CHAPTER VIII 



Thermodynamics of Ocean Currents 



The preceding description of the effect of the wind, especially the 

 discussion of the secondary effect of the wind in producing currents in 

 stratified water, may leave the impression that the wind is all-important 

 in the development of the ocean currents and that thermal processes can 

 be entirely neglected. Such an impression, however, would be very mis- 

 leading. In discussing the secondary effect of the wind, it was repeatedly 

 mentioned that the development of the currents caused by a redistribution 

 of mass by wind transport would be checked partly by mechanical proc- 

 esses and partly by thermal processes. Surface waters that were trans- 

 ported to higher latitudes would be cooled, and thus a limit would be set 

 to the differences in density which could be attained; upwelling water 

 would be heated when approaching the surface, and at a certain vertical 

 velocity a stationary temperature distribution would be established at 

 which the amount of heat absorbed in a unit volume would exactly balance 

 the amounts lost by eddy conduction and by transport of heat through the 

 volume by vertical motion. The establishment of a stationary tempera- 

 ture distribution within upwelling water would check the effect of upwell- 

 ing on the horizontal distribution of density. 



The above examples serve to emphasize the importance of the thermal 

 processes in the development of the currents, but an exact discussion 

 of the thermodynamics of the ocean is by no means possible. So far, 

 the principles of thermodynamics have found very limited application 

 to oceanographic problems, but this does not mean that the thermal 

 processes are unimportant compared to the mechanical. 



Thermal Circulation. The term ''thermal circulation" will be 

 understood to mean a circulation that is maintained by adding heat to a 

 system in certain regions and by cooling it in other regions. The char- 

 acter of the thermal circulation in the ocean and the atmosphere has been 

 discussed by V. Bjerknes and collaborators. Their conclusions can be 

 stated as follows: If within a thermal circulation heat shall be trans- 

 formed into mechanical energy, heating must take place under higher 

 pressure and cooling under lower pressure. Such a thermodynamic 

 machine will run at a constant speed if the mechanical energy that is 



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