What Is Salinity (Geophysical)?

Salinity, in its geophysical sense, is the mass concentration of dissolved inorganic salts in a natural water body, most commonly expressed for seawater as practical salinity units (PSU) on the Practical Salinity Scale 1978 or, more recently, as absolute salinity in grams per kilogram under the TEOS-10 thermodynamic equation of state. The quantity is fundamental to oceanography and geochemistry because, together with temperature and pressure, it determines the density of seawater and therefore the buoyancy forces that drive large-scale ocean circulation. Typical open-ocean values cluster near 35 PSU, with the Red Sea exceeding 40 and the Baltic falling below 10.

Salinity reflects the balance of evaporation, precipitation, river discharge, ice formation, and ice melt at the sea surface, and of advection and mixing beneath it. As a result, salinity fields serve as a tracer of the global water cycle and a sensitive indicator of regional climate variability.

Measurement and Instrumentation

In-situ salinity is measured indirectly through electrical conductivity, which together with temperature and pressure yields practical salinity via the PSS-78 algorithm. Conductivity-temperature-depth (CTD) instruments, autonomous Argo profiling floats, and glider-mounted sensors form the core of the global sampling network, supplying vertical profiles from the surface to roughly 2,000 meters. Laboratory calibration against IAPSO Standard Seawater anchors the measurement chain to an internationally agreed reference, and the UNESCO-endorsed TEOS-10 thermodynamic equation of state provides the formulas used to convert conductivity readings into absolute salinity and derived quantities such as density and sound speed.

Satellite Remote Sensing

Since 2009, satellite radiometers operating in the L-band near 1.4 GHz have mapped sea surface salinity from orbit by exploiting the sensitivity of sea-surface brightness temperature to dissolved salt content. The European Space Agency's SMOS mission, NASA's Aquarius mission (2011 to 2015), and the ongoing NASA Soil Moisture Active Passive (SMAP) mission all contribute gridded salinity products. These observations provide near-global coverage on roughly weekly timescales, revealing features such as the salty subtropical gyres, the fresh tropical rain bands, and the seasonal freshwater plumes from the Amazon and Ganges-Brahmaputra systems. Sensitivity is reduced in cold polar water and biased within about 50 kilometers of coastlines due to land contamination.

Role in Ocean Circulation and Climate

Salinity gradients, together with temperature gradients, drive the thermohaline component of global ocean circulation, including deep-water formation in the North Atlantic and the Southern Ocean. Freshening or salinification of high-latitude surface waters alters stratification and can modulate the strength of the Atlantic Meridional Overturning Circulation, with downstream consequences for regional climate. Work summarized by the NOAA Ocean Climate Monitoring program shows that the observed contrast between salty and fresh ocean regions has intensified over recent decades, consistent with an accelerated hydrological cycle under surface warming. Salinity also influences sea ice formation, since brine rejection during freezing produces dense surface water that sinks and ventilates the deep ocean.

Applications

Salinity has applications across a range of geophysical and engineering disciplines, including:

• Operational ocean forecasting and numerical weather prediction, where salinity fields constrain density and mixed-layer depth
• Climate research, particularly studies of the water cycle, sea level budget, and ocean heat uptake
• Fisheries and marine ecology, where salinity structure controls stratification, nutrient supply, and habitat distribution
• Estuarine and coastal engineering, including harbor design, desalination intake siting, and dredging impact assessment
• Underwater acoustics and naval systems, since sound speed depends on salinity along with temperature and pressure
• Geochemistry of evaporite deposits, submarine groundwater discharge, and paleo-ocean reconstructions from sediment cores