Waves 



127 



on courses toward land. Most of the 

 records, however, show a complex form sim- 

 ilar to that of surface waves and strongly 

 suggestive of interference between two or 

 more sets of internal waves. 



In a study of slicks off the La Jolla region 

 Ewing (1950) found that these thin films of 

 oily organic matter are associated with the 

 troughs of internal waves (Fig. 113). Lateral 

 movement of the slicks toward shore indi- 

 cated that the internal waves were of the 

 progressive type with dimensions similar to 

 those of Ufford. Bending of the slicks to 

 conform with the shore showed that the in- 

 ternal waves became refracted in the same 

 manner as surface waves. Further analogy 

 with surface waves is shown by breaking in- 

 ternal waves, or internal surf, reported by 

 Defant (1950a) in the Mediterranean and 

 observed by R. E. Stevenson (personal com- 

 munication) in a line of bathythermograph 

 lowerings in San Pedro Bay in 1954. Dur- 

 ing 1950 a 6-day series of measurements 

 were made by three ships of the California 

 Cooperative Oceanic Fisheries Investigation 

 to test the suggestion by Defant (1950a, 

 \950b) that internal waves of tidal period 

 exist and are capable of causing erroneous 

 interpretations of currents from dynamic 

 topography. The three ships were stationed 

 in a line 40, 160, and 320 nautical miles off 



Point Sur (about 50 miles north of Chart I). 

 At the innermost ship internal tides of ampU- 

 tude 10 to 15 meters were detected at depths 

 between 12 and 110 meters (Reid, 1956). 

 Although the period was tidal and the wave 

 in phase with the tide at the coast, the 

 asymmetry of highs and lows typical of the 

 tide curve was lacking. At the two outer 

 ships nonperiodic fluctuations of similar 

 amplitude were measured. Since bathy- 

 thermograph lowerings were made only at 

 1-hour intervals, the measurements could 

 not show the short-period internal waves 

 measured by Ufford. 



An internal tide was also found at a depth 

 of 6 1 meters (200 feet) at a station near the 

 northwest end of Santa Catalina Basin, 

 where temperature measurements were made 

 at 1 to 2 hour intervals for 25 hours (Emery, 

 1956Z)). Unlike the one off Point Sur, this 

 wave lagged about 2.5 hours behind the tide 

 ashore (Fig. 114). More interesting, how- 

 ever, are waves measured below the sill of 

 the basin; these exhibit an intermediate 

 period of 2 hours and possibly another of 

 18 hours. The amplitude was as much as 

 200 meters (650 feet). It is suggested that 

 these deep oscillations of temperature indi- 

 cate the presence of one or more standing 

 internal waves having a node near the cen- 

 ter of the basin (Fig. 115). Such waves also 



SLICK 



SLICK 



1310 HRS. 



Figure 1 13. Temperature (°F) structure under a series of sea surface slicks measured with a fixed series of thermister 

 beads at an anchor station off Mission Beach June 12, 1958. The position of each slick corresponds to the trough of 

 an internal wave where the isotherms are sharply depressed. The slicks and associated internal waves move shoreward 

 in this area at speeds as great as 0.25 meter/sec. Courtesy of E. C. LaFond, U. S. Navy Electronics Laboratory, San 

 Diego. 



