Salinity, the amount of dissolved salt in water, is shaped by a surprisingly wide range of forces. In the ocean, the global average sits around 34.7 practical salinity units (psu), but actual values swing from as low as 7 in the Baltic Sea to nearly 40 in the Red Sea. What creates that variation comes down to a balance between processes that add salt or remove freshwater and processes that dilute saltwater with freshwater inputs.
Evaporation and Precipitation
The single biggest driver of ocean salinity is the balance between evaporation and precipitation. When water evaporates from the sea surface, the salt stays behind, concentrating in the remaining water. When rain falls on the ocean, it dilutes the surface, lowering salinity. The pattern this creates is remarkably consistent: subtropical oceans (roughly 20° to 30° from the equator) have the highest surface salinities because evaporation far outpaces rainfall in those hot, dry belts. Tropical waters near the equator and polar regions, where precipitation or meltwater is abundant, tend to be fresher.
This evaporation-minus-precipitation balance also drives seasonal shifts. In tropical convergence zones, where heavy monsoon rains fall for months at a time, surface salinity can drop noticeably during wet seasons. In the western North Pacific and North Atlantic, high evaporation rates during warmer months push salinity upward. Away from coastlines and ice, evaporation and precipitation are the dominant forces controlling how salty the surface ocean becomes.
River Runoff and Freshwater Discharge
Major rivers inject enormous volumes of freshwater into the ocean, creating low-salinity plumes that can extend hundreds of kilometers offshore. The Amazon River, the world’s largest by discharge, peaks at around 276,000 cubic meters per second. That outflow is so massive that its freshwater signal is detectable across a wide swath of the tropical Atlantic. Even anomalies in the Amazon’s flow, variations of about 50,000 cubic meters per second from average, measurably shift surface salinity across the region. The Congo River, peaking near 56,000 cubic meters per second, has a similar effect off the west coast of Africa.
Closer to shore, the impact is even more pronounced. Estuaries and coastal zones experience dramatic salinity gradients where river water mixes with seawater. These gradients shift with the seasons: spring snowmelt or monsoon rains swell rivers, pushing fresher water farther out to sea, while dry seasons allow saltwater to creep upstream.
Ice Formation and Melting
Sea ice plays a dual role. When seawater freezes, most of the dissolved salt is expelled from the forming ice crystals in a process called brine rejection. The leftover brine is denser and saltier than the surrounding water, so it sinks. If conditions are right, this heavy, salty water cascades down the continental slope and into the deep ocean, carrying salt, nutrients, and dissolved gases far below the surface. This process is one of the engines that drives deep ocean circulation.
Melting works in reverse. When sea ice or glacial ice melts, it releases relatively fresh water that lowers surface salinity. This is playing out on a large scale around Greenland, where accelerating ice sheet melt has been freshening the North Atlantic. Satellite and ocean profile data from 1993 to 2020 show that salinity near Greenland’s coast has dropped, with the decrease extending from the surface down to more than 2,000 meters. Year-to-year swings in North Atlantic surface salinity have grown dramatically since around 2005, with especially low values recorded near Greenland in 2008, 2010, 2015, and 2017.
Ocean Currents and Mixing
Water doesn’t stay put after evaporation or rainfall changes its salt content. Ocean currents constantly redistribute salinity. Wind-driven surface currents (called Ekman transport) push water masses horizontally, spreading salty or fresh water far from where it originated. Outside the tropics, this horizontal movement of water is actually the dominant control on seasonal salinity changes, outweighing local evaporation and rainfall.
Vertical mixing matters too. Deeper water with different salt concentrations can be stirred upward into surface layers by wind, waves, or density-driven overturning. Salinity and temperature together determine seawater density: saltier water is denser, and colder water is denser. Small differences in density from place to place create pressure gradients that drive large-scale ocean circulation, sometimes called the global conveyor belt. This system moves water between the surface and the deep ocean over centuries, redistributing salt and heat worldwide.
How Salinity Changes With Depth
Salinity isn’t uniform from surface to seafloor. In many parts of the ocean, there’s a distinct layer called the halocline where salinity changes sharply over a relatively short depth range. Above the halocline, surface processes like rain, river input, and evaporation control salt levels. Below it, deep water retains the salinity it carried when it sank from the surface, sometimes in a completely different part of the ocean.
The halocline acts as a barrier. In regions with heavy freshwater input, like the Arctic or near large river mouths, a strong halocline can trap warmer, saltier water below a fresher surface layer. This layering, called stratification, limits the exchange of heat and nutrients between surface and deep water. When stratification weakens, nutrients rise and surface conditions can change rapidly.
Why Some Seas Are Extreme
Geography amplifies or dampens the forces described above, which is why enclosed or semi-enclosed bodies of water often sit at the extremes. The Red Sea averages about 39.8 psu, the highest of any major sea. It’s surrounded by desert with almost no river input and intense evaporation year-round, so salt just keeps concentrating. The Baltic Sea, by contrast, averages only about 7.2 psu. It receives massive freshwater inflow from rivers across northern Europe and has a narrow connection to the Atlantic that limits saltwater exchange.
The open ocean falls between these extremes but still varies by latitude. The saltiest surface waters sit in the subtropical belts, while equatorial and polar waters are fresher. The pattern mirrors the global distribution of rainfall and evaporation almost exactly.
Factors Affecting Salinity on Land
Salinity isn’t only an ocean issue. Groundwater and soil salinity affect agriculture, drinking water, and ecosystems worldwide. On land, salinity is driven by a different but overlapping set of forces.
Geological sources matter first. Some rock formations and soil layers are naturally rich in soluble salts. As water moves through these formations, it dissolves minerals and carries them into aquifers. In arid regions, high evaporation and low rainfall concentrate these salts further because there isn’t enough rain to flush them deeper into the ground or dilute them.
Human activity accelerates the problem. Excessive groundwater pumping for irrigation can draw salty water upward from deeper zones into shallower freshwater aquifers. Heavy use of chemical fertilizers adds dissolved salts to the soil. Poor drainage traps salty irrigation water near the surface, and over time, saline zones expand laterally and vertically. In coastal areas, over-extraction of groundwater lowers the water table enough for seawater to intrude inland, contaminating wells that were previously fresh.
How Salinity Is Measured
Modern salinity measurement relies on electrical conductivity. Saltier water conducts electricity more readily, so scientists lower instruments called CTDs (conductivity, temperature, and depth sensors) into the water column and calculate salinity from the conductivity reading, adjusted for temperature and pressure. The result is expressed in practical salinity units on a scale established in 1978. A reading of 35 psu, for example, means the water conducts electricity at the same rate as a standard solution containing 35 grams of salt per kilogram of water. Satellites can also estimate surface salinity from space by measuring microwave emissions from the ocean surface, which change with salt concentration.