DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 73 



Wiist points out that such an empirical relationship is found because 

 the surface saHnity is mainly determined by three processes : decrease of 

 salinity by precipitation, increase of salinity by evaporation, and change 

 of salinity by processes of mixing. If the surface waters are mixed with 

 water of a constant salinity, and if this constant salinity is denoted by 

 So, the change of salinity due to mixing must be proportional to So — S, 

 where S is the surface salinity. The change of salinity due to processes 

 of evaporation and precipitation must be proportional to the difference, 

 E — P. The local change of the surface salinity must be zero; that is, 



^ = a(S, - S,) + h{E - P) = 0, 



or 



S = So + A:(^ - P). (V, 2) 



As this simple formula has been established empirically, it must be 

 concluded that the surface water is generally mixed with water of a 

 salinity that is, on an average, 34.6°/oo. This value represents approxi- 

 mately the average value of the salinity at a depth of 400 to 600 m, and 

 it appears therefore that vertical mixing is of great importance to the 

 general distribution of surface salinity. This concept is confirmed by 

 the fact that the standard value of the salinity differs for the different 

 oceans. For the North Atlantic and the North Pacific, Wiist obtains 

 similar relationships, but the constant term. So, has the value SS.SO^^/oo 

 in the North Atlantic Ocean and 33.70°/oo in the North Pacific Ocean. 

 The corresponding average values of the salinity at a depth of 600 m are 

 35.5°/oo and 34.0°/oo, respectively. For the South Atlantic and the 

 South Pacific Oceans, Wiist finds that So = 34.507oo and 34.64%o, 

 respectively, and that the average salinity at 600 m in both oceans is 

 about 34.5°/oo. In these considerations the effect of ocean currents on 

 the distribution of surface salinity has been neglected, and the simple 

 relations obtained indicate that transport by ocean currents is of minor 

 importance as far as average conditions are concerned. The difference 

 between evaporation and precipitation, E — P, is, on the other hand, of 

 primary importance, and, because this difference is closely related to the 

 circulation of the atmosphere, one is led to the conclusion that the average 

 values of the surface salinity are, to a great extent, controlled by the 

 character of the atmospheric circulation. 



The distribution of surface salinity of the different oceans is shown in 

 chart 3, in which the general features that have been discussed are 

 recognized, but the details are so closely related to the manner in which 

 the water masses are formed and to the types of currents that they cannot 

 be dealt with here. 



Periodic Variations of the Surface Salinity. Over a large area, 

 variations in surface salinity depend mainly upon variations in the 



