DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 91 



have been formed by vertical mixing and must have been formed by 

 lateral mixing, probably along ct surfaces. An example of modification 

 of a water mass by vertical mixing is found in the South Atlantic where 

 the Antarctic Intermediate Water flows north. This water, near its 

 origin, is characterized by a low salinity minimum, but the greater the 

 distance from the Antarctic Convergence the less pronounced is the 

 salinity minimum (fig. 60, p. 213). This change probably cannot be 

 accounted for by lateral mixing, but Defant has shown that it can be 

 fully explained as a result of vertical mixing. 



Wiist has introduced a different method for the study of the spreading 

 out and mixing of water types — the "core method." By the ''core" 

 of a layer of water is understood that part of the layer within which 

 temperature or salinity, or both, reach extreme values. Thus, in the 

 Atlantic Ocean, the water that flows out from the Mediterranean has a 

 very high salinity and can be traced over large portions of the Atlantic 

 Ocean by means of an intermediate salinity maximum which decreases 

 with increasing distance from the Strait of Gibraltar. The layer of 

 salinity maximum is considered as the core of the layer in which the 

 Mediterranean Water spreads, and the decrease of the salinity within 

 the core is explained as the result of processes of mixing. In this case a 

 certain water type, the Mediterranean Water, enters the Atlantic Ocean 

 and loses its characteristic values, owing to the mixing, but can be 

 traced over long distances. The spreading of the water can also be 

 described by means of a T-S curve, one end point of which represents 

 the temperature and salinity at the source region and the other end point 

 the temperature and salinity in the region where the last trace of this 

 particular water disappears. Having defined such a T-S curve, one can 

 directly read off from the curve the percentage amount of the original 

 water type that is found in any locality. The core method has proved 

 very successful in the Atlantic Ocean, and is particularly applicable in 

 cases in which a well-defined water type spreads out from a source region. 



It has been mentioned that differences in density in a horizontal 

 direction can exist only in the presence of currents, and therefore the 

 relation between density distribution and currents must now be dis- 

 cussed. The character of the currents in the different oceans must also 

 be examined, because knowledge of the oceanic circulation is necessary 

 to an understanding of the interaction between the atmosphere and the 

 sea. 



