Salinity of ocean waters

All natural water, even rain, holds some dissolved salts. Salinity is the total dissolved salt in sea water, measured as grams of salt in 1,000 g (1 kg) of sea water, written as parts per thousand (‰ or ppt). Salinity of 24.7‰ is taken as the upper limit of brackish water.

Factors affecting salinity:

• Evaporation (raises it) and precipitation (lowers it) control surface salinity.
• River inflow of fresh water lowers it in coastal areas; in polar regions, freezing and thawing of ice change it.
• Wind moves water from one area to another.
• Ocean currents carry saltier or fresher water.
• Salinity, temperature and density are linked.

Horizontal distribution:

• Normal open ocean: 33-37‰. The land-locked Red Sea: as high as 41‰. Estuaries and the Arctic: 0-35‰, varying by season. Hot, dry inland waters can reach 70‰.
• Pacific: varies with its huge size; lower (35‰ to 31‰) in the west of the northern part from Arctic meltwater.
• Atlantic: average about 36‰, highest at 15°-20° latitude, peaking at 37‰ between 20° N and 30° N.
• Indian Ocean: average about 35‰. The Bay of Bengal is less salty (great river inflow); the Arabian Sea more salty (high evaporation, little fresh water).
• Seas: the North Sea is fairly salty despite its high latitude (the North Atlantic Drift brings salty water); the Baltic and Black Seas are low (huge river inflow); the Mediterranean is high (strong evaporation). Inland waters such as the Dead Sea and the Great Salt Lake are far saltier than any ocean.

Vertical distribution. Surface salinity rises with evaporation or loss of water to ice, and falls with fresh-water inflow; at depth it is much more stable. Generally salinity increases with depth, and there is a distinct zone, the halocline, where it rises sharply. Saltier water is denser and sinks below fresher water, so the ocean becomes layered by salinity.

Example (new): salt pans along the Gujarat coast use the same idea as a hot, dry sea: sea water is let into shallow beds where strong sun evaporates it, raising salinity until salt crystals form.