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.
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.
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.
