OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF MASS 105 



between the two water masses must be horizontal, but, in the region where 

 the water of low density flows with a velocity v' , the boundary surface 

 must slope, the steepness of the slope being determined by (VI, 25). 

 These conditions are shown schematically by the block diagram in fig. 24, 

 where it is supposed that the denser water reaches the surface on the left- 

 hand side of the current. This figure brings out an important relation- 



Fig. 24. Isobaric surfaces and currents within water that in 

 part extends as a wedge over resting water of greater density. 



ship between the current and the distribution of mass: The current flows 

 in such a direction that the water of low density is on the right-hand side of the 

 current and the water of high density is on the left-hand side. This rule 

 applies to conditions in the Northern Hemisphere, since in the Southern 

 Hemisphere the water of low density will be to the left and the water of 

 high density will be to the right. 



Practical Methods for Computing Ocean Currents 



"Relative" Currents. As has already been stated (p. 99), oceano- 

 graphic observations can give information only as to the relative topog- 

 raphies, and therefore only as to the corresponding ''relative" velocities. 

 When using the term ''relative" velocity, it should be borne in mind that 

 the actual velocities (relative to the earth) are not obtained by adding a 

 constant value to the "relative," but that the value that has to be added 

 in order to obtain the actual velocities varies from one vertical to another, 

 depending upon the unknown slope of the isobaric surface that has been 

 used as a reference surface. 



The contour lines of the isobaric surfaces represent the stream lines 

 of the relative motion, but they are not, as a rule, identical with trajec- 

 tories, even if the reference surface is level so that the computed velocities 

 represent the actual motion. If such is the case, the contour lines will be 

 trajectories only if the motion is stationary — that is, if dVx/dt = dvy/dt = 

 (p. 93). Because it has been assumed that Vx = Vy = 0, it follows that 



