Define density. Identify those properties of seawater that density is dependent on. Recognize how density effects seawater stability.
The density of seawater is dependent on salinity, temperature, and pressure. At constant temperature and pressure, density varies with salinity. A temperature of 32°F and an atmospheric pressure of 1,013.2 mb are considered standard for density determination. At other temperatures and pressures the effects of thermal expansion and compressibility are used to determine density. The density at a particular pressure affects the buoyancy of various objects, notably (specialmente) submarines. Density is defined as mass per unit volume, and is expressed in grams per cubic centimeter.
The greatest changes in density of seawater occur at the surface. Here, density is decreased by precipitation, runoff from land, melting of ice, or heating. When the surface water becomes less dense, it tends to float on top of the more dense water below. There is little tendency for the water to mix; therefore, the condition is one of stability. The density of surface water is increased by evaporation, the formation of sea ice, and cooling. If the surface water becomes more dense than the water below, it sinks to a level having the same density. Here, it tends to spread out to form a layer, or to increase the thickness of the layer of which it has become a part. As the more dense water sinks, the less dense water rises, and a convective circulation is established. The circulation continues until the density becomes uniform from the surface to a depth at which a greater density occurs. If the surface water becomes sufficiently dense, it sinks all the way to the bottom. If this occurs in an area where horizontal flow is unobstructed, the water that has descended spreads to other regions, creating a dense bottom layer. Since the greatest increase in density occurs in polar regions, where the air is cold and great quantities of ice form, the cold, dense polar water sinks to the bottom and then spreads to lower latitudes.
This process has continued for such a long period of time that the entire ocean floor is covered with this dense polar water. This explains the layer of cold water at great depths in the ocean.