Ocean salinity

1. Meaning, composition and measurement

Salinity expresses the concentration of dissolved salts in water, rather than the total quantity of salt in an ocean basin. Average seawater contains roughly 35 grams of dissolved salts per kilogram of seawater. This is conventionally represented as 35 parts per thousand, or 35‰, and is equivalent to approximately 3.5% by mass. Confusing parts per thousand with percentage is a common examination error: normal seawater is not 35% salt.

The dominant dissolved ions are chloride, sodium, sulphate, magnesium, calcium and potassium. Chloride contributes about 55% and sodium about 31% of the total dissolved salts by mass. In ordinary open-ocean water, the relative proportions of the major ions remain nearly constant even when total salinity changes. This principle of constant proportions does not necessarily hold in isolated saline lakes or waters strongly modified by chemical reactions.

Modern instruments commonly determine Practical Salinity from electrical conductivity, with corrections involving temperature and pressure. Practical Salinity is dimensionless, although PSU is widely encountered in teaching material. The modern TEOS-10 framework uses Absolute Salinity, expressed in grams per kilogram, for thermodynamic calculations. For basic geographical comparisons, values near 35 are usually adequate, but the measurement conventions should not be treated as formally identical.

• Salinity concerns dissolved salts; suspended sediment does not form part of the salinity measurement.
• Oceanic salts originate partly from continental weathering and river transport, while seafloor hydrothermal processes both supply and remove particular dissolved substances.

2. Processes controlling salinity

Surface salinity mainly reflects the freshwater balance and the movement of water. Evaporation removes water while leaving most dissolved salts behind, increasing salinity. Precipitation and river discharge add relatively fresh water and dilute seawater. Thus, evaporation exceeding precipitation favours higher salinity, whereas precipitation exceeding evaporation favours lower salinity. Temperature influences evaporation, but temperature alone cannot explain salinity patterns because humidity, wind and freshwater supply also matter.

Freezing and melting have contrasting effects. During sea-ice formation, most salt is excluded from the growing ice and enters the surrounding water through brine rejection. The remaining seawater becomes saltier and potentially denser. Melting sea ice introduces relatively fresh water and usually lowers near-surface salinity. Iceberg and glacier melt also freshen seawater. Sea ice is not completely salt-free because some brine can remain trapped within it.

Currents transport saline or fresh water away from its source region, while winds, waves and turbulence mix contrasting water masses. Basin geometry matters as well: restricted exchange through a narrow strait can preserve unusually high or low salinity. Consequently, coastal salinity can vary sharply over short distances and seasons. Monsoon rainfall, river floods and changes in circulation can outweigh the broad latitudinal pattern near continental margins.

• Useful diagnostic: assess evaporation minus precipitation, then river input, ice processes and water exchange.
• Evaporation changes concentration rather than adding salt; dilution reduces concentration without necessarily removing salt.

3. Horizontal distribution and regional contrasts

Across the open ocean, surface salinity commonly peaks in the subtropical belts, broadly around 20°–30° latitude in both hemispheres. Descending air associated with subtropical high-pressure systems suppresses rainfall, while evaporation remains substantial. Equatorial waters are warm but receive abundant convectional rainfall, so their surface salinity is generally lower than that of the subtropics. This is why the hottest ocean waters are not automatically the saltiest.

Higher latitudes generally have lower surface salinity because evaporation is weak and precipitation, runoff and seasonal melting supply freshwater. However, sea-ice formation can locally increase salinity, especially in polar shelf waters. Broad global patterns therefore coexist with strong seasonal and regional exceptions. The Atlantic is generally saltier than the Pacific, reflecting basin-scale differences in freshwater balance, atmospheric moisture transport and ocean circulation rather than simply differences in ocean size.

In the northern Indian Ocean, the Arabian Sea is generally saltier at the surface than the Bay of Bengal. Strong evaporation and relatively smaller freshwater inputs favour Arabian Sea salinity. The Bay receives heavy monsoon rainfall and discharge from rivers including the Ganga–Brahmaputra–Meghna system, Mahanadi, Godavari and Irrawaddy. The resulting fresh surface layer is especially prominent in the northern bay and changes seasonally.

Restricted seas illustrate the importance of local water budgets. The Red Sea has salinity commonly around 40‰ or higher in its northern part because of aridity, strong evaporation and negligible perennial river inflow. The Mediterranean is also more saline than the adjacent Atlantic. Conversely, the Baltic is brackish because freshwater supply is large relative to evaporation and exchange with the North Sea is restricted.

• The Dead Sea and Great Salt Lake are inland saline lakes, not parts of the ocean.
• Regional salinity figures are approximate: location, depth, season and observational method affect reported values.

4. Vertical distribution, halocline and density

Vertical salinity profiles differ between regions. In many subtropical areas, relatively saline surface water overlies less saline water, so salinity decreases downward through part of the upper ocean. In freshwater-influenced high-latitude regions, a low-salinity surface layer often overlies saltier water, producing an increase with depth. Neither pattern should be presented as a universal rule for all oceans.

A halocline is a zone where salinity changes markedly with depth. It differs from a thermocline, which is defined by a temperature gradient, and a pycnocline, which is defined by a density gradient. These layers may overlap but are not synonymous. Below the strongly variable upper ocean, salinity usually changes more slowly, although distinct intermediate and deep-water masses retain characteristic temperature–salinity signatures.

Increasing salinity raises seawater density when temperature and pressure are held comparable. Cooling also generally increases the density of seawater under normal oceanic conditions. Cold, saline water can therefore sink and help form deep or bottom water. Nevertheless, overturning circulation is not driven by salinity alone: atmospheric forcing, winds, mixing and basin geometry are also essential. A fresh surface cap can stabilise the water column and inhibit vertical convection.

• Temperature–salinity diagrams help oceanographers identify and trace water masses.
• Salinity lowers the freezing point: typical seawater begins freezing near −1.9°C at surface pressure.

5. Climatic, ecological and observational significance

Salinity connects the ocean to the global water cycle. Changes in evaporation, rainfall, river flow and ice melt leave salinity signatures, but currents can transport those signatures far from their origin. Salinity observations therefore help diagnose freshwater redistribution rather than providing a simple local rainfall record. Along with temperature, salinity controls density structure and influences how heat, oxygen and nutrients move through the ocean.

Low-salinity surface layers can restrict mixing with deeper waters. In the Bay of Bengal, salinity stratification can help create a barrier layer that limits entrainment of cooler subsurface water. This affects upper-ocean heat storage and can influence cyclone–ocean interactions, although cyclone intensity also depends on atmospheric conditions and other ocean properties. Freshwater inflow and saline intrusion similarly shape estuarine habitats, fisheries and the availability of usable coastal water.

Measurements come from ship-based sampling, conductivity–temperature–depth instruments, moored sensors and autonomous Argo floats. Most standard Argo floats profile approximately the upper 2,000 metres. Satellite missions such as ESA’s SMOS and NASA’s SMAP estimate sea-surface salinity from microwave observations. These observations are complementary: satellites provide broad surface coverage, while in-water instruments reveal vertical structure and support calibration and validation.

• For map-based questions, connect subtropical dry belts, major river mouths, polar ice zones and restricted sea passages with expected salinity patterns.
• For statement questions, reject absolute claims such as salinity always increasing with depth or equatorial waters always having maximum salinity.

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