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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-F) (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 balances - 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.

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