Ocean salinity is a fundamental property of seawater, defined as the concentration of dissolved salts within the water. The ocean is a vast, complex global salt solution, with an average salinity of about 35 parts per thousand. Understanding this measurement governs the physics, chemistry, and biology of the marine environment. Salinity is a powerful driver of global ocean systems and climate patterns.
Defining and Measuring Ocean Salinity
Salinity is defined as the amount of solid material, measured in grams, dissolved in one kilogram of seawater. The average open ocean holds approximately 35 grams of dissolved salt per kilogram of water, expressed as 35 parts per thousand (ppt). Modern oceanographers quantify this property using the Practical Salinity Scale (PSS-78), which yields the dimensionless Practical Salinity Unit (PSU). On this scale, 35 ppt is referred to as 35 PSU.
The dissolved solids are primarily ionic forms of common minerals; six ions account for over 99% of all sea salts. Chloride ions (Cl⁻) and sodium ions (Na⁺) are the most abundant, forming sodium chloride (common table salt). Other dissolved components include sulfate, magnesium, calcium, and potassium ions. Salinity is typically measured indirectly by assessing the water’s electrical conductivity, as the dissolved ions allow electricity to pass through more easily.
Sources and Modifiers of Salinity
Ocean salt originates from two long-term geological processes. Rivers deliver much of the salt by slowly eroding and dissolving minerals from continental rocks as they flow to the sea. The other major source is the Earth’s crust beneath the ocean floor, where hydrothermal vents release mineral-rich fluids. As seawater seeps into the crust near volcanically active zones, it is superheated, dissolving metals before being expelled back into the ocean.
While these geological sources determine the ocean’s total salt inventory, local salinity levels are constantly modified by the global water cycle. Evaporation, which removes pure water vapor and leaves the salt behind, is the most significant process that increases surface salinity. Conversely, the addition of freshwater through precipitation (rain or snow) dilutes the surface water and decreases salinity. Runoff from major rivers, such as the Amazon, also introduces large volumes of fresh water that significantly lower salinity near coastal areas and river mouths.
The freezing and melting of sea ice in polar regions also modify salinity. When seawater freezes, the salt is largely excluded from the ice crystals (brine rejection). This concentrated, salty water then sinks, increasing the salinity of the deep water below. When sea ice melts, it releases fresh water back into the ocean, creating a low-salinity surface layer.
Salinity’s Influence on Ocean Density and Circulation
Salinity and temperature are the two main factors determining seawater density, which governs the ocean’s vertical movement. Saltier water is denser than less salty water, and colder water is denser than warmer water. These density differences create the deep-ocean circulation system known as the thermohaline circulation (“thermo” for temperature, “haline” for salt). The circulation is driven by the sinking of cold, dense water masses in specific regions of the ocean.
In the polar regions, the formation of sea ice creates extremely cold, dense, saline water through the brine rejection process. This dense water sinks to the ocean floor, pulling surface water behind it and initiating a global-scale flow pattern often called the global conveyor belt. This massive, slow-moving current transports water throughout the world’s ocean basins, a journey that can take centuries. The thermohaline circulation acts as a global heat distribution system, carrying warm surface water from the tropics toward the poles and redistributing nutrients across the planet.
The density stratification created by salinity and temperature differences also affects marine life by controlling the mixing of water layers. Small changes in surface salinity, particularly in high-latitude regions, can alter the rate at which water sinks. By influencing deep-ocean currents, salinity regulates global climate patterns and the distribution of heat.
Global Patterns of Salinity Variation
Salinity is not uniformly distributed across the globe, with distinct patterns appearing based on the balance between evaporation and precipitation. The highest surface salinities are generally found in the subtropics, around 20° to 30° latitude north and south. In these regions, high temperatures and minimal rainfall lead to intense evaporation that concentrates the salt. Specific enclosed areas, such as the Mediterranean Sea and the Red Sea, exhibit high salinities (sometimes exceeding 40 ppt) due to limited freshwater inflow and high evaporation rates.
In contrast, the lowest salinities occur near the equator and in the high-latitude polar regions. Equatorial areas experience heavy rainfall, which dilutes the surface water despite the high temperatures. At the poles, the surface water is freshened by the melting of sea ice and glacial runoff. The North Pacific, which receives significant freshwater input from rivers and high precipitation, is generally less saline than the North Atlantic.
Scientists monitor these global salinity patterns because they serve as an indicator of changes in the global water cycle. Satellite missions and oceanographic instruments track shifts in the ocean’s saltiness to better understand the effects of climate change. For instance, the freshening of polar waters due to increased ice melt suggests a greater influx of freshwater, which can slow down the density-driven ocean circulation.