Brief notes on the Factors Controlling the Density of Ocean Water

Temperature:

Temperature is the most important of all the factors that has an important influence on the density of ocean water. In the open ocean, in general, it is the temperatures that determine the distribution pattern of the density. That is why the temperature and density distribution in the open sea show similar characteristics.

There is a gradual and regular increase in the density of sea water from the equator towards the poles. In spite of the high surface salinities in the subtropical regions, there is no increase in the thermally produced low density of the sea water there.

It is undoubtedly true that if there are large differences in the densities of adjacent water masses they cannot remain in equilibrium. The horizontal variations in the surface salinities result in large – scale water movements in the oceans.

The densities of ocean water at the atmospheric pressure and 35 %o salinity have been shown at different temperatures.

Salinity:

Salinity is another very important factor that influences the ocean water density. At a certain temperature the density increases with increasing salinity. This is due to the fact that the components of salinity have a greater density than the distilled water. It is a well-known fact that at 4°C the density of pure water is 1.00g/cm³.

But when some salt is dissolved in it, the salt water thus created will have higher density. This is so because the salt water is heavier than the pure water. At the same temperature, therefore, the sea water will be denser than the pure water which always floats upon it.

Now it is clear that any further increase in the amount of salinity of the sea water results in the corresponding increase in its density, even though the temperature does affect the density. It is to be borne in mind that at 4°C temperature with 35%o salinity the density of sea water is 1.0278 g/cm³.

However, if the salinity is reduced to 20%o, then its relative density will also be reduced to 1.016g/cm³. In the Caspian Sea, the Red Sea, and the Mediterranean Sea the salinity is high due to the higher rate of evaporation.

Pressure:

Pressure is another variable which controls the density of sea water, because the pressure and compressibility of sea water are very closely related. Any increase in the air pressure will bring the different layers of water closer together due to mutual pressure.

Naturally it results in an increase in the density, since volume of the water decreases. It is worthwhile to remember that there exists a very close interrelationship between the temperature, the pressure, and the salinity of sea water. The higher the temperature, the lower is the air pressure.

Similarly, the degree of salinity directly affects the density. However the density decreases with decreasing air pressure. On the contrary, in case of any decrease in air temperature and increase in air pressure the density is bound to be high.

If the density of the surface water is higher than that of the adjacent subsurface layer, vertical convection occurs leading to the formation of a layer of homogeneous water.

In areas where intensive cooling occurs at the surface, the vertical convection currents move to greater depths until the density from the surface to the bottom attains homogeneity.

In case the increase in density at the surface stops before the convection currents have penetrated to the bottom, the process may result in the formation of water that spreads at an intermediate level.

Convergence and divergence:

The processes of convergence and divergence exert an important influence on the density distribution in the oceans. Vertical movement of water masses are due either to differences in the density at various depths as pointed out earlier or to the meeting of two types of currents converging upon one another, thus causing sinking which must be counter-balanced elsewhere by ascending currents.

In areas where convergence occurs, water of the relatively higher density moves downwards. On the other hand, in areas of divergences, the ascending motion of bottom water of higher density and lower temperature towards the surface. This process is called upwelling. This process is generally the result of offshore winds.

The divergences are most clearly observed off the west coasts of the continents, where the trade winds drive the warm surface water westwards.

Similarly, in the westerlies this process occurs along the east coast of the continents. Thus, the loss of water along the sea coast must be compensated by ascending currents which bring bottom water at the surface.

Other Minor Factors:

In addition to the above stated major factors, there are some other minor factors which also affect the density of the sea water. Rainwater, the surface run-off brought by the rivers flowing into the oceans, and melt water from the ice and snow deposited on the ground are such minor factors as contribute to lower down the density.

On the contrary, evaporation, cooling of the surface water and the process of ice formation tend to increase the density of the ocean water.

It is worthwhile to bear in mind that the differences in the density of ocean water are mainly responsible for controlling the pattern of oceanic circulation. Besides the ocean currents, the density difference is one of the major factors that cause various kinds of the vertical as well as horizontal motions in the lower layers of the ocean water.