CHAPTER VI 



Ocean Currents Related to the Distribution of Mass 



Introduction 



The ocean currents may be conveniently divided into three groups: 

 (1) currents that are related to the distribution of density in the sea, (2) 

 currents that are caused directly by the stress which the wind exerts 

 on the sea surface, and (3) tidal currents and currents associated with 

 internal waves. 



To the first class belong the well-known, large-scale ocean currents 

 such as the Gulf Stream, the Kuroshio, the Equatorial Currents, the 

 Benguela Current, and others. All of these currents transport great 

 amounts of water. Their courses at the surface are known from ships' 

 observations (p. 45), and at subsurface depths their character has, in a 

 few localities, been determined from direct measurements of currents 

 from anchored vessels, and in many more localities they have been 

 ascertained from determinations of temperatures and salinities. 



The wind drift also transports water in one and the same direction 

 over large areas if it blows prevailing from one direction. The wind 

 drift is of fundamental importance to the development and maintenance 

 of the ocean circulation, and will therefore be dealt with in a separate 

 chapter. 



In contrast to these two types of currents the currents that are 

 associated with tides and internal waves run alternatingly in opposite 

 directions or are rotating. Although such currents may attain high 

 velocities and are oceanographically significant, they are of no direct 

 importance to the circulation of the ocean waters or to the interaction 

 between the ocean and the atmosphere and will therefore not be discussed 

 here. 



In general, all three types of currents are present at the same time, 

 and this makes it virtually impossible to obtain knowledge of the ocean 

 currents on an entirely empirical basis. If such knowledge were to be 

 obtained, it would be necessary to conduct measurements from anchored 

 vessels at numerous localities for long periods and at many depths. By 

 means of such series the different types of periodic currents could be 

 examined, and by averaging they could be eliminated and the other types 



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