Density of Seawater. Stability in the Ocean

The density of seawater varies with changes in temperature, salinity, and, to a minor extent, pressure. Most changes in density occur at the sea surface where solar heating, cooling, precipitation, and evaporation cause the greatest fluctuations in temperature and salinity.

As pressure increases, the density of seawater increases; however, this density increase is slight because seawater is practically incompressible. It is only in the greatest ocean depths that this pressure effect is measurable, and even there the changes are small.

When ions are added to a fixed volume of water, its mass increases. In just this way, an increase in salinity produces an increase in the density of seawater (Fig. 6-1). Salinity changes also affect the temperature at which seawater freezes. As water salinity increases, water must be colder to freeze (Fig. 6-2). Thus, seawater freezes over a range of temperatures rather than at a single temperature. For seawater of average salinity, freezing begins at approximately —2°C. As ice begins to form, however, the remaining brine becomes more saline (sea ice is essentially free of ions) and must reach an even lower temperature to freeze.

Figure 6-1. Curve showing the variation of density with salinity for water of 10°C

Figure 6-2. Curve showing the variation of freezing point of water as a function of salinity

The effect of temperature on the density of seawater is more complex than that of pressure or salinity. The density of most seawater (where salinity is greater than 25 parts per thousand) decreases with an increase in temperature (Fig. 6-3), so that cold seawater is more dense than warmer water that has the same salinity. Seawater with salinity less than 25 parts per thousand (brackish) becomes less dense if cooled below the temperature at which it reached maximum density.

Figure 6-3. Variation of density of seawater (35 %o) with temperature

The combined effects of temperature and salinity on the maximum density and freezing point of water are demonstrated in Fig. 6-4. Pure water reaches its maximum density at 4°C, but saline water reaches its maximum density at a lower temperature. Most seawater (i.e., seawater having a salinity greater than 25 parts per thousand) has no maximum density peak, because it freezes before this peak is reached. In other words, the density of seawater increases with decreasing temperature until it freezes.

Figure 6-4. Effect of salinity on the temperature of maximum density and the initial freezing point of seawater

Several generalizations about density in the world ocean should be remembered: Seawater has a density between 1.02 and 1.03; in the ocean, density normally increases with depth. Seawater becomes less dense by either warming or dilution (precipitation or melting of ice), or both. Seawater becomes more dense when it is cooled or when its salinity is increased (by evaporation or formation of ice), or both. Pressure has no effect on salinity. Seawater density is more sensitive to temperature fluctuations than to salinity fluctuations (except near 0°C).

Stability in the Ocean. Stability in the ocean is governed by the depth distribution of density. Density, in turn, is governed by the distribution of temperature and salinity. Not only is density more sensitive to temperature changes than salinity changes, but the range of temperatures in the ocean is greater than the range of salinity values. Therefore, temperature is generally the most important factor in determining stability in the ocean.

Several combinations of temperature and salinity can lead to stability, instability, or indifferent stability in the ocean. Table 6-1 shows several examples of such combinations.

Table 6-1. Stability in the Ocean

Typical density profiles in the open ocean are illustrated in Fig. 6-5. These profiles are largely the result of the temperature distribution. The shape of the salinity profile depends upon geographic location. Areas where the salinity decreases with depth are usually areas where temperature decreases rapidly with depth so that stability is maintained.

Figure 6-5. Typical density profiles from three regions of the world ocean. Note scale distortion of depth to emphasize the surface regions of the column. (Data after Muromtsev, 1958)

The typical density profile describes warm water lying over cold water in a stable configuration. Quite different temperature and salinity conditions produce stability in fiords, estuaries, and landlocked seas. In Puget Sound and the Black Sea, for example, cold fresh runoff from rivers lies over warmer saline water flowing in from the sea. This causes extreme stability and is referred to as the freshwater lid effect.

In the open ocean, the shape of the salinity profile is governed by geographic effects on the balance of evaporation and precipitation. In the subtropical zones (20 to 30 latitude), evaporation exceeds precipitation, so surface water has the highest salinity in the world ocean. The profile of salinity of this surface water shows a decrease with depth.

In high latitudes and near the equator, precipitation exceeds evaporation, so the opposite salinity profile prevails, because the water is diluted near the surface For the entire world ocean, however, evaporation must exceed precipitation since the amount of water on earth is virtually constant (addition of juvenile water is negligible in this case). In other words, total runoff from land plus total precipitation must equal total evaporation.

The evaporation-precipitation balance causes important local effects in estuaries, bays, and lagoons at the periphery of the world ocean. These effects are discussed in detail in the section on inshore oceanography in Chap. 11 (.page 318).

Although an unstable density configuration cannot persist in nature, it does occur in a seasonal manner in middle and high latitudes. When instability occurs, a vertical circulation, or overturn, takes place, so that stability is restored. Figure 6-6 shows temperature, salinity, and density profiles taken at the four seasons in the open ocean.

During the summer (Fig. 6-6C), heating and evaporation lead to stability. In the fall (Fig. 6-6D), cooling at the surface causes the saline water to become more dense than underlying water, so overturn takes place. Mixing by storms tends to destroy the thermocline and enhance the overturn. This process continues throughout the winter (Fig. 6-6A), especially if ice forms at the surface, because the freezing of seawater increases the salinity of the residual brine. By spring (Fig. 6-6B), the warming of surface water and development of a thermocline reestablish a stable density profile that persists through the following summer.

At the continental shelf off Antarctica, ice forms on the surface of the sea every year. Here, therefore, there is an overturn every winter, because the freezing of seawater removes relatively pure water and leaves extremely saline and cold, dense brine that promptly sinks.

In summary, stability occurs when warm water lies over cool water as a result of solar heating at the surface, or when fresh water overlies saline water because of river outflow or excessive precipitation. Instability is usually caused by saline water overlying less saline water as a result of evaporation or freezing at the surface. Instability also results when cool water lies over warmer water because of seasonal cooling at the surface.

Indifferent stability exists when the temperature and salinity are uniform with depth. In the open ocean, this condition is invariably caused by mixing as a result of overturn, wind action on the sea surface, and currents. In local areas, such as Puget Sound, tidal currents are instrumental in producing mixing that leads ultimately to indifferent stability.