354 



The Representation of Oceanic Movements nad Kinematics 



in both cases. Reference should be made to Vol. II for further discussion of these 

 current diagrams. It may be mentioned here that the components, phases and ampli- 

 tudes of the inertia waves correspond closely to those given by the theory of oscilla- 

 tions (see Chap. XIII, 6). Since the current diagram deviates only slightly from a circle 

 the fine dashed circle in Fig. 147); for this the amplitudes of both components must be 



Fig. 147. Current measurements at the anchor station of the "Altair", 16-20 June 1938. 

 Left side: current card of the semi-diurnal tide according to the smoothed values of the 

 individual hours and the current ellipse according to the values of the harmonic analysis. 

 Right side: the same for the 17-hourly inertia wave (the dashed circle indicates the theoretical 



inertia circle). 



the same, and furthermore, the phase of the ^-component must lag one-quarter of a 

 period behind that of the iV-component. The observations give an amplitude ratio of 

 M4 and 12-6 h + 4-25 h = 16-85 h as compared with 17-0 h which is a difference of 

 only 0-15 h. These properties of the 17 h wave confirm that it is a pure inertia wave. 

 Decomposition of current data by means of other methods. After the elimination of 

 the periodic components there still remain other more aperiodic effects superimposed 

 on the basic current, which is almost constant in time. These deviations may be due 

 to various causes such as piling up of water at shores (Anstau) or variable wind stress. 

 The wind especially is liable to give rise to drift currents in the surface layers, the 

 direction and strength of which depend on that of the wind and change with it. The 

 observed current in these cases can be looked upon as the resultant of thedrift current 

 and the basic current. If the latter alone is required the two must be separated by a 

 special procedure. Nansen (1902) gave a suitable method for this which was used in 

 the evaluation of the ice-drift observations of the "Fram". If intervals of time, for 

 which the wind resultant is zero, are taken together and the effect of the wind on the 

 water therefore considered very small, then the resultant current for the total interval 

 will be due to the basic current alone. In that way he found by analysis of six rather 



