154 THERMODYNAMICS OF OCEAN CURRENTS 



tion that it becomes greater than the density of the underlying strata, the 

 surface water must sink and be replaced by water from some subsurface 

 depth. The vertical currents that arise in this manner are called vertical 

 convection currents. They are irregular in character and should not be 

 called ^'currents" if this term is defined as motion of a considerable body of 

 water in a definite direction. 



The depth to which vertical convection currents penetrate depends 

 upon the stratification of the water. A mass of surface water whose 

 density has been increased by cooling or evaporation sinks until it meets 

 water of the same density. If mixing with neighboring water masses 

 takes place, it sinks to a lesser depth. When vertical convection currents 

 have been active for some time, an upper layer of homogeneous water is 

 formed, the thickness of which depends upon the original stratification 

 of the water, the intensity of the convection currents, and the time the 

 process has lasted. Thus, an upper homogeneous layer can be formed in 

 two different manners — either by the mechanical stirring due to wind, or 

 by the effect of thermohaline vertical convection currents. 



The vertical convection currents as a rule are of greater importance in 

 higher latitudes. In latitudes where an excess of evaporation is found, 

 the heating of the surface is often so great that the decrease of the surface 

 density by heating more than balances the increase by evaporation. In 

 these circumstances the surface salinity will be greater than the salinity 

 at a short distance below the surface. The formation of deep and bottom 

 water by vertical convection currents is dealt with elsewhere (p. 89). 



