Internal Waves 



563 



regularly in agreement with theoretical principles and without considerable 

 disturbance. This does not seem to be the case at springtide when the tidal 

 currents are stronger. The repeated serial observations of the "Dana" from 

 14 to 15 July 1928, which were carefully analysed by Jacobsen and Thomson, 

 show that the internal wave, which was linked with the surface tide, had 

 decayed completely. In the layer between 70 and 150 m the wave is almost 

 turning over. Several values show an instability in the stratification. The 

 wave profile is very asymmetrical and the internal wave assumes the form 

 of an internal bore. It probably traverses the entire strait in the region of 

 the strong tidal currents at springtide. Then it vanishes in the adjoining sea 

 where the depth is so much greater and where the discontinuity in the density 

 is entirely missing, whereas it was well developed in the strait. Figure 234 



Solor hours 

 LAW. Codix HWCodix LW. Codix KW. Codix 



-2 2 4 9 8 10 I 2| 14 16 



•4 



Codix 



2 +2 +4 ±6 -4 -2 +2 

 HWCodix LW. Codix HWCodix 



LuP° r hours 



Fig. 234. "Dana"' Anchor Station in the Strait of Gibraltar <£ = 35°57-5'N. I = 5°21'W. 



14-15 July 1928. Isopycnals (kg/m 3 ). Below is given the tide curve for Cadix. The arrows 



indicate the direction and velocity of the total current (which is the sum of the basis and 



tidal current) in the upper layers in the centre of the Strait at springtide. 



presents the course of the isopycnals (kg/m 3 ) for the entire duration of the 

 observations up to a depth of 300 m and the unstable internal tide wave within 

 the strait. Also entered in Fig. 234 are the tide curves of Cadix and the curves 

 for the total surface current (main current plus tidal current) for the middle 

 of the Straits. 



A number of additional cases of obvious instability of internal tide waves 

 were shown by Defant (1948, p. 52) in the observations of the "Meteor" 

 anchor stations on the West African Shelf between the Canary and the Cape 

 Verde Islands. A particularly interesting case was the anchor Station 366 



36" 



