DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 85 



In middle and lower latitudes, two more convergences are found — the 

 Subtropical and the Tropical Convergences. These are not so well 

 defined as the Antarctic Convergence, but must be considered more as 

 regions in which converging currents are present. The Subtropical 

 Convergence is located in latitudes in which the density of the upper 

 layers increases rapidly toward the poles. The sinking water therefore 

 has a higher density the farther it is removed from the Equator, and will 

 spread out at the greatest depths. 



In the Tropics the density of the surface water is so low that, regard- 

 less of how intense a convergence is, water from the surface cannot sink to 

 any appreciable depth, but spreads out at a short distance below the 

 surface. A sharp boundary surface develops between this light top layer 

 and the denser water at some greater depth. 



In order to account for the general features of the density distribution 

 in the sea, emphasis has been placed on descending motion of surface 

 water, but evidently regions must exist in which ascending motion pre- 

 vails, because the amount of water that rises toward the surface must 

 exactly equal the amount that sinks. Ascending motion occurs in regions 

 of diverging currents (divergences), which may be present anywhere in 

 the sea but which are particularly conspicuous along the western coasts 

 of the continent, where prevailing winds carry the surface waters away 

 from the coast. There the upwelling of subsurface water takes place, a 

 process that will be described when dealing with specific areas. The 

 upwelling brings water of greater density and lower temperature toward 

 the surface, and therefore exercises a wide-spread influence upon condi- 

 tions off coasts where the process takes place but where the water rises 

 from depths of less than a few hundred meters. Ascending motion takes 

 place on a large scale around the Antarctic Continent, particularly to the 

 south of the Atlantic Ocean, where rising deep water replaces water that 

 contributes to the formation of the Antarctic bottom water and of the 

 water that sinks at the Antarctic Convergence. 



It is evident from these considerations that in middle and lower 

 latitudes the vertical distribution of density to some extent reflects the 

 horizontal distribution at or near the surface between the Equator and 

 the poles. It is also evident that, in general, the deeper water in any 

 vertical column is composed of w^ater from different source regions, and 

 once was present in the surface layer somewhere in a higher latitude. 

 Owing to the character of the currents, these generalizations are, how- 

 ever, subject to modifications within different ocean areas, and these 

 modifications will be discussed when dealing with the different oceans. 



Subsurface Distribution of Temperature and Salinity 



The general distribution of temperature is closely related to that of the 

 density. In high latitudes the temperature is low from the surface 



