Salinity

The salinity of water in the surface layer of the World Ocean depends mainly on evaporation and atmospheric precipitation. Surface salinity is greatly influenced in coastal regions by the fresh water outflows from rivers, and in polar regions by the processes of freezing and thawing of ice. Below the surface layer, the salinity field is formed as a result of the interaction between the transport of salts by currents and diffusion by the intermixing of waters. High salinity (>35) is encountered in surface waters at tropical latitudes, where evaporation is greater than at other latitudes. The lowest average salinity of open oceanic waters (~29) is observed in the summer in the Arctic Ocean. In coastal regions with significant river run off, salinity can range from near zero to 20. The salinity of deep and near bottom waters in the oceans is about 35. Salinity and temperature together affect the density of water. Many physical characteristics depend on density distribution, for example, water exchange processes, intermixing and sound transmission.

Salinity is an extremely important property of sea water, physically and biologically. Pure water has a maximum density at a temperature of 4 degrees C; upon further cooling it becomes slightly less dense and tends to rise within the water column, explaining the common formation of ice on fresh water bodies in winter. Sea water, on the other hand, when its salinity is greater than 24.7, continues to become more dense below 4 degrees, causing it to sink in the water column. This explains the relative rarity of ice on sea water, except at the poles. This division of behavior at a salinity of 24.7 has been mentioned as defining the upper limit of "brackish water," one of the great salinity regimes of biological and physical significance (although there are other definitions). Other regimes and some widely cited salinity values are: Fresh water (40).

The Figure has been selected to show salinity contours for April, a month in the northern hemisphere where seasonal runoff and precipitation is normally high, affecting coastal salinities.