564 



Internal Waves 



(13-16 April 1937, 10°16'N., 16°38'W.; 60 repeated casts in 1 hourly intervals). 

 In Fig. 235 the principal isothermes have been averaged from five periods 

 of the semi-diurnal to one period. This figure demonstrates clearly the asym- 

 metrical profile of the internal tide wave in the vertical displacement of the 

 isotherms. The extreme values are shifting with increasing depth, the internal 

 wave is retarded more and more, especially the maximum in the deeper layers 

 lags several hours after the maximum in smaller depths. 



Ancor Stat. 366 



Fig. 235. Isotherms at "Meteor" Anchor Station 366. Average from five periods. 



All these facts indicate that the wave profile of internal waves can decay 

 under certain conditions, which can lead finally to internal breakers (surf). 

 One should not think that these internal breakers should have the same 

 energy and have the same speed as the surf of the regular surface waves. 

 This internal surf takes place very slowly, because of the small velocity of 

 the internal waves. Such internal breakers can be produced in a wave tank 

 in a two-layer system. There the phenomenon can be followed closely be- 

 cause of the slowness of the occurrence. A photograph of experiments of 

 such kind will be found in DEFANTj(19r3, p. 83; 1929, p. 51). Similar phe- 

 nomena will occur with internal waves in nature. 



The question arises as to what causes an internal tide wave to such a turbu- 

 lent process of degeneration and finally to break down internally. When the 

 amplitude of internal waves becomes great and cannot be considered as small 

 compared to the depth of the water, then the wave changes its profile during 

 its travel. The slope at the front of the wave becomes steeper, and flattens 

 out at the rear of the wave; the wave profile becomes asymmetrical. The 

 increase of this asymmetry leads to the breakers and the surf. Under similar 

 conditions, internal waves will behave the same way as surface waves where 

 the amplitudes were great at shallow depth. Both circumstances are doubtless 

 contributing factors in forcing an asymmetry of the wave profile. 



