Density of Water Masses in Ocean, Vertical and Horizontal Density Distribution 193 



Table 79. Density a^ and potential density oq at ''Meteor'' station 310 

 (19-3° N., 25-0° W.) 



Atmospheric isentropic analysis requires an investigation of conditions on a sur- 

 face of constant entropy. In the atmosphere, provided there is no condensation, these 

 surfaces are identical with surfaces of constant potential temperature and also with 

 surfaces of constant potential density. For oceanic water the relationships between 

 entropy, potential temperature and potential density are not so straightforward as 

 for atmospheric air and in particular, under normal conditions the surfaces of con- 

 stant entropy, constant potential temperature and constant potential density in the 

 sea are not identical sets of surfaces. It can easily be understood that especially the 

 surfaces of equal potential temperature are not identical with surfaces of equal po- 

 tential density by considering the complete dependence of the latter on the locally 

 varying salinity which plays only a minor role in the calculation of the potential 

 temperature. Thus for an investigation of the spreading of the water masses neither 

 one of these surfaces can be favoured, since each satisfies certain conditions which 

 seem to be necessary for such considerations, but are not sufficient to give any of the 

 two methods a special preference. It is thus equally incorrect to denote the method 

 of using surfaces of equal potential density as reference surfaces as "isentropic" 

 method because they have nothing to do with entropy which for sea-water is difficult 

 to define. 



Since there is, as previously pointed out, very little'difference between the potential 

 density ae and the density Cf (down to a pressure of 1000 decibars or a depth of 1000 

 m), instead of strictly "isentropic analysis" simply the distribution of the oceano- 

 graphic factors on surfaces of constant a^ has been studied. The method is thus quite 

 simple in practice, but its usefulness is rather limited if one considers strictly its proper 

 limits of applicability, and it offers little advantage over the "core layer" method and 

 other similar methods which will be discussed later. The displacement of water masses 



