160 WATER MASSES AND CURRENTS OF THE OCEANS 



The Suhantarctic Water Mass occupies the region between the Antarc- 

 tic Convergence and the southern Subtropical Convergence, and repre- 

 sents a transition from the Antarctic Circumpolar Water Mass to the 

 Central Water Masses of the southern oceans. The corresponding 

 Subarctic Water Masses are found to the north of the northern Subtropical 

 Convergences and are formed by mixing, winter cooling, and excess 

 precipitation. 



In the Pacific Ocean an Equatorial Water Mass is present between the 

 Central Water Masses of the North and South Pacific. In the Indian 

 Ocean a similar Equatorial Water Mass is present, but it is lacking in the 

 North Atlantic. 



In the North and South Pacific the Subarctic and Suhantarctic Water 

 Masses penetrate toward the Equator along the west coasts of North and 

 South America, where their character is changed because of heating and 

 evaporation at the surface. 



Fig. 40 shows the character of the water masses that have been dis- 

 cussed, their regions of formation, and their distribution. The chart in 

 the upper part of the figure and the temperature-salinity curves in the 

 lower part should together illustrate the concepts that most water masses 

 are formed at the sea surface and sink and spread in a manner which 

 depends upon their density in relation to the general distribution of 

 density in the oceans. 



Water masses of corresponding character are present in the different 

 oceans, but, nevertheless, a marked contrast exists between the Atlantic 

 and Pacific Oceans. In the Atlantic the Central Water Masses of the 

 North and South Atlantic dominate. No Equatorial Water exists, and 

 Subpolar Water Masses are of small extension. In the Pacific, on the 

 other hand, a large Equatorial Water Mass is present, and, along the 

 western coasts of North and South America, Subpolar Waters advance 

 toward the Equator. Equally striking is the fact that in both the North 

 and South Pacific two Central Water Masses are present. The existence 

 of these water masses is probably related to the fact that over both the 

 North and South Pacific two high-pressure areas are generally present. 

 In the South Pacific, two high-pressure areas appear on the charts of 

 average pressure distribution in winter. In the North Pacific, two 

 pressure areas often show up in daily weather maps; these areas do not 

 appear on charts for seasons, however, because of the variable location 

 of the eastern pressure cell, but the wind systems characteristic of the two 

 high-pressure areas are recognized by the influence they exert on the 

 oceanographic conditions. 



Table 17 contains the average salinities of the Central Water Masses 

 at different temperatures and the maximum deviations from the averages, 

 according to the curves in fig. 40. It is seen from the table and the figure 

 that the Central Water Masses of the South Atlantic, the Indian, and the 



