3 Seawater Properties
Learn how seawater’s composition, salinity, temperature, and density shape ocean layers and influence sinking, rising, and mixing.
Composition and
Seawater is a solution of water and dissolved substances. About of seawater by mass is water; most of the remainder is dissolved salts. The most abundant dissolved ions are chloride and sodium, with sulfate, magnesium, calcium, and potassium also present. Seawater also contains dissolved gases, nutrients, and trace substances.
varies across the ocean and with depth. Evaporation removes water but leaves salts behind, increasing . Rainfall, river inflow, and melting ice add freshwater and generally lower surface . When sea ice forms, much of the salt is excluded from the ice, making nearby seawater saltier.
Temperature across the ocean
Sunlight heats the ocean’s surface, so surface temperature varies with latitude, season, and weather. In much of the ocean, temperature decreases with depth: sunlight warms the upper water, while deep water receives little direct solar heating. Surface water can be very cold near polar regions and is generally warmer in tropical regions.
Under typical ocean conditions, cooler seawater is denser than warmer seawater. Seawater also freezes at a lower temperature than freshwater because dissolved salts lower its freezing point.
and water movement
Seawater depends mainly on temperature and , and also on pressure, especially at great depth. Pressure rises with depth. Increasing increases , decreasing temperature increases , and increasing pressure slightly increases .
A parcel of water that is denser than its surroundings tends to sink, while less-dense water tends to rise. Cold, salty water can sink below warmer or fresher water. These movements contribute to the ocean’s vertical circulation.
and ocean layers
describes the layering of ocean water caused by differences in properties such as temperature, , and . Because denser water tends to lie below less-dense water, layers can remain distinct until winds, currents, or turbulence mix them.
A typical ocean profile has a surface mixed layer, a transition zone, and a deep layer. These layers are not fixed or identical throughout the ocean.
Mixed, transition, and deep layers
Wind and waves mix the upper ocean in the surface mixed layer, making its temperature and other properties relatively uniform. The layer’s depth changes with location and season.
In the transition zone, temperature or may change rapidly with depth. A is a zone of rapid temperature change, a is a zone of rapid change, and a is a zone of strong vertical change.
Below the transition zone, the deep layer is generally cold and comparatively uniform in temperature. Its properties still vary among ocean regions and water masses.
How layers change and mix
Seasonal heating can strengthen surface layering, while winter cooling and stronger mixing can weaken it. Freshwater entering at the surface can create a less-salty layer above denser water.
For example, a warm, relatively fresh parcel and a cold, salty parcel will generally have different densities: the cold, salty parcel will generally be denser and tend to sink beneath the warm, fresh parcel. If wind or turbulence mixes the water, the boundary between the parcels becomes less distinct.