Ocean Salinity: Definition, Factors, and Climate Connections

Salinity refers to the amount of dissolved salt in a body of water, such as a river, lake, or the ocean. In the ocean, the water has an average salinity of between 35 and 37 grams of salt per kilogram of water (35 and 37 g/kg, also written as 33%o and 37%o). Salinity is lower in areas of the sea where rivers enter the ocean because the freshwater of the rivers dilutes the seawater. Water is considered brackish, such as that in estuaries, if its salinity rises to 24.7 g/kg. Most of the salt in seawater is sodium chloride; other salts include magnesium chloride, calcium sulfate, and potassium sulfate. The majority of these salts originate in minerals on the land and are dissolved and washed into the sea by rivers and other flowing water. Salinity influences the temperature, density, evaporation, movement, and other characteristics of seawater. It also influences the distribution of fish and other living organisms in the ocean.

DENSITY, SURFACE LAYER, AND DEPTH. The amount of salt in seawater determines the temperature at which the water freezes. The more salt that water contains, the lower the water’s freezing point. The temperature and salinity of seawater help determine its density. As the temperature of seawater decreases, the density increases. As the salt content of seawater increases, so does the density. The density and salinity of seawater can continue to increase after temperatures sink below the freezing point, such as during sea ice formation.

The salinity of the surface layer of the ocean is the most studied region of marine salinity. Surface-layer salinity varies according to the extent of evaporation and precipitation (the main drivers of the water cycle), inflows from rivers, freezing and thawing of ice, ocean currents, winds, and water temperature and density. These factors cause maximum salinity levels of about 36 to 37 g/kg, to occur in the North and South Atlantic Oceans. Relatively high salinity levels also occur in the Southeast Pacific Ocean, the Arabian Sea, and the Mediterranean Sea. Salinity tends to decrease close to the equator because of the dilution effects of frequent heavy rains and near the poles (to as low as 30 g/kg) because of the dilution effects of meltwater from polar ice. The most saline open body of water is the Red Sea (about 40 g/kg), where evaporation is especially high and precipitation and freshwater inflow especially low. Salinity is even greater in isolated bodies of salt water with no freshwater inflow and limited current mixing. The most saline body of water is the Dead Sea, where salinity approaches 300 g/kg.

Salinity varies with the depth of the sea. At and near the surface, salinity increases as water evaporates or becomes frozen as ice, and it decreases as freshwater from rivers or melted ice enters the sea. At lower depths, salinity remains more constant because it is less affected by evaporation, freezing, or freshwater inflows. Deeper water tends to be higher in salinity and denser than shallow water. Between the light, shallow water and the heavy, deep water layers is a relatively thin intermediate layer known as the halocline, in which salinity changes abruptly.

CLIMATE-RELATED CHANGES. Long-term changes in the ocean’s salinity and currents affect regional climates as well as marine life. Conversely, changes in climate produce effects on sea salinity. Scientists with the National Oceanic and Atmospheric Administration (NOAA) have closely monitored ocean salinity within a few meters of the surface since the early 2000s using robotic Argo floats. Data from these devices indicate, according to a 2014 NOAA report, that regional changes in salinity have occurred since about 2004. Specifically, seawater has become increasingly salty in the Western Indian Ocean, in the western and central tropical Pacific Ocean, and in the eastern subtropical Pacific Ocean of both hemispheres. At the same time, much of the North Atlantic Ocean and eastern tropical South Pacific Ocean has become less salty and fresher. Many scientists attribute these changes to global warming, which has caused precipitation to increase in rainy areas and evaporation to increase in dry areas, thereby making relatively fresh areas of the ocean fresher and relatively salty areas of the ocean saltier. Continued climate change is expected to accelerate the global water cycle, further exaggerating these regional effects of greater freshness or greater saltiness.

In 2011, NASA launched its first satellite designed to study ocean surface salinity. The instruments on the Aquarius satellite can detect the microwave emissions of the top two centimeters (about one inch) of ocean water. These emissions vary depending on water temperature and saltiness. These and other advanced instruments are providing scientists around the world with increasingly useful and more precise data to help them better understand salinity and other physical properties of the ocean and how they relate to global climate change.

FURTHER READING: National Aeronautics and Space Administration. 2016. “Salinity.” https://science.nasa.gov/earth -science/oceanography/physical-ocean/salinity/. Accessed December 8, 2016.

National Oceanic and Atmospheric Administration. 2014. “2013 State of the Climate: Ocean Salinity.” https://www.climate.gov/news-features/understanding-climate/2013-state-climate -ocean-salinity. Accessed December 8, 2016.

National Weather Service, National Oceanic and Atmospheric Administration. “‘A Funny Bath’— The Dead Sea.” http://oceanservice.noaa.gov/education/yos/resource/JetStream/ocean/dead_max .htm/. Accessed December 8, 2016.

National Weather Service, National Oceanic and Atmospheric Administration. “Sea Water.” http:// www.srh.noaa.gov/jetstream/ocean/seawater.html. Accessed December 8, 2016.

PMF IAS—Geography Notes for General Studies. 2016. “Ocean Salinity—Vertical, Horizontal Distribution.” http://www.pmfias.com/ocean-salinity-vertical-horizontal-distribution/. Accessed December 8, 2016.

Science Learning Hub. 2010. “Ocean Salinity.” http://sciencelearn.org.nz/Contexts/The-Ocean-in -Action/Science-Ideas-and-Concepts/Ocean-salinity. Accessed December 8, 2016.

Soos, Andy. 2012. Environmental News Network. “The Change in Ocean Salinity.” http://www.enn .com/climate/article/44324. Accessed December 8, 2016.