82 DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 



corresponding intensive mixing does not take place, the heat is not 

 distributed over a thick layer, and, consequently, the range of the 

 temperature near the surface is much greater. 



Diurnal Variation of Temperature in the Upper Layers. 

 Knowledge as to the diurnal variation of temperature at depths below the 

 surface is very scanty. It can be assumed that the depth at which the 

 diurnal variation is perceptible will depend greatly upon the stratification 

 of the water. A layer of sharp increase of density a short distance below 

 the free water surface will limit the conduction of heat to such an extent 

 that diurnal variation of temperature will be present above the boundary 

 layer only. 



On the Meteor expedition, hourly temperature observations were made 

 at the surface and at a depth of 50 m at a few stations in the Tropics 

 where an upper homogeneous layer was present which had a thickness of 

 70 m. Defant has shown that in these cases the diurnal oscillation of 

 temperature at subsurface depths is in agreement with the laws that have 

 been derived on the assumption of a constant heat conductivity. At 

 50 m the amplitude of the diurnal variation was reduced to less than two 

 tenths of the amplitude at the surface, and the maximum occurred about 

 6.5 hours later. 



The diurnal variation of sea temperature is, in general, so small that 

 it is of little importance to the physical and biological processes in the sea, 

 but knowledge of the small variations is essential to the study of the 

 diurnal exchange of heat between the atmosphere and the sea (p. 69). 

 The data that are available for this purpose, however, are very inadequate 

 at the present time. 



Distribution of Density 



The distribution of the density of the ocean waters is related to two 

 features. In a vertical direction the stratification is generally stable 

 (p. 87 and p. 100), and in a horizontal direction differences in density can 

 exist only in the presence of currents. The general distribution of 

 density is therefore closely related to the character of the currents, but for 

 the present purpose it is sufficient to emphasize the fact that in every 

 ocean region water of a certain density that sinks from the sea surface 

 tends to sink to and spread at depths where that density is found. 



Since the density of sea water depends on its temperature and salinity, 

 all processes that alter the temperature or the salinity influence the den- 

 sity. At the surface the density will be decreased by heating, addition of 

 precipitation, melt-water from ice, or by run-off from land, and will be 

 increased by cooling, evaporation, or formation of ice. If the density 

 of the surface water is increased beyond that of the underlying strata, 

 vertical convection currents arise that lead to the formation of a layer 

 of homogeneous water. Where intensive cooling, evaporation, or freezing 



