Salinity
Learning Objective:
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Define salinity; recognize its effect on seawater density; state its
ranges in the open ocean and the major factor that controls it.
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The term salinity is related often to the amount of salt in the
water. In oceanography, salinity is defined as "the total amount of
dissolved solids in seawater." Salinity is measured in parts per thousand
by weight, and is symbolized ‰. The measurement gives us the grams of
dissolved material per kilogram of seawater.
The salinity values of ocean water range between 33‰ and 37‰, with an
average of 35 ‰. In the open ocean, surface salinity is decreased by
precipitation, increased by evaporation, and changed by the vertical mixing and
inflow of adjacent water. Near shore, salinity is generally reduced by river
discharge and fresh-water (acqua dolce) runoff from land. In the colder waters that freeze and
thaw (scongelano), salinity generally increases during periods of ice formation and decreases
during periods of ice melt.
Latitudinally, surface salinity varies in a similar manner in all oceans.
Maximum salinity values occur between 20° and 23° north and south, whereas
minimum salinity values occur near the equator and toward higher latitudes. The
controlling factor in average surface salinity distribution is the latitudinal
differences in evaporation and precipitation. Exceptions to this statement do
occur, and local variations should be expected, especially near the mouth of the
larger river systems and in the Atlantic coastal water of the United States,
Labrador, Spain, and Scandinavia. The best known region of strong horizontal
salinity gradients is the Grand Banks region, where warm, saline Gulf Stream
water mixes with the colder, less saline water of the Labrador Current. Here,
water with a salinity value as low as 32‰ may possibly override or lie
adjacent to water having a salinity value greater than 36‰. A similar
situation prevails in the Pacific Ocean, where the Kuroshio and Oyashio currents
mix.
At latitudes poleward of 40° north and south, where precipitation generally
exceeds evaporation, salinity values tend to increase with depth. Usually during
summer, these positive salinity gradients are accompanied by strong negative
temperature gradients and result in very stable water, especially in the coastal
regions. These strong, shallow salinity (and temperature) gradients persist
through the summer.
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