Internal Waves 



559 



internal waves. The bottom configuration at this point shows a submarine 

 barrier. Therefore, only the upper water-masses of the fjord are connected 

 with the free ocean. This submarine sill or barrier, which rises nearly to the 

 level of the discontinuity surface, is an obstacle for the development of the 

 horizontal currents of the seiches and these currents cause a disturbance at 

 the boundary surface, which now travels as an internal wave with the period 

 of the seiche along the discontinuity surface into the fjord. Zeilon (1934) 

 could support this explanation by very convincing experiments in the wave 

 tank. He also demonstrated that in an oscillating stratified water-mass a dis- 

 turbance in the bottom configuration could generate internal waves which 

 then travel along the discontinuity surface of density. Zeilon found in other 

 experiments that internal waves can be formed when a tidal current is ad- 

 vancing toward a wide shelf. If the discontinuity surface is in about the same 

 depth as the outer limit of the shelf, an internal wave is formed on the edge 

 of the shelf and it travels toward the coast in shallow water and also toward 

 the open ocean in deeper water. The two illustrations in Fig. 233 demonstrate 



Fig. 233. Tidal current producing internal waves at the edge of the shelf. At b the internal 

 wave hreaks at the shelf (Zeilon). 



Zeilon's experiments. The greatest amplitudes will be found near the shelf 

 in shallow water. Submarine breakers occur during the propagation on the 

 shelf, which can be clearly recognized in the illustration "b". 



Further causes for generation of internal waves are the more or less sudden 

 appearance of a strong surface current, which can be caused by a gust of 

 wind (similar to Sandstrom's experiments, see p. 528), or by the occurrence 

 of a bottom current. The place where the outside disturbance acts upon the 

 water-mass is in both cases the source of the internal waves, which proceed 

 along the boundary surface in the direction of the disturbing current. 



