Download Ocean Salinity Why does salt content matter in the oceans?

Physical Properties: Ocean Salinity
Why does salt content matter in the oceans?
One of the major differences between oceans and lakes is that oceans have a much higher
concentration of dissolved materials in their water. Where did this dissolved material come
from, especially since much of it is common salt (sodium chloride)? Why does it matter that
ocean waters are not evenly salty?
The dissolved salts in the oceans primarily came from the continents. As rainwater from
storms and meltwater from snow travel across the land surface, the water slowly dissolves salts
contained in rocks and soils. Over millions and millions of years, the salts travel to and
accumulate in the ocean basins. Whenever ocean water evaporates, it leaves behind the
dissolved salts, since water vapor gas cannot carry dissolved materials. Undersea volcanoes and
hydrothermal vents also contribute dissolved salts to the oceans. Freshwater typically contains
less than 0.5 parts per thousand of dissolved salts, while ocean waters often have an average of
35 parts per thousand (ppt) of dissolved salts. If you had 1,000 grams of water and 35 of those
grams were salt, that would be a salinity of 35 ppt.
Ocean salinity tends to be lower where there is a lot of surface runoff entering the ocean
from landmasses, such as areas with lots of freshwater rivers flowing into the ocean. It also
tends to be lower in areas where the amount of rainfall exceeds the amount of evaporation.
Salinity tends to be higher in the centers of oceans (far from dilution by coastal runoff) and in the
subtropics where evaporation rates exceed precipitation rates.
Ocean salinity turns out to be an important factor in the vertical movement of water in the
oceans, and drives a large circulation along the ocean basin bottoms that connects all of the large
oceans of the world. Consider this graphic:
As salinity decreases due to lots of rainfall, the water gets less dense and rises to the ocean
surface. Likewise, as temperature increases, the density of water decreases and it rises toward
the surface. Conversely, as water salinity increases due to high evaporation, it gets denser and
tends to sink; as water cools, it gets denser and tends to sink. Very cold, very saline water
always tends to sink. What does this mean for ocean circulation?
The differences in thermal properties and salinity have developed a global circulation
known as the thermohaline circulation. Warm surface waters flow from the Pacific and Indian
Oceans into the Atlantic Ocean, travel northward and cool as they approach the latitude of
Greenland. This cooled, salty water then sinks to the bottom of the Atlantic and returns to the
Indian and Pacific Oceans in what has been described as a “global conveyor belt” of ocean
currents. The time it takes water to flow through the bottom of the basins has been calculated at
up to 1,000 years; surface gyres, by contrast, may complete a circuit around a basin in a few
short years.
Sea surface salinity has been linked to weather and climate events such as the El
Nino/Southern Oscillation pattern. A recently-launched NASA satellite, Aquarius, is actually
designed to monitor and investigate the salinity of the global oceans, as well as precipitation, sea
ice concentrations, and water vapor in the atmosphere. The data sets from Aquarius may help
scientists better predict the onset, duration, and strength of climatic events that are driven by the
oceans.