THERMODYNAMICS OF OCEAN CURRENTS 153 



be in the same direction as those caused by heating and cooling, or they 

 may be in opposite directions. When examining the circulation that 

 arises because of the external factors that influence the density of the 

 surface waters, one must take changes of both temperature and salinity 

 into account and must consider not the thermal but the thermohaline 

 circulation. For this reason, Bjerknes' theorem is better formulated as 

 follows: // a thermohaline circulation shall produce energy, the expansion 

 must take place at a greater depth than the contraction. In this form, the 

 theorem can be used to determine if, within any given circulation, energy 

 is gained or lost through thermohaline changes. 



If thermal and haline circulations are separated, in some instances 

 they work together and in others they counteract each other. The 

 greatest heating takes place in the equatorial region, where, owing to 

 excess precipitation, the density is also decreased by reduction of the 

 salinity. In the latitudes of the subtropical anticyclones the heating is 

 less, and, in addition, the density of the water is increased by excess 

 evaporation. Between the Equator and the latitudes of the subtropical 

 anticyclones, conditions are therefore favorable for the development of a 

 strong thermohaline circulation. North and south of these latitudes the 

 haline circulation will, however, counteract the thermal, because the 

 density is decreased by excess precipitation but increased -by cooling. 

 There a weak thermohaline circulation might be expected. 



In the absence of a wind system, one might expect a slow thermohaline 

 circulation directed from the Equator to the poles at the surface and in the 

 opposite direction at some subsurface depth. This circulation would be 

 modified by the rotation of the earth and by the form of the ocean basins, 

 but nothing can be said as to the character of the system of currents that 

 would be developed under such conditions. It is probable, however, that 

 the existing current system bears no similarity to the one that would 

 result from such a thermohaline circulation, but is mainly dependent upon 

 the character of the prevailing winds and the extent to which the circula- 

 tion maintained by the wind is checked by the thermal conditions. In 

 other words, the wind system tends to bring about a distribution of 

 density that is inconsistent with the effect of heating and cooling, and the 

 actual distribution approaches a balance between the two factors. These 

 two factors — the wind and the process of heating and cooling — are 

 variable, however, in time and space, and therefore a stationary distribu- 

 tion of density with accompanying stationary currents does not exist. 

 Only when average conditions over a long time and a large area are 

 considered can they be regarded as stationary. 



Vertical Convection Currents. The thermohaline circulation is 

 of small direct importance to the horizontal current, but is mainly respon- 

 sible for the development of vertical convection curreiits. Wherever the 

 density of the surface water is increased so much by cooling or by evapora- 



