SECT. 5] INTERNAL WAVES 747 



The occurrence of visible slicks is contingent upon proper wind, lighting, 

 sufficient organic matter on the water, and the nature of internal waves. The 

 concentration of surface film depends on the interrelation of internal wave 

 height and period. The average depth of the internal wave and its relation to 

 water depth also influences the type of circulation, and thus has a bearing on 

 the formation of slicks. 



A surface slick was sometimes observed over the trough of the depression in 

 the thermocline. On other occasions a slick wandered to a position nearly over 

 the crest of a wave. However, in 85 out of 105 cases the slick was on the descend- 

 ing thermocline somewhere between the crest and the following trough (Fig. 17). 

 This relationship is believed to be the result of water circulation created by 

 internal waves. 



The significant motion is a surface convergence over the trailing slope of the 

 internal wave. Although the maximum expansion of the surface layer Avas 

 over the trough, the slicks were normally found at the active surface con- 

 vergence zone. 



i. Relation to tide 



Many observers, notably Helland-Hansen and Nansen (1909), Defant (1932), 

 Uiford (1947), LaFond (1949), Rudnick and Cochrane (1951) and Arthur (1954), 

 have noted that internal temperature fluctuations sometimes have a nearly 

 tidal periodicity. Tidal period and phase relationships have, therefore, been 

 compared with internal waves and sea-temperature structure measured in 

 various waters around the world (LaFond and Rao, 1954). 



Haurwitz (1954) has questioned whether these observations refer to strictly 

 periodic components of internal observations. Unless a long series of measure- 

 ments are available, it is impossible to distinguish periodic from less regular 

 variations if there are irregular fluctuations (as there always are in observations 

 of internal motions). 



On the basis of Haurwitz's criterion there are only a few places in the ocean 

 where it has been possible to show with fair certainty that tidal periodic 

 internal waves exist. The most remarkable example is due to Reid (1956), who 

 has found lunar, semi-diurnal fluctuations of large amplitude off" the coast of 

 California (Fig. 18). 



While it is not certain to what extent periodic variations are present, there is 

 no doubt that large variations of temperature of a quasi-periodic nature close 

 to the semi-diurnal frequency are present in the sea. Below we shall discuss an 

 example of quasi-semi-diurnal oscillations which clearly implies that the 

 driving forces are related to the tide. 



Wide fluctuation of internal and surface tide phases were established by 

 the observations of Lee and LaFond (1960) from the NEL oceanographic 

 tower. The depth of an isotherm, recorded for seven consecutive days, was 

 plotted with reference to the phase lag of observed high surface tide (Fig. 19). 



This observation showed that the relationship may change phase daily, but 



