Ocean Currents in a Non-homogeneous Ocean 



493 



("Nullflache"), quite apart from the fact that short series of current measurements are 

 almost always strongly disturbed by the tides. Thus the essential data needed to 

 decide about the position of the "zero level" is largely lacking. The effort to utiUze 

 the observations as fully as possible and to determine the pressure differences as good 

 as possible, at least in the upper layers, has led to place the zero surface as deep as 

 possible. This choice was also suggested by the generally rapid decrease in the velocity 

 of the currents with depth. 



Over the entire area under consideration most investigators have thus usually placed 

 the zero level at a constant dynamic depth and as deep as possible (as far as the water 

 depth and the observations available allowed), and from this have derived the absolute 

 topography of the pressure surfaces and that of the physical sea level from the relative 

 topographies. Table 136 presents a summary of all the depths selected for the zero 

 level by different investigators. The differences of more than 1000 m indicate that these 

 are pure assumptions for which there is no firm basis. However, all investigators have 

 been aware of the inadequacy of this procedure and have regarded the selection made 

 purely as a make-shift. The assumption of a zero level at a constant large depth will, 

 of course, conceal all currents in the layers just above and below this depth, and these 



Table 136. Depth of the ''zero level" {Nullflache'') in the Atlantic Ocean according to the 

 assumption of difl'erent investigators 



are thus falsified if by chance the zero level selected does not correspond with the 

 actual position of such a level. On the other hand, the deeper the zero level is placed, 

 the less will it disturb the pressure conditions at the sea surface. 



To obtain a correct idea of the deep current, it is not sufficient to assume a constant 

 depth for the zero level. Such an assumption, moreover, does not correspond to the 

 dynamics of the ocean currents in nature and, as has been stressed by Ekman (1939) 

 takes no account of the topography of the sea bottom. These problems of dynamic 

 oceanography have been dealt with by Dietrich (1937 a, c), who has thrown light on a 

 number of aspects of them. The zero level, more suitably could be called "reference- 

 surface" and has to be placed at such a depth where the velocity component at right 

 angles to the dynamic section under consideration is zero. It must, of course, closely 

 adapt to the mass structure of the entire oceanic area, since this is in fact a conse- 

 quence of the currents and is closely connected with them. In these circumstances it is 

 to be expected, especially when larger areas of the sea are taken into consideration, 

 that the reference-level for the reduction of relative into absolute topography must be 



