504 



Ocean Currents in a Non-homogeneous Ocean 



method thus avoids the difficulty that the horizontal pressure gradient should vanish 

 at the bottom. Groen (1948) has somewhat modified this method by assuming that 

 only the slope of the isosteres in the imaginary water body is identical at each depth 

 with that at the bottom slope. The values of the anomaly at the bottom slope and the 

 distribution of the slopes over the entire space completely determine, however, the 

 entire distribution of the specific volume. No special assumption about the distribution 

 of 8 is required. The difference from the previous assumption cannot be very large but 

 the method is more correct. Therefore, all the methods described here give results which 

 essentially do not differ from each other. 



7. Remarks About the Observational Material Necessary for a Dynamic Computa- 

 tion and Critical Discussion of the Procedure 



In order to understand the importance of the absolute topographies of the isobaric 

 surfaces it must be realized that a knowledge of these allows a complete evaluation of 

 the field of motion at individual depths. The current vectors parallel in this field the 

 isohypses of the pressure surfaces, and the velocities are inversely proportional to the 

 distances between them as well as to the sine of the latitude. In the Northern Hemi- 

 sphere for an observer looking downwards along the slope of the pressure surfaces 

 (Fig. 233) the current will flow to the right. A practical formula for the numerical 

 evaluation of topographies is 



„ = Jt_ f_ (XV.26) 



2(x) sm cf) An 



from which v is obtained in m/sec if AD is entered in dynamic metres and An in ordinary 

 metres. 



Fig. 233. Schematic representation of an absolute dynamic topography with the corre- 

 sponding velocities and diagrams of forces (G, gradient force; C, Coriolis force). 



The following prerequisites for the use of this formula should be borne in mind: 



(1) the topographical charts must approximate the actual state within the particular 

 oceanic area at a definite time as accurately as possible, 



(2) the currents must be steady, 



(3) it must be possible to disregard the effect of friction. 



