periods are shown in figure 4. They show the number of occurrences of periods 

 between 2 and 22 minutes in percent. The number of waves contained in each 

 histogram is about 120. The figure is in good agreement with the theory given 

 above, both distributions are peaked in the period range of 4-to-lO minutes and 

 decrease towai'd periods larger than 20 minutes. Final conclusions, however, are 

 possible only after the spectral analysis of the entire data. A better agreement 

 between these histogi'ams cannot be expected because the internal waves are 

 strongly dependent on the changing local mean stratification, ^ whereas the swell 

 is governed by quite different sources. 



INTERNAL WAVES 



Figure 4. Histograms of periods of swell modulations (left) and of internal 

 waves (right) at the NEL Tower. 



CONCLUSIONS 



1. Resonant interaction between amplitude-modulated swell and internal 

 waves may create internal waves of 2-to-20-minute periods. Internal waves with 

 amplitudes of l-to-3 meters can be produced within an interaction time of 15 

 minutes if the stratification of the water is such that 



pi -'k^l'^/fco^ -w./ 



is an eigenvalue of the internal wave equation (k^, k2, wi, and co2 being wave 

 numbers and frequencies of the modulated swell). 



2. Internal waves due to amplitude-modulated swell have the same charac- 

 teristics as the modulation. Specifically, they have the same wavelength and 

 period and travel in the same direction as the amplitude modulation. 



26 



