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ICP Website Curator: Robert B. Schmunk — NASA Official: Gavin A. Schmidt


General Characteristics of the World's Oceans: 3

Ocean Salinity

As mentioned before 96.5% of the ocean is water. The other 3.5% is salinity. Salinity is the total amount of dissolved solids (in grams) in 1000 grams (1 kg) of water, and is described as parts per thousand. These solids form ions (charged molecules) because of interactions between lattices and water molecules. Because of the charge that these salts carry, salinity is determined by measuring the seawater's conductivity. The most abundant ion in seawater is chlorine, followed by sodium, sulfate, magnesium, calcium, and potassium. These components make up 99.36% of the oceans salinity. Seawater also contains dissolved gases, such as carbon dioxide, nitrogen, and oxygen.

Like temperature, salinity also has an effect on water. As salinity increases vapor pressure decreases and osmotic pressure increases (important for marine organisms) and lowers the temperature of initial freezing (0°C), creating a no fixed freezing point. Salinity also has an effect on the viscosity of water. This is because certain ions like sodium and potassium shift the equilibrium toward water's unstructured phase, while others like magnesium prefer the structured portion. The image below shows sea surface salinity.

Levitus (94) annual mean ocean surface salinity

This image was generated based on the all observed data until 1994. The pink and red represent high levels of salinity and the dark blue shows extreme low rates of salinity. The high salinity seawater is in secluded areas and the subtropics (Sargasso Sea, Persian Gulf, Red Sea, and the Mediterranean Sea), while the lowest seem to be in the north (Baltic Sea and Gulf of Bothnia) and around the continents.

The distributions of salinity are quite different from temperature. High concentrations are usually in the center of the ocean basins away from the mouths of rivers, which pour in fresh water. They are also in sub-tropical regions due to high rates of evaporation (clear skies, little rain, and prevailing winds) and even higher where there is landlocked sea of dry regions. Low salinity is in the high latitudes. This is because of the lower evaporation rates and the melting of ice which dilutes the water. Basically, there are low rates of salinity where precipitation is greater than evaporation, mainly in the coastal or equatorial regions.

Levitus (94) annual zonal mean ocean salinity

The diagram above illustrates the average sea salinity as you go deeper into the ocean. The y-axis depicts depth in meters and the x-axis, latitude. The black area indicates average bathymetry over longitude. The unusually deep loop of salinity around 10 to 50 degrees North is partly caused by the extreme salinity levels of the Mediterranean.

Although the distributions of temperature and salinity differ markedly, they are closely related. For example, a varying freezing point, created by increased salinity, has an effect on temperature. Another similarity is that both properties are affected by evaporation; ocean temperature is cooled with evaporation while ocean salinity is increased. Even below the ocean's surface, both characteristic relationships can be used to identify water masses, which have fixed temperatures and salinities.

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