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.