558 



Internal Waves 



waves. But Defant (1932, p. 295; 1934, p. 310) proved theoretically, that tide 

 waves at inner boundary surfaces cannot be caused by such forces. The neces- 

 sary conditions are never fulfilled under the given circumstances within the 

 ocean, and it has to be stated that the occurrence of inner tide waves can 

 hardly be caused by forces of such kind. 



The importance of meteorological influences on the formation of internal 

 waves was discussed by Wedderburn in his work on Scottish lakes. M.Pet- 

 tersson (1920, p. 32) used similarly the daily variations in the vertical dis- 

 tribution of the water-masses at the oceanographic station Borno (Gullmar- 

 fjord), to find possible relations with simultaneous meteorological conditions. 

 The comparison of the variations in the depth of the isohalines with the 

 corresponding force of the winds in the direction of the fjord (NE.-SW.) 

 shows immediately that the curves for most of the cases run parallel (see 

 Fig. 232). A more exhaustive investigation of the correlation between the 



NE 



-25 cm 



I 5 9 13 17 21 25 29 2 6 10 14 18 22 26 2 6 10 14 18 22 26 30 

 1911 Jon. Feb. March 



Fig. 232. Relationship between the variations in depths of the isohalines 31 and 32 % 



and the surface and the force of the wind in the direction of the Gullmarfjord. Borno, 



1 January 30 March 1911 (H. Pettersson). 



vertical displacements and the wind-force confirmed numerically this meteoro- 

 logical influence. Where the wind blows from the ocean towards the fjord 

 the surface water piles up at the inner end of the fjord. This is accompanied 

 by a lowering of the boundary surface between upper and lower layer of the 

 water (mostly with a delay of 1-2 days). Conditions are reversed with the 

 wind blowing towards the sea. Not all oscillations can be traced back to 

 these simple occurrences. But the influence of the wind remains the principal 

 influence for the formation of internal waves, at least in the case of more 

 or less enclosed water-masses. 



While investigating the seiches of the Gullmarfjord Zeilon (1912) found 

 the main period of the uninodal wave to be HOmin (see Chapter VI. 3a, 

 p. 182). Internal waves of the same period occurred at the discontinuity 

 surface in the fjord. Zeilon, thinks that the currents in the Kattegat near the 

 entrance of the fjord are the outer cause for the generation of these secondary 



