86 DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 



to the bottom. The bottom and deep waters that spread out from high 

 latitudes retain their low temperature, but in middle and lower latitudes 

 a warm top laj^er is present, the thickness of which depends partly on the 

 processes of heating and cooling at the surface and partly on the charactei' 

 of the ocean currents. This upper layer of warm water is separated from 

 the deep water by a layer of transition within which the temperature 

 decreases rapidly with depth. From analogy with the atmosphere, 

 Defant has applied the terms ''troposphere" and ''stratosphere" to two 

 different parts of the ocean. "Troposphere" is applied to the upper 

 layer of relatively high temperature which is found in middle and lower 

 latitudes and within which strong currents are present, and "strato- 

 sphere" is applied to the nearly uniform masses of cold deep and bottom 

 water. This distinction is often a very useful one, particularly when 

 dealing with conditions in lower latitudes, but it must be borne in mind 

 that the terms are based on an imperfect analogy with atmospheric condi- 

 tions and that only some of the characteristics of the atmospheric tropo- 

 sphere and stratosphere find their counterparts in the sea. 



So far, we have mainly considered an ideal ocean extending to high 

 northerly and high southerly latitudes. Actually, conditions may be 

 complicated by communication with large basins that contribute to the 

 formation of deep water, such as the Mediterranean Sea, but these cases 

 will be dealt with specifically when considering the different regions. 

 Conditions will be modified in other directions in the Indian and Pacific 

 Oceans, which are in direct communication with only one of the polar 

 regions, and these modifications will also be taken up later. Here it must 

 be emphasized that the general distribution of temperature is closely 

 related to the distribution of density, which again is controlled by external 

 factors influencing the surface density and the type of deep-sea circulation. 



The general distribution of salinity is more complicated than that of 

 temperature. Within the oceanic stratosphere the salinity is very 

 uniform, but within the troposphere it varies greath^, being mainly 

 related to the excess of evaporation over precipitation. The distribution 

 of surface sahnity, already discussed (p. 71), is in general characteristic 

 of the distribution within the troposphere, as is evident from the vertical 

 section in figs. 60 (p. 213) and 61 (p. 216). 



Water Masses 



The T-S Diagram. Water masses can be classified on the basis of 

 their temperature-salinity characteristics, but density cannot be used 

 for classification, because two water masses of different temperatures and 

 salinities may have the same density. For the study of water masses, it is 

 convenient to make use of the temperature-salinity (T-S) diagrarn, which 

 was introduced by Helland-Hansen. Helland-Hansen pointed out that 

 when in a given area the temperatures and corresponding salinities of the 



