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Ocean salinity results from a balance between evaporative loss, precipitation input, river discharge, and ice formation/melting. Salinity patterns create density contrasts that fuel thermohaline circulation, with regional salinity variations reflecting local hydrology and climate conditions.
You already know from ocean density and thermal stratification that seawater density depends on both temperature and salinity, and that density differences drive the ocean's vertical structure. Now consider the salinity side of that equation in detail. Salinity — the total mass of dissolved salts per kilogram of seawater, typically expressed in practical salinity units (PSU) — averages about 35 PSU globally, but varies significantly from place to place. Understanding why requires thinking about salinity as a budget: processes that add or remove fresh water change the salt concentration of what remains.
The two dominant controls on surface salinity are evaporation and precipitation. Evaporation removes pure water from the ocean surface, leaving salt behind and increasing salinity. Precipitation adds fresh water, diluting the salt and decreasing salinity. This is directly analogous to the water cycle you studied as a prerequisite — the same atmospheric processes that move water from ocean to atmosphere and back also reshape the ocean's salt distribution. In the subtropical ocean basins (around 20–30° latitude), where dry descending air from the Hadley cell drives intense evaporation and little rain falls, surface salinity is highest — often exceeding 37 PSU. Near the equator, where the Intertropical Convergence Zone delivers heavy rainfall, and at high latitudes, where precipitation exceeds evaporation, surface salinity drops below 34 PSU. The global pattern of surface salinity is essentially a mirror of the pattern of evaporation minus precipitation.
Two additional processes act as significant freshwater sources and sinks. River discharge injects large volumes of fresh water near coastlines, creating pronounced low-salinity plumes — the Amazon River, for example, depresses surface salinity across thousands of square kilometers of the tropical Atlantic. At high latitudes, sea ice formation is a powerful salt source: when seawater freezes, most of the dissolved salt is expelled from the growing ice crystal in a process called brine rejection, leaving behind cold, extremely salty water that is dense enough to sink to the ocean floor. Conversely, when sea ice melts in spring, it releases relatively fresh water that caps the surface and freshens the upper ocean.
These salinity contrasts matter because they directly affect density and therefore circulation. The dense, salty water produced by subtropical evaporation and polar brine rejection feeds the deep limb of the thermohaline circulation — the global conveyor belt that moves water masses between ocean basins over centuries. Changes in the freshwater budget — increased glacial meltwater, shifts in precipitation patterns, or altered river runoff — can weaken or redirect these density-driven flows. Salinity is not just a chemical property of seawater; it is a dynamical variable that links the atmosphere's water cycle to the ocean's deepest circulation.