Variations occur in ocean salinity due to several factors. The most common factor is the relative amount of evaporation or precipitation in an area. If there is more evaporation than precipitation then the salinity increases (since salt is not evaporated into the atmosphere). If there is more precipitation (rain) than evaporation then the salinity decreases. Another factor that can change the salinity in the ocean is due to a very large river emptying into the ocean. The runoff from most small streams and rivers is quickly mixed with ocean water by the currents and has little effect on salinity. But large rivers (like the Amazon River in South America) may make the ocean have little or no salt content for over a mile or more out to sea. The freezing and thawing of ice also affects salinity. The thawing of large icebergs (made of frozen fresh water and lacking any salt) will decrease the salinity while the actual freezing of seawater will increase the salinity temporarily. This temporary increase happens in the first stages of the freezing of seawater when small ice crystals form at about minus 2 degrees Centigrade. These tiny, needle-like ice crystals are frozen freshwater and the salts are not part of them so the liquid between these crystals becomes increasingly salty to the point of it being a brine. Eventually though, as seawater freezes, the ice crystals trap areas with brine and the entire large piece of frozen seawater (ice floe) is salty.

Variations in salinity