372 



The Representation of Oceanic Movements and Kinematics 



played here by the compensation requirement which is a result of the continuity law. 

 Since water is almost completely incompressible it cannot accommodate a widening 

 or contracting of the stream lines by contraction or expansion and movements normal 

 to the flow direction or even counter currents are set up to a much greater extent than 

 in air movements in order to avoid the formation of empty space. The nature of these 

 counter movements can only be fully explored empirically by observations in nature 

 or by special suitable experiments. Experiments of this sort have been made extensively 

 by Krummel (191 1, p. 470) and have been used for a clarification of many phenomena 

 exhibited by the pattern of the ocean currents. The results of that shown in Fig. 161 

 are particularly instructive. The resemblance of the experimental current system to 

 that in the Central Atlantic can readily be seen ; this system consists of the two wind 



Fig. 161. Experimentally produced current patterns (simulation of the current system in the 

 central part of the Atlantic Ocean) (according to Krummel). 



drifts induced at the sea surface by air currents, and the corresponding circulations 

 to the north and the south as well as the (equatorial) counter current between them. 

 At the projecting peak on the left-hand side of the experimental tank representing 

 land (Cape San Roque) the current intensity was surprisingly large (corresponding to 

 the Guayana Current). 



Standing vortices are formed at coastal bays, in which the flow always shows such 

 a sense of rotation that the current on the seaward side follows the main current while 

 that on the landward side is opposed to it. Hydrodynamically such a vortex can be 

 stationary, but it will always have the same water mass circulating within it and there 

 will be no water transfer from the main current to the vortex. In nature this is usually 

 not the case. Pulsations in the main current will always affect the intensity and the 

 extent of the stationary vortex and will thereby lead to a renewal of the water circulating 

 in it. Such replacement currents in bays and small gulfs are termed "neer currents" 

 and are always present at any reasonably irregular coast consisting of small bays and 

 projecting land. An example is shown in Fig. 162. 



The compensation requirement need not always to be satisfied by horizontal trans- 

 ports, but vertical movements are also sometimes involved and give rise to very charac- 

 teristic oceanographic phenomena (upwelling). 



