OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF MASS 109 



Actual Currents. So far, we have considered mainly the ''relative " 

 currents associated with the relative field of pressure, but the ultimate 

 goal must be to determine the actual currents. The problem of deter- 

 mining actual currents can be dealt with in two steps. In the first 

 place, one can consider whether there are reasons to assume that the 

 actual currents are determined completely by differences in density. If 

 this question is answered in the affirmative, one can then consider what 

 reference surface should be used in order to find the actual motion. 



The first question can be approached in the following manner: If the 

 distribution of mass remains stationary, the flow must always be parallel 

 to the isopycnals, because, if this condition is not fulfilled, the distri- 

 bution of mass will be altered by the motion. On the assumptions made, 

 the flow is always parallel to the isobars, and it follows, therefore, that 

 under stationary conditions isopycnals and isobars must be parallel at 

 all levels. It also follows that the isobars and isopycnals at one level 

 must be parallel to those at all other levels. This rule is identical with 

 the "law of the parallel solenoids" of Helland-Hansen and Ekman. The 

 motion that has to be considered when dealing with distortion of the 

 field of mass depends, however, on the total field of pressure, and this 

 total field must evidently have the same geometrical shape as the internal 

 field if the law of the parallel solenoid shall be fulfilled. The total field 

 is composed of the internal and the slope fields (p. 103), and consequently 

 these fields must coincide if the law of the parallel solenoids shall be 

 fulfilled. It is very unlikely, however, that a slope field of such character 

 develops, for which reason parallel isopycnals and isobars strongly 

 indicate that a slope field is absent. 



The study of large-scale conditions in the ocean has shown that over 

 large areas the isotherms, isohalines, and, consequently, the isopycnals 

 are parallel at different levels and that their direction coincides with the 

 direction of the relative isobars or the contour lines of the isobaric 

 surfaces. This empirical result strongly supports the view that the large- 

 scale currents are largely determined by the internal distribution of mass. 

 Even in small areas a similar arrangement is often found, but many 

 exceptions are encountered there which clearly demonstrate that station- 

 ary conditions do not exist, and which may be interpreted as indicating 

 that the currents are not determined entirely by the distribution of mass. 



The next question that arises is whether it is possible in the ocean to 

 determine a surface along which the velocity is zero, so that actual 

 velocities are found when the "relative" motion is referred to this surface. 

 Such a surface need not be an isobaric surface but may have any shape. 



One school of oceanographers points out that the deep waters of the 

 oceans are nearly uniform and that the isopycnal surfaces there are 

 nearly horizontal. It is therefore assumed that in the deep water 

 the isobaric surfaces are also nearly horizontal and that actual currents 



