Ocean Water: Density

Temperature, salinity and pressure work together to determine water density (weight of water divided by the amount of space it occupies). Cold, salty water is much denser than warm, fresher water and will sink below the less dense layer.

The ocean waters can be divided into three layers, depending on their densities. Less dense waters form a top layer called the surface mixed zone. The temperature and salinity of this layer can change often because it is in direct contact with the air. For example, water evaporation could cause an increase in salinity, and a cold front could cause a drop in temperature.

The next layer is the pycnocline, or transition zone. The density here does not change very much. This transition zone is a barrier between the surface zone and a bottom layer, allowing little water movement between the two zones.

The bottom layer is the deep zone, where the water remains cold and dense. The polar regions are the only places where deep waters are ever exposed to the atmosphere because the pycnocline is not always present.