PROBLEM 



Investigate and report on the nature of internal waves of 2-to-20-minute 

 periods at the U. S. Navy Electronics Laboratory (NED Oceanographic Research 

 Tower. Specifically, study their relation to surface waves. 



RESULTS 



1. Resonant interaction between amplitude-modulated swell and internal waves 

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

 tudes of 1-3 meters can be produced within an interaction time of 15 minutes if 

 the stratification of the water is such that 



\k , — fe ,, fco , — cl) ,^ ) 



is an eigenvalue of the internal wave equation (ftj, ko, coi, and (02 being wave 

 numbers and frequencies of the modulated swell). 



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

 tics as the modulation. Specifically, they have the same wavelength and period 

 and travel in the same direction as the amplitude modulation. 



3. The resonance process is most efficient in the case of a modulation which 

 travels in the same direction as the carrier wave (the main constituent) of a 

 swell. The creation of internal waves of this type is strongly dependent on the 

 stratification of the water. 



4. Preliminary evaluations of temperature and swell records from the NEL 

 Tower show good agreement between periods of swell modulation and internal 

 wave periods. 



5. It is likely that internal waves ai-e created on the entire continental shelf 

 off Southern California during times of favorable stratification. This depends on 

 tides and wind. 



Bo RECOMMENDATIONS 



1. Verify, with additional measurements at the NEL Tower, the theory that 

 internal waves are produced by swell. 



2. Acquire, for complete analysis, long records of temperature fluctuations and 

 surface waves in order to compute spectra with high resolution and statistical 

 confidence. 



