1 1 6 OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF AAASS 



the average depth of no motion, and the transports above and below that 

 depth are in opposite directions. 



The procedure is illustrated in fig. 28, in which is plotted the difference 

 ADa — ADb at Meteor station 36 off South America and station 23 off 

 South Africa, both in about latitude 29°S. Assuming no current at a 

 depth of 3500 m, one would obtain a transport to the north proportional 

 to the area ABC, or equal to 44.5 million mVsec. Such a transport to 

 the north is impossible, and the velocity must therefore be zero at some 

 other depth. The average value of AD a — ADb is 0.090 dynamic meters, 

 and this value is found at a depth of 1325 m. If this depth is selected 

 as a depth of no motion, the transport to the north becomes proportional 

 to the area DBE and is equal to 20.7 million mVsec; below 1325 m the 

 transport is directed to the south, is proportional to the area ECF, and 

 is also equal to 20.7 million m^/sec. The transport above the depth of no 

 motion can be written in the form 



where d is the depth above which the transport was first calculated and h 

 is the depth of no motion. 



Only the average depth of no motion can be determined in this manner ; 

 generally, the depth of no motion is not constant, but a variation in depth 

 can be found by studying the distribution of temperature and salinity 

 in the section. It is rational to assume that a surface of no motion 

 coincides with isothermal and isohaline surfaces, and that therefore, in 

 a section, the depth of no motion follows the isotherms and isohalines. 

 On that assumption one finds that in the example which has been dis- 

 cussed the depth of no motion rises from about 1450 m off South America 

 to about 1200 m off South Africa. 



In conclusion it is necessary to emphasize that, so far, nothing has 

 been stated as to cause or effect. It is obvious that all the equations 

 which have been used state only that, on the assumptions made, a definite 

 relationship exists between the currents and the distribution of mass. 

 In order to discuss the cause of the ocean currents, it is necessary to 

 examine the manner in which the distribution of mass is maintained. 

 It is found that the distribution of mass is controlled by processes of 

 radiation at the sea surface, the exchange of heat with the atmosphere, 

 the evaporation from the sea surface, and the character of the prevailing 

 winds. The processes of heating and cooling are all important in estab- 

 lishing and maintaining differences in density, but the wind renders the 

 energy that is needed for maintaining the circulation. In contrast to the 

 atmosphere the ocean is an inefficient thermodynamic machine that 

 converts very small amounts of heat to kinetic energy. 



