Salinité

Salinité, known in English as salinity, is the measure of the concentration of dissolved salts in a solution, most commonly in seawater. It reflects the total dissolved inorganic salts, with sodium and chloride ions constituting the largest share. In oceanography, salinité is a key determinant of water density and plays a major role in driving ocean circulation, vertical mixing, and the distribution of marine life. Values vary with latitude, season, and proximity to land.

The standard scale for salinité in marine contexts is the Practical Salinity Scale (PSS-78), which expresses salinity in practical salinity units (PSU). PSU are dimensionless and numerically close to the mass fraction of dissolved salts in seawater; typical average seawater has a salinité of about 35 PSU (approximately 3.5% dissolved salts by mass). Freshwater generally has salinité below 0.5 PSU, while coastal and estuarine waters can vary widely due to river input and evaporation. Modern formulations, such as Absolute Salinity and the TEOS-10 framework, provide more rigorous links between salinité, density, and other thermodynamic properties, but PSU remains the common reference for many datasets and observations.

Salinité affects several physical and chemical properties of seawater, including density, temperature of melting and freezing, and the solubility of gases and minerals. Global patterns show higher salinité in subtropical regions due to evaporation, and lower salinité in polar regions and near rivers and rainfall. Local salinité can fluctuate with tides, river discharge, precipitation, and ice formation, influencing habitat suitability, nutrient cycling, and carbon exchange with the atmosphere.

Applications of salinité range from climate research and ocean modeling to desalination planning and water quality assessment. The term derives from the Latin sal, salt.