Internal Waves 



567 



In the upper wave the direction of the basic currents is the same in both 

 water-masses; in the lower wave the direction of the basic currents is opposite. 

 In both cases the velocity of the upper water-mass is the greater one, as it 

 is always the case in nature. It will be immediately recognized that the asym- 

 metry of the wave profile reminds one of the slopes of internal tide waves. 

 When the asymmetry of the wave increases it will cause finally powerful inter- 

 nal processes of turbulence. In the case of dynamic instability, the boundary 

 surface curls up into vortices of the same direction, which insert themselves 

 between the two layers which have different motions. Figure 237 shows, 



Fig. 237. Transformation of a dynamic unstable internal wave (Rosenhead). 



according to Rosenhead (1932, p. 170), the transformation of an unstable 

 wave. The final result will be the formation of a mixing layer, caused by 

 the continued vortex motion. There will be then a more or less continuous 

 transition of the density and of the velocities from the lower to the upper layer. 

 The discontinuity of temperature and of salinity decreases rapidly on con- 

 tinental shelves by approaching the shore and then mostly disappears inside 

 a stripe off the coast completely. This is due to the circumstance that the 

 decreasing depth of the ocean causes the internal tide waves to become so un- 

 stable that the discontinuity layer disappears. This process is the basic pre- 

 requisite for the process of upwelling of cold water along the coasts. 



6. Stationary Internal Wave-like Displacements 



The oceanographic survey of various regions of shallow depth (on the 

 continental shelf or straits) as well as in the open ocean of not too large 

 a depth, showed frequently vertical cross-sections with striking wave-like dis- 

 tributions of the isotherms and isohalines and of course of the isOpycnals. 

 They were apparently of a stationary character. Merz (1935), for instance, 

 found in the western part of a temperature cross-section through the South 

 Atlantic Ocean in 35° S., large vertical variations of the isotherms, which 



