The Representation of Oceanic Movements and Kinematics 



351 



-6 -4 



cm/sec 



4 6 



cm/sec 



Fig. 144. Frequency distribution of scattering of the north and east component e and -q for 

 the point cloud of the current measurements of Fig. 142 (the full lines indicate the Gaussian 



frequency distribution). 



ellipse. In place of it Thorade used the scatter circle the radius of which is given simply 

 by p^ = m^^ + m^. This circle is quite sufficient for the characterization of the scatter 

 of a point cloud of current values. The probabihty that an observation will fall within 

 the scatter circle is with sufficient accuracy about 2/3, that is, about 2/3 of all observed 

 values will fall within the scatter circle. In the case previously mentioned (see Fig. 

 143). p = 3-76 cm/sec; the actual number falUng within the scatter circle is 103 of 

 the 150 values, which is about 2/3. 



Elimination of periodic components. The variations in speed and direction of ocean 

 currents often include periodic components superimposed on the mean current {the 

 basic current). The basic current because it is often obtained by elimination of the 

 periodic components is therefore sometimes rather unsuitably called "residual current". 



The basic current need usually not to be constant either in direction or velocity, but 

 these changes are mostly aperiodic and of long duration and therefore differ consider- 

 ably from the periodic components. The presence of these components is shown par- 

 ticularly well by graphical representation of the individual vectors in a progressive 

 vector diagram. A constant basic current plotted in this way will give a straight line, 

 while a wavy or spiral trajectory indicates the presence of periodic components. 

 Figure 145 shows a case of this type. Generally a water transport occurs directed to- 

 wards west-south-west, but it is not uniform and shows wavy fluctuations to the north 

 and the south (period of these oscillations about 14-15 h). 



The periodic components can be eliminated by taking a mean over the periods 

 present; the periodic components then cancel out giving the average basic current. 

 Thus, the case of Fig. 145 gives a mean displacement over the entire period of 2-0 

 nautical miles towards the south and 7-5 nautical miles to the west in 24 h or a basic 

 current of W. 15° S., 16-7 cm/sec. 



The calculation process for such a separation of observed current values, taken over 

 a long interval into the basic current, and the periodic components can be illustrated 



