152 THERMODYNAMICS OF OCEAN CURRENTS 



true. Consider a vessel filled with water. Assume that heating at the 

 surface takes place at the left-hand end and that, toward the right-hand 

 end, the heating decreases, becoming zero at the middle of the vessel. 

 Bej^ond the middle cooling takes place, reaching its maximum at the 

 other end (fig. 39c). Under these conditions the heated water to the 

 left will have a smaller density than the cooled water to the right, and 

 will therefore spread to the right. Owing to the continuity of the 

 system, water must rise near the left end of the vessel and sink near the 

 right end, thus establishing a clockwise circulation that, at the surface, 

 flows from the area where heating takes place to the area where cooling 

 takes place. When stationary conditions have been established, the 

 temperature of the water to the left must be somewhat higher than the 

 temperature of the water to the right, owing to conduction from above. 



This circulation is quite in agreement with Bjerknes' theorem, because 

 at the surface the water that flows from left to right is being cooled, since 

 it flows from a region where heating dominates into a region where cooling 

 is in excess. On the other hand, on the return flow, which takes place at 

 some depth below the surface, the water is being warmed by conduction, 

 because it flows from a region of lower temperature to a region of higher 

 temperature. Thus, the circulation is such that the heating takes place 

 at a greater depth than the cooling. This circulation, however, cannot 

 become very intensive, particularly because the heating within the return 

 flow must take place by the slow process of conduction. 



If the oceanic circulation is examined in detail, many instances are 

 found in which the vertical circulation caused by the wind is such that the 

 thermal machine runs in reverse, meaning that mechanical energy is 

 transformed into heat, thus checking the further development of the wind 

 circulation. When upwelling takes place, the surface flow will be directed 

 from a region of low temperature to a region of high temperature, and the 

 subsurface flow will be directed from high to low temperature. The 

 thermal machine that is involved will consume energy and thus counteract 

 a too-rapid wind circulation. Thus, in the Antarctic the thermal 

 circulation will be directed at the surface from north to south and will 

 counteract the wind circulation, which will be directed from south to 

 north. On the other hand, systems are found within which the thermal 

 effect tends to increase the wind effect and within which the increase of 

 the circulation must be checked by dissipation of kinetic energy. 



Thermohaline Circulation. So far, only thermal circulation has 

 been considered, but it must be borne in mind that the density of the 

 water depends on both its temperature and its salinity, and that in the 

 surface layers the salinity is subject to changes caused by evaporation, 

 condensation, precipitation, and addition of fresh water from rivers. In 

 the open ocean the changes in density are determined by the excess or 

 deficit of evaporation over precipitation. These changes in density may 



