182 



WATER MASSES AND CURRENTS OF THE OCEANS 



layer. The discontinuity layer is explained mainly as a result of heating 

 due to the absorption of radiation from the sun and forced mixing due to 

 wind within the water. Since this water comes from higher latitudes, it 

 has a relatively low temperature. As the mixing penetrates to greater 

 and greater depths, the difference in density between the warm surface 

 waters and the colder subsurface strata is concentrated in a thinner and 

 thinner layer within which the stratification becomes more and more 

 stable. When the stratification has become so stable that the layer takes 

 the character of a discontinuity surface, further mixing is inhibited, 

 because the eddy conductivity is greatly reduced by the exceedingly great 



Fig. 49. A. Topography of the discontinuity surface in the equatorial region of 

 the Atlantic, shown by depth contours in meters, and corresponding currents. B. 

 Salinity in the layer of maximum salinity below the discontinuity layer and assumed 

 currents. Regions without salinity maximum shaded. (Both representations after 

 Defant.) 



stability (p. 20), and the amounts of heat that are conducted downward 

 become so small that they are carried away by weak currents. Additional 

 absorption of solar energy is used mainly for evaporation; the final tem- 

 perature that is attained will depend upon the rate of evaporation, and is 

 therefore determined by the interaction between the atmosphere and the 

 ocean. 



In a given locaHty the thickness of the upper warm layer depends not 

 only upon the intensity of mixing but also upon the character of the 

 circulation in the top layers, because in the presence of currents the dis- 

 continuity surface cannot remain horizontal, but must slope (p. 103). On 

 the basis of the Meteor data and all other observations available from the 

 tropical parts of the Atlantic Ocean, Defant has been able to construct a 

 chart showing the topography of the discontinuity surface between 

 latitudes 25°N and 25°S (fig. 49). In the presence of a sloping discontinu- 

 ity surface the flow of the water above the surface, relative to the under- 

 lying water masses, must, in the Northern Hemisphere, be in such 

 direction that the surface sinks to the right of an observer looking in the 

 direction of flow, and in the Southern Hemisphere such that the surface 

 sinks to the left. On the basis of these rules, arrows have been entered 

 showing the direction of flow, which, in general, agrees well with the 

 observed average surface currents in the tropical regions of the Atlantic. 



