, let us see how the principle of conservation of salt may be applied on a relatively small scale.
6.2.1 PRACTICAL APPLICATION OF THE PRINCIPLES OF CONSERVATION AND
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205 and similarly,
F V 1 = Sl
S~
1 (6.5b)
These equations give us a means of calculating the rates of flow across A 1 and A2, if the mean salinity at each section and the rates of evaporation, precipitation and run-off are known. Also, the average volume of water flowing through the sections per unit time is in each case equal to the cross- sectional area x the mean current velocity, i.e. V1 - A~
~1
and V 2 - A 2 E 2.Thus, if areas A j and
A2
are known, fil and fi2, the average velocities of the currents flowing through the sections, may be estimated.In deriving Equations 6.3 to 6.5, we assumed that salt is carried into and out of the channel only by advection of the mean current; the effect of, say, an eddy of exceptionally high or low salinity could not be taken into account.
Also, in estimating average values of the different parameters, any variations resulting from tidal flow would have to be allowed for.
Nevertheless, this application of the principles of continuity and of conservation of salt provides an extremely useful tool in the study of semi- enclosed bodies of water. Here, we will demonstrate how the principles may be applied to flow into and out of the Mediterranean (Figure 6.13).
At the Straits of Gibraltar, Atlantic water of relatively low salinity flows into the Mediterranean Sea, while high salinity Mediterranean water flows out at depth.
Figure 6.13(b) suggests that S1 is between 36.25 and 36.5, and that $2 is between 37.0 and 38.0. The average values are about 36.3 for S1 and 37.8 for S~.
Figure 6.13 West-east sections across the Gibraltar sill of (a) temperature (~ and (b) salinity, showing the inflow of Atlantic water in the upper layers and the outflow of Mediterranean Water (shown in blue) at depth. (The vertical exaggeration is about x 75.)
Direct current measurements in the upper layers of water in the Straits of Gibraltar indicate that Vl is of the order of 1.75 x 106m3s -1, which suggests that application of the principles of continuity and conservation of salt can produce reasonably reliable results.
The value you calculated in Question 6.5(b) indicates that it takes about 70 years for all the water in the Mediterranean to be replaced. This is the residence time of water in the Mediterranean. The term 'flushing time' is also used, particularly in connection with flow in estuaries; it is useful because it is a measure of the extent to which pollutants are likely to accumulate.
As mentioned earlier, the evaporation-precipitation cycle is not the only mechanism whereby salinity may be changed. At high latitudes, the formation of ice and the addition of meltwater from glaciers and sea-ice have a significant effect on the salinity of seawater. In Section 6.3, we will see how the removal of freshwater and/or heat from surface water drives the deep thermohaline circulation.
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 onh' 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-eonservative 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 M e d i t e r r a n e a n W a t e r 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, fight to the sea-floor at more than 2000m depth. The
homogeneous water mass so formed has a salinity of more than 38.4 and a temperature of about 12.8 ~ As it leaves the Straits of Gibraltar at depth,
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~~ ~ > ~'~ ~ ~= ~'~.~ _ ~q-~'_ ~'~ ~ o q_q. =. 0 "-,1Within 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.