Ocean Currents in a Non-homogeneous Ocean 



501 



{b) Conversion of Relative to Absolute Topographies Using a Reference-level of 

 Varying Depth 

 For a reference-level of constant depth there is little difficulty in the conversion from 

 relative to absolute topography (see p. 21 1). The differences in dynamic depth represent 

 at the same time also differences in the physical sea level and in the level of individual 

 isobaric surfaces, respectively. If the depth of the dynamic reference level varies from 

 place to place this simple procedure can no longer be used ; the individual differences 

 of dynamic depths above or below the reference level must be coupled or inter- 

 connected one to the next in a suitable way in order to construct step wise the surfaces 

 of equal pressure (Dietrich, 1937 a). To determine the absolute topography of a 

 pressure surface Pq above the reference level, three oceanographic stations A, B, C 

 were chosen Sind Pa,Pb,Pc are the pressures at the points on the reference level at which 

 by necessity the pressure surfaces parallel the level surfaces (Fig. 230). 



Fig. 230. To the method of transforming relative into absolute dynamic topographies. 



If the stations are sufficiently close to each other, the dynamic reference level can 

 as an approximation by broken up into a step wise course. Along the section from 

 A to B the mean pressure will be given by 



i (Pa + Pb) 



and that between C and D by 



Pa, 6 



Pb, c = HPb+Pc)' 



The dynamic height differences 8^ and S^ of the isobaric surface p^ over the mean 

 reference level can be determined in the usual way for stations A and B, as well as the 

 differences in dynamic height b\ and Sc of the isobaric surface Pq above the reference- 

 level between B and C. Then S„ + Sj, and S^ + S'b + Sc are the vertical deviations 

 of the isobar /7o from the level surface, running through the point A. The conversion 

 can thus be made quite simply for dynamic sections. If the stations are distributed over 

 a larger oceanic region, then the condition has to be satisfied that the absolute values 

 calculated along different paths (sections) must lead to the same value. Defant 

 (1941 b) has developed a triangle method which has been found very useful in the 

 determination of the absolute topography of the Atlantic Ocean from a large network 



