162 WATER MASSES AND CURRENTS OF THE OCEANS 



western South Pacific Oceans are very similar, as should be expected, 

 because they are formed in regions in which the external influences — that 

 is, the atmospheric circulation and the processes of heating and cooling — 

 are similar. The corresponding water mass of the eastern South Pacific 

 is of lower salinity, probably because of admixture of the low-salinity 

 Subantarctic Water of the Peru Current. Such admixture may also be 

 responsible for the fact that the Central Water of the western South 

 Pacific has a slightl}^ lower salinity than the Central Waters of the Indian 

 and South Atlantic Oceans. 



The Central Waters of the North Atlantic and the North Pacific 

 Oceans are quite different, the former having a very high and the latter 

 a very low salinity. The contrast probably results from the different 

 character of the ocean circulation and the differences in the amounts of 

 evaporation and precipitation, especially in high latitudes, which are 

 intimately related to the distribution of land and sea. 



The Central Water Masses are all of small vertical extension, particu- 

 larly in the North Pacific Ocean, where their thickness over large areas is 

 only 200 to 300 m. In all oceans the greatest thickness of the Central 

 Water Masses is found along the western boundaries, reaching 900 m in 

 the Sargasso Sea region of the North Atlantic. 



The Central and Equatorial Water Masses are covered by a surface 

 layer 100 to 200 m thick, within which the temperature and the salinity 

 of the water vary greatly from one locality to another, depending upon 

 the character of the currents and the exchange with the atmosphere, and 

 within which great seasonal variations occur in middle latitudes. The 

 surface layer, the Central Water Masses, and the upper portions of the 

 Equatorial Water Masses together form the oceanic troposphere (p. 86). 



The Subantarctic Water between the Central Water Masses of the 

 southern oceans and the Antarctic Convergence has nearly the same char- 

 acter all around the earth, and is therefore considered as belonging to the 

 waters of the Antarctic Ocean. It is of low salinity and is probably 

 formed by mixing and vertical circulation in the region between the 

 Subtropical and the Antarctic Convergences. In the North Atlantic the 

 corresponding Subarctic Water is found only in a small region and is of 

 relatively high salinity, but in the North Pacific it is of wide extension and 

 of low salinity. The Subarctic Water must be formed by processes that 

 differ from those that maintain the Subantarctic Water. In the southern 

 oceans the Antarctic Convergence represents a continuous and well- 

 defined poleward boundary of the Subantarctic Water, but in the 

 northern oceans the corresponding Arctic Convergence is found in the 

 western parts of the oceans only, and in large areas there is no marked 

 poleward boundary of Subarctic Waters. This contrast between south 

 and north must be related to the differences in the distribution of land and 

 sea and is reflected in the character of the waters. The Subarctic Waters 



