494 Ocean Currents in a Non-homogeneous Ocean 



a surface of locally varying depth. The determination of its form and the different 

 factors that must be considered for fixing its position in oceanic space is not an easy 

 task. It should be stressed that the choice of such a surface is always more or less 

 subjective, and such an assumption can only be made plausible by giving proper 

 weight to all the different view points which are in question. 



{a) Determination of the Topography of the Reference-Level 



A first attempt was made by Dietrich in an investigation of the dynamics of the Gulf 

 Stream to introduce a reference-level of variable depth by investigating characteristic 

 features in the distribution of oxygen in order to fix the reference-level. He thus 

 accepted the widely held view that the layers showing the intermediate oxygen minima 

 (see Pt. I, p. 66 and following pages) are at the same time also layers of very weak 

 motion or of no motion at all, and could thus be regarded as motionless boundary 

 layers between individual components of the deep-sea circulation. However, Rossby 

 (1936 a), ISELiN (1936) and especially Wattenberg (1938) and Sverdrup (1938 M 

 have questioned this assumption and have expressed doubts about the suitability of 

 these oxygen minima as reference-levels. In the upper layers of the ocean the oxygen 

 distribution can be regarded, on the one hand, as a consequence of thermal and bio- 

 chemical oxygen consumption, and on the other hand, of the renewal of the water 

 masses by horizontal advection. The intermediate minima are thus regions of parti- 

 cularly strong oxygen consumption and can hardly be regarded as completely motion- 

 less layers. The results obtained by Dietrich for the currents in the Gulf Stream on the 

 basis of this assumption are not such as to give confidence in reference-levels derived 

 from the oxygen minimum. Even the customary division of the water masses of an 

 ocean, pictured by major longitudinal and transverse section> and allowing for the 

 characterization of the different water bodies, is scarcely suitable for the determination 

 of the topography of the reference-level. Even though they may be practical and useful 

 in giving a general qualitative picture of the meridional and zonal velocity components 

 of the ocean currents. 



Defant (1941 b) has gone a quite different way in order to determine the dynamic 

 reference-level in the Atlantic, which avoids the use of any particular boundary layer 

 between the individual water types and makes use only of dynamic evaluations of 

 observational data, which must be closely connected with the structure of the water 

 masses of the particular area. The differences in dynamic depth of the pressure values 

 between two neighbouring stations give, by means of equation (XV. 12), a relative 

 measure of the velocity difference perpendicular to the cross-section between the sea 

 surface and the corresponding depth. When these differences are plotted in an appro- 

 priate co-ordinate system (ordinate :pressures; abscissa :difference in dynamic depth) 

 they give a relative vertical velocity profile at right angles to the section between the 

 two stations (Fig. 226). This profile cannot be converted to an absolute velocity profile 

 without knowing the zero point on the abscissa. By comparison of a large number of 

 difference-curves for neighbouring pairs of stations it shows in most cases that in 

 each profile there is a layer of considerable vertical thickness in which the differences 

 in dynamic depth are constant or almost constant. If the zero point of the abscissa 

 scale is placed outside of this layer then the entire layer must have a constant velocity. 



