446 



General Theory of Ocean Currents in a Homogeneous Sea 



For the upper layers when the depth of the sea is great, one obtains in close approxi- 

 mation 



u=U 



U cos ft and v=Usmft, (XIII.66) 



which indicates a simple harmonic inertia motion with an amplitude U. In reality 

 of course a decrease always occurs in a spiral form but when the depth is large this 

 decrease is extremely small. These oscillations can be found by observation as far 

 down as near to the sea bottom. Figure 1 93 shows the course in the velocity components 

 at the surface, for a middle depth and for a layer near to the bottom in the case where 



20 



-10 

 20 



10 

 



8 4? 



Pendulum, hr 



8 



Fig. 193. Gradient current in the ocean for an ocean depth hlD = -j, and in fact for z = 

 (surface), z/A = 0-5 and 0-9. (Values for the north and east components for «/C/ and vllJ, 



according to Fr. Defant). 



h\D = f . It can be seen that due to small damping the amplitude of the inertia 

 oscillation is still quite large in the mid-depth. Calculations can also be made for the 

 case of a gradually developing sea surface slope; the amplitude of the inertia oscilla- 

 tion produced depends on the rate at which this slope develops but the character of the 

 oscillation is still kept. 



(c) Inertia Currents in Ocean Currents 



The preceding discussion leads to the expectation that inertia currents will be of 

 frequent occurrence in ocean currents, but a considerable time passed before their 

 existence was actually proved. This was due to the circumstance that in order to prove 

 the presence of cum sole, turning current variations of this type corresponding to their 

 period, current measurements from an anchored ship over an interval of several days 

 were needed. Measurements of this type are only seldom made and are associated 

 with considerable difficulties which have been overcome only in recent times. The 

 first current measurements in which the presence of inertia currents was suspected, 

 was the long series of measurements made by Helland-Hansen and Ekman (1931) 

 in the trade wind region of the Eastern North Atlantic. At the anchor station with the 

 longest observational period (141 h, 30-2° N., 14-0° W.) there were, besides oscillations 

 with tidal periods and also others with inertial oscillation, periods with 23-844 mean 



