What Is Sea Surface Salinity?

Sea surface salinity (SSS) is the concentration of dissolved salts in seawater at or near the ocean surface, expressed in practical salinity units (PSU) or, equivalently, parts per thousand. Open ocean surface salinity ranges from roughly 33 to 38 PSU, with fresher values in high-latitude seas receiving abundant precipitation and river input, and higher values in the subtropics where evaporation exceeds precipitation. SSS is a defining parameter for ocean dynamics: together with temperature, it determines seawater density, which drives the thermohaline circulation that redistributes heat and carbon around the planet.

The field draws from physical and chemical oceanography, microwave remote sensing, and geophysics. In-situ instruments such as CTDs (conductivity-temperature-depth profilers) and the global Argo float network provide direct salinity measurements, while satellite missions have opened the possibility of synoptic global coverage.

Satellite Measurement Techniques

The salinity of seawater modulates the L-band (1.4 GHz) microwave emission of the sea surface, making spaceborne radiometry the primary remote sensing approach. The relationship between L-band brightness temperature and salinity is weaker than for soil moisture, but sufficient for open-ocean retrievals with target accuracy near 0.2 PSU. The European Space Agency's SMOS mission, launched in 2009, carried the MIRAS L-band synthetic aperture radiometer and demonstrated that global SSS could be monitored from orbit. NASA's SMAP satellite, designed primarily for soil moisture, provides complementary SSS retrievals at roughly 40-km resolution with global coverage approximately every three days. Both missions deliver gridded daily and monthly SSS datasets through archives such as NASA's Physical Oceanography DAAC (PO.DAAC). Accuracy degrades for water colder than about 5 degrees Celsius, where the dielectric sensitivity of seawater to salinity drops sharply, and within about 50 km of coastlines where land contamination biases the brightness temperature.

Ocean Circulation and the Global Water Cycle

SSS acts as a natural tracer of the global water cycle because precipitation dilutes salinity and evaporation concentrates it. Regions of high SSS in the subtropical Atlantic coincide with net evaporation zones that export freshwater vapor to higher latitudes, while the low-salinity tongue extending westward across the equatorial Pacific traces the influence of the Intertropical Convergence Zone rainfall. NOAA CoastWatch SSS products provide satellite-derived data used to track freshwater lens dynamics after tropical cyclones and monitor changes in the Amazon River plume. SSS gradients also affect biological productivity, as buoyancy-driven stratification from freshwater input can suppress nutrient upwelling and alter phytoplankton bloom timing. Long-term trends in SSS, toward higher salinity in already-salty regions and lower salinity in fresh regions, are a diagnostic indicator of an accelerating hydrological cycle under climate change. NASA's Technical Reports Server documents comparisons of satellite and in-situ SSS observations that have guided retrieval algorithm improvements across missions.

In-Situ Observation Networks

Shipboard thermosalinographs continuously sample surface water pumped from the ship's intake, providing high-spatial-resolution along-track salinity records in regions remote from Argo float coverage. Surface drifters equipped with conductivity sensors add near-surface salinity observations in equatorial and coastal zones. Research vessels conduct periodic cross-basin transects under programs such as the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP), validating satellite retrievals and anchoring the long-term climate record.

Applications

Sea surface salinity has applications in a wide range of disciplines, including:

• Global hydrological cycle monitoring and freshwater flux estimation
• Ocean circulation and thermohaline dynamics research
• Tropical cyclone intensity and track forecasting
• Marine ecosystem and fisheries management
• Climate model initialization and validation