THERMODYNAMICS OF OCEAN CURRENTS 



151 



produced by the thermal circulation equals the energy that is expended 

 in overcoming the friction. 



In the ocean, ''higher pressure" can generally be replaced by ''greater 

 depth/' and "lower pressure" by "smaller depth." Applied to the ocean 

 the theorem can be formulated as follows: If within a thermal circulation 

 heat shall be transformed into mechanical energy, the heating must take 

 place at a greater depth than the cooling. 



This theorem was demonstrated experimentally by Sandstrom before 

 it was formulated by Bjerknes. In one experiment, Sandstrom placed 

 a "heater" at a higher level and a "cooler" at a lower level in a vessel 

 filled with water of uniform temperature (fig. 39a). The heater consisted 

 of a system of tubes through which warm water could be circulated, and 

 the cooler consisted of a similar system through which cold water could 

 be circulated. When warm and cold water was circulated through the 

 pipes, a system of vertical convection currents developed and continued 



w 



Fig. 39. Types of circulation induced in water by different placement of warm 

 (W) and cold (C) sources. 



until the water above the heater had been heated to the temperature 

 of the circulating warm water and the water below the cooler had been 

 cooled to the temperature of the circulating cold water. When this state 

 had been reached and a stable stratification had been established, all 

 motion ceased. 



In a second experiment (fig. 39b), Sandstrom placed the cooling sys- 

 tem above the heating system. In this case the final state showed a 

 circulation mth ascending motion above the heating unit and with 

 descending motion below the cooling unit. Thus, a stationary circulation 

 was developed because the heating took place at greater depth than 

 the cooling. 



From these experiments and from Bjerknes' theorem, it is immediately 

 evident that conditions in the oceans are very unfavorable to the develop- 

 ment of thermal circulations. Heating and cooling take place mainly at 

 the same level — namely, at the sea surface, where heat is received by 

 radiation from the sun during the day when the sun is high in the sky, or 

 lost by long-wave radiation into space at night or when the sun is so low 

 that the loss is greater than the gain, and where heat is received or lost by 

 contact with air. 



Because heating and cooling take place at the surface, one might 

 expect that no thermal circulation could develop in the sea, but this is not 



