Density Stratification in the Ocean

The temperature of seawater is fixed at the sea surface by heat exchange with the atmosphere. The average incoming energy from the Sun at the Earth s surface is about four times higher at the equator than at the poles. The average infrared radiation heat loss 

Because the seawater signatures of temperature and salinity are acquired by processes occurring at the air-sea interface, we can also state that the density characteristics of a parcel of seawater are determined when it is at the sea surface. This density signature is locked into the water when it sinks. The density may be modified by mixing with other parcels of water, but if the density signatures of all the end member water masses are known, and there aren t too many of them, this mixing can be unraveled and the proportions of the different source waters to a given parcel can be determined. 

To a first approximation, the vertical density distribution of the ocean can be described as a threelayered structure. The surface layer is the region from the sea surface to the depth having a temperature of about 10° C. The transition region where the temperature decreases from 10 to 4° C is called the thermocline. The deep sea is the region below the thermocline. 

9-4 Simulated three-dimensional T-S diagram of the water masses of (a) the Pacific, (b) the Indian, and (c) the Atlantic oceans. Apparent elevation is proportional to volume. Elevation of highest peak corresponds to (a) 26.0 x 10 , (b) 6.0 X 10 , and (c) 4.6 x 10 km per bivariate class 0.1 C x 0.01%o. Reproduced from Worthington (1981) with the permission of MIT Press. 

Because temperature (T) and salinity (S) are the main factors controlling density, oceanographers use T-S diagrams to describe the features of the 

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The subject of geophysical fluid dynamics encompasses two effects not usually considered in other areas of fluid mechanics rotation of the fluid and the existence of density stratifications in the oceans and atmosphere. Together, these lead to very important and fascinating qualitative changes in the fluid motions. A useful account of some of the phenomena encountered in rotating and stratified fluids can be found in Chaps. 15 and 16 of DJ Tritton, Physical Fluid Dynamics 2nd ed. (Oxford University Press, Oxford, 1988). 

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