What Is Ocean Salinity?

Ocean salinity is the concentration of dissolved salts in seawater, expressed as grams of dissolved solids per kilogram of seawater (g/kg) or in practical salinity units (PSU), a dimensionless measure based on electrical conductivity ratios. The global average surface salinity is approximately 35 PSU, though it varies from below 30 PSU in polar regions and near river mouths to above 40 PSU in enclosed basins such as the Red Sea. Salinity is one of two primary variables, alongside temperature, that determine seawater density and therefore govern ocean circulation on a global scale.

The study of ocean salinity connects physical oceanography, geochemistry, and climate science. Understanding salinity distribution and its changes over time requires measurements from shipborne instruments, autonomous floats, moored sensors, and satellite platforms.

Physical and Chemical Basis of Salinity

Seawater contains a predictable mixture of dissolved ions, dominated by chloride (about 55 percent by mass) and sodium (about 31 percent), with smaller contributions from sulfate, magnesium, calcium, and potassium. This ionic composition remains nearly constant in proportion across the open ocean, a property known as the constancy of seawater composition, which allows salinity to be inferred reliably from electrical conductivity measurements. Salinity increases where evaporation exceeds precipitation, as in the subtropical gyres, and decreases where precipitation, river discharge, or glacial meltwater input is high. The addition of freshwater from melting polar ice is a particular concern in the context of climate change: observations compiled by NASA's Salinity Science program show a measurable long-term freshening of high-latitude North Atlantic water associated with accelerating ice sheet loss.

Measurement and Monitoring

The standard measurement of salinity at sea uses a CTD instrument that records conductivity, temperature, and depth simultaneously, computing salinity from the conductivity and temperature values via the Practical Salinity Scale 1978 (PSS-78). The Argo float program, which operates more than 3,900 autonomous profiling floats globally, provides sustained salinity measurements down to 2,000 meters depth, transmitting data to shore via satellite after each surfacing cycle. Satellite-based microwave radiometers extend salinity observation to the global ocean surface; the NASA Aquarius mission, launched in 2011, and the Soil Moisture Active Passive (SMAP) satellite, launched in 2015, have produced multi-year records of surface salinity with spatial resolution on the order of 40 to 150 kilometers. The NOAA National Centers for Environmental Information archive global salinity and temperature observations for climate research.

Role in Ocean Circulation and Climate

Salinity influences ocean circulation because it controls the density of seawater alongside temperature. In the North Atlantic, cooling of saline surface water drives deep water formation: dense, salty water sinks and flows southward as North Atlantic Deep Water, pulling warm tropical surface water northward in what is termed the Atlantic Meridional Overturning Circulation (AMOC). A reduction in surface salinity from excess freshwater input can weaken this density-driven sinking, potentially slowing or destabilizing the AMOC and altering the climate of adjacent landmasses. Salinity also affects sound speed in the ocean, with implications for acoustic communication and sonar performance. NOAA's thermohaline circulation educational resource illustrates how salt concentration and temperature jointly initiate the deep-ocean conveyor belt.

Applications

Ocean salinity has applications in a range of fields, including:

• Climate modeling and prediction of overturning circulation strength
• Freshwater cycle and hydrological balance assessment
• Fisheries management and marine ecosystem monitoring
• Sonar and underwater acoustic system design and calibration
• Desalination process engineering and coastal water resource planning