INTRODUCTION 



It was shown by E. C. LaFondl-3 and 0. S. Lee^ that internal waves of 

 2-to-20-minute periods are a dominant feature in the thermal structure of the sea 

 around the NEL Oceanographic Research Tower. From extensive measurements, 

 E. C. LaFond derived that 50 percent of all waves had periods greater than 7.3 

 minutes and 50 percent of waves had heights of more than 5.6 feet (170 centi- 

 meters). Wave heights of more than 20 feet (6 meters) can sometimes be observed 

 in water only 60 feet (18 meters) deep. 



These short-period oscillations are only one part of the entire internal 

 wave spectrum. They ai'e superimposed on longer fluctuations of the mean ther- 

 mocline, with changes mainly due to internal tides and wind. 5 Nevertheless, 

 these waves in the 2-to-20minute range are the most striking fluctuations 

 besides the internal tides. Their occurrence seems to depend on several factors. 

 There is neither a close relationship to the surface tides nor to the internal 

 tides, but the changing stratification due to the tides seems to be of importance. 



Most likely, these waves are long-crested, progressive waves traveling 

 toward shore with a velocity of 20-to-40 feet per minute (lO-to-20 centimeters 

 per second). The measurements by 0. S. Lee^ indicate a beamwidth of only 

 ±15 degrees. This agrees with former measurements by C. W. Ufford,6 G. 

 Ewing,7 and E. C. LaFond. 1 C. S. Cox^ got similar results. They ai-e supported 

 by observations of sea surface slicks, which are often closely related to internal 

 waves. According to 0. S. Lee,'^ the mean speed is 27 feet per minute (13.7 

 centimeters per second) and the direction 85 degrees. 



Figure 1 gives an example of these waves measured on 4 October 1966, 

 1900-2000. The temperatui-e fluctuations are shown for thermistors 3 to 21. The 

 distance between the thermistors is 2.5 feet, thermistor 3 being 5 feet above the 

 bottom and. thermistor 21 about 20 feet below sea surface. The water depth is 

 60 feet. 



After a calm period of several hours the waves start to occur at about 1900. 

 The isotherm depth decreases during the next half hour and during that time high- 

 amplitude internal waves are present. The period is not quite independent of 

 depth. Shorter periods are generally observed near the surface than in deeper 

 layers, but all waves are of first mode. This seems typical for the area. 



The origin of these waves is rather obscure. There are no obvious mete- 

 orological or tidal forces that could produce the regular wave trains. If there 

 were a constant coupling between the tidal phase and the occurrence of these 

 waves, one would be inclined to interpret them as an adaptation of the changing 

 mean stratification. But this is not possible. The only reason for their creation 

 therefore seems to be surface waves. 



F. K. Balis has shown that, in the case of a two-layered model, resonance 

 is possible for second order interactions between surface and internal boundai-y 

 waves. S. A. Thorpe 10 extended the theory to wave interactions in a contin- 

 uously stratified fluid. He showed that a transfer of energy from surface to 

 internal waves may occur, and an internal wave generation mechanism will exist. 

 The theory has been applied to situations which might be realized in the labora- 

 tory, but an application to natural conditions has not been attempted. For tank 

 experiments (under somewhat extreme conditions) he found that the internal wave 

 amplitude will be equal to that of the surface waves after an interaction time of 

 only 28 seconds. 



