AEROGRAPHER'S MATE 3 & 2 



with similar characteristics and distinguishing 

 between them in terms of "air masses." The 

 same concept may be applied to bodies of water 

 with similar characteristics. However, in the 

 ocean there exists a more steady state of the 

 environment than that found in the atmosphere. 

 Therefore, water masses are found to remain 

 within more well-defined areas than air masses 

 are. 



In middle and low latitudes the arrangement 

 of water masses vertically is such that one can 

 distinguish between the surface layer, the 

 upper water, the intermediate water, the deep 

 water and in some localities, the bottom water. 

 In high latitudes, the intermediate water is 

 often lacking, and the upper water is similar 

 to the deep water. 



FORMATION 



The density of water increases with depth. 

 However, in a horizontal direction, the density 

 generally increases toward the polar regions. 

 Surface water of a given density which is 

 sinking in higher or middle latitudes will sink 

 until it reaches a level of constant density and 

 then begin to spread out horizontally. As a 

 result, in middle latitudes, the vertical and 

 horizontal distribution of density will be nearly 

 the same during those seasons when surface 

 waters are most frequently subsiding. Water 

 masses may generally be said to form either 

 through the process of sinking surface waters 

 or by subsurface mixing. 



The vast majority of water masses are 

 formed at the surface of the sea. They then 

 sink and spread outward from their source 

 regions in a manner that depends on their 

 density in relation to the density of the sur- 

 rounding oceans. This is true of nearly all water 

 masses with the exception of the equatorial 

 water masses of the Indian and the Pacific 

 Oceans, which are formed by the process of 

 subsurface mixing. Water masses do not normally 

 form at the surface in low latitudes. 



DISTRIBUTION 



Central Water Masses 



Figure 16-10 is provided as a reference 

 for the source regions of the various water 

 masses discussed in the following paragraphs. 



The Central Water masses are normally 

 found in relatively low latitudes although their 

 source region is in the region of the subtropi- 

 cal convergences (between 35° and 40° North 

 and South of the equator). 



The Central Water masses of the South 

 Atlantic, the Indian Ocean, and the western 

 South Pacific Ocean have similar properties 

 since in the regions where they are formed, 

 circulation, heating, and cooling processes are 

 also similar. The water masses of the eastern 

 South Pacific have lower salinity than those 

 found in the western portion, probably because 

 of the mixing of the low salinity Subantarctic 

 Water with that of the Peru Current. Similar 

 mixing processes also contribute to the Central 

 Waters of the western South Pacific which in 

 a like manner have slightly lower salinity than 

 the Central Water of the Indian and South 

 Atlantic Oceans. 



The Central Waters of the North Atlantic 

 and the North Pacific Oceans vary considerably, 

 primarily due to the high salinity content of 

 the North Atlantic as compared to the rela- 

 tively low salinity of the North Pacific. These 

 differences are probably due to variations in 

 the ocean circulation, amounts of precipitation, 

 and evaporation as a result of land and sea 

 distribution, especially in the higher latitudes. 



The vertical extent of the Central Water 

 masses are generally relatively shallow, with 

 the greatest thicknesses being observed along 

 their western boundaries. They may reach 900 

 meters in the Sargasso Sea region of the 

 North Atlantic. 



The opposing Central Water masses in the 

 equatorial parts of the North and South Atlantic 

 Ocean are separated by a transitional region 

 which consists of intermediate properties as 

 a result of mixing between the two regions. 

 However, in the Pacific Ocean the separation 

 between the Central Water masses is well 

 defined since they are separated by another 

 water mass referred to as Equatorial Water. 



Equato i Water Masses 



Equatorial Water probably forms on the 

 Southern side of the Equator in the Pacific 

 since it is similar to the water mass of that 

 ocean and has a higher salinity than any of 

 the water masses of the North Pacific Ocean. 



392 



