Density of seawater and pressure

The density of a material is given in units of mass per unit volume and expressed in kilograms per cubic metre in the SI system of units. In oceanography the density of seawater has been expressed historically in grams per cubic centimetre. The density of seawater is a function of temperature, salinity, and pressure. Because oceanographers require density measurements to be accurate to the fifth decimal place, manipulation of the data requires writing many numbers to record each measurement. Also, the pressure effect can be neglected in many instances by using potential temperature. These two factors led oceanographers to adopt a density unit called sigma-t (σ_t). This value is obtained by subtracting 1.0 from the density and multiplying the remainder by 1,000. The σ_t has no units and is an abbreviated density of seawater controlled by salinity and temperature only. The σ_t of seawater increases with increasing salinity and decreasing temperature.

The relationship between pressure and density is demonstrated by observing the effect of pressure on the density of seawater at 35 psu and 0 °C. Because a one-metre (three-foot) column of seawater produces a pressure of about one decibar (0.1 atmosphere), the pressure in decibars is approximately equal to the depth in metres. (One decibar is one-tenth of a bar, which in turn is equal to 10⁵ newtons per square metre.)

Values associated with the change in seawater density with depth are listed in the table.

Increasing density values demonstrate the compressibility of seawater under the tremendous pressures present in the deep ocean. If seawater were incompressible, each cubic centimetre of water in the water column would expand, and density values at all depths would be equal. If the average pressure occurring at a depth of 4,000 metres (about 13,100 feet, the approximate mean depth of the ocean) was somehow replaced with the average pressure that occurred at 2,000 metres (about 6,600 feet) and the area of the oceans remained constant, there would be an average sea level rise of about 36 metres (120 feet).

The temperature of maximum density and the freezing point of water decrease as salt is added to water, and the temperature of maximum density decreases more rapidly than the freezing point. At salinities less than 24.7 psu the density maximum is reached before the ice point, while at the higher salinities more typical of the open oceans the maximum density is never achieved naturally. For example, at 5 psu a density maximum is found between 0 and 10 °C (32 and 50 °F). (Its actual position is at 3 °C [37.4 °F], where the σ_t value is 4.04 for 5 psu.) This ability of low-salinity water and, of course, fresh water to pass through a density maximum makes them both behave differently from marine systems when water is cooled at the surface and density-driven overturn occurs.

During the fall a lake is cooled at its surface, the surface water sinks, and convective overturn proceeds as the density of the surface water increases with the decreasing temperature. By the time the surface water reaches 4 °C (39.2 °F), the temperature of maximum density for fresh water, the density-driven convective overturn has reached the bottom of the lake, and overturn ceases. Further cooling of the surface produces less dense water, and the lake becomes stably stratified with regard to temperature-controlled density. Only a relatively shallow surface layer is cooled below 4 °C. When this surface layer is cooled to the ice point, 0 °C, ice is formed as the latent heat of fusion is extracted. In a deep lake the temperature at depth remains at 4 °C. In the spring the surface water warms up and the ice melts. A shallow convective overturn resumes until the lake is once more isothermal at 4 °C. Continued warming of the surface produces a stable water column.

In seawater in which the salinity exceeds 24.7 psu, convective overturn also occurs during the cooling cycle and penetrates to a depth determined by the salinity and temperature-controlled density of the cooled water. Since no density maximum is passed, the thermally driven convective overturn is continuous until the ice point is reached where sea ice forms with the extraction of the latent heat of fusion. Since salt is largely excluded from the ice in most cases, the salinity of the water beneath the ice increases slightly, and a convective overturn that is both salt- and temperature-driven continues as sea ice forms.

The continuing overturn requires that a large volume of water be cooled to a new ice point dictated by the salinity increase before additional ice forms. In this manner, very dense seawater that is both cold and of elevated salinity is formed. Such areas as the Weddell Sea in Antarctica produce the densest water of the oceans. This water, known as Antarctic Bottom Water, sinks to the deepest depths of the oceans. The continuing overturn slows the rate at which the sea ice forms, limiting the seasonal thickness of the ice. Other factors that control the thickness of ice are the rate at which heat is conducted through the ice layer and the insulation provided by snow on the ice. Seasonal sea ice seldom exceeds about 2 metres (about 7 feet) in thickness. During the warmer season, melting sea ice supplies a freshwater layer to the sea surface and thereby stabilizes the water column (see sea ice).

Surface processes that alter the temperature and salinity of seawater contribute to the process of driving the vertical circulation of the oceans. Known as thermohaline circulation, it continually replaces seawater at depth with water from the surface and slowly replaces surface water elsewhere with water rising from deeper depths.