The ocean is a very important element of the climate system which covers about 71% of the Earth's surface and has an average depth of roughly 3700 m. Sea water is composed of 96.5% water and 3.5% dissolved salts, particles, gases and organic matter. The most important of these components are chloride and sodium, which represent about 85% of the dissolved material. Although the total quantity of dissolved salts varies from place to place, their relative contribution is very stable in sea water. Rather than specifying each of the components, it is thus very convenient to define a bulk salinity as the total amount of dissolved material (in grams) in a kilogram of sea water. This dimensionless salinity is then given in ‰ (parts per thousand). However, in practice, measuring the total amount of dissolved material is difficult. The salinity scale is thus based nowadays on the conductivity of sea water and given in psu (practical salinity unit). For simplicity, this new scale has been chosen so that the salinity in psu is very close to that in ‰.
The density of sea water increases with salinity as well as with pressure (thus with depth as the ocean is also in hydrostatic equilibrium on a large-scale), while it decreases with increasing temperature. In a very simplified picture, it is often considered that temperature dominates the density changes at high temperatures, while the role of salinity is larger at low temperatures. Salinity also influences the freezing point of sea water which, at the surface, decreases from 0oC for pure water to -1.8oC at a salinity of 35 psu.