738 liAFOND [chap. 22 



c. Speed 



The speed of internal waves was determined by measuring vertical oscilla- 

 tions simultaneously in three locations (UflFord, 1947a; Lee, 1961a), and 

 deduced from the movement of their associated sea-surface slicks (see page 746). 



Time-lapse films of surface slicks off southern California, ^ showed that 

 internal waves moved toward shore at speeds of 0.11 to 0.6 knots. Other 

 measurements from anchored ships with range markers indicated an average 

 speed of 0.31 knots. More recent measurements averaged 0.27 knots in 60-ft 

 deep water. 



d. Direction 



The shape of internal waves varied Avith shoreward movement and with the 

 refraction as they moved into shallower water. Nearly all internal waves pro- 

 ceeded from a west to southwest direction at the Mission Beach location. 



e. Currents 



Relative currents in the open sea are usually computed from the distribution 

 of mass. Current along an isobaric surface is essentially a function of the geo- 

 potential, or dynamic slope of the isobaric surface. If motion is taken to be 

 negligible at a particular depth, or an isobaric surface to be level, the dynamic 

 slope of the upper isobaric surface can be determined from variations of specific 

 volume along the isobaric layer. The current at the upper surface, relative to 

 any possible current at the lower surface, can thus be established (LaFond, 

 1951). 



Under the influence of internal waves, however, the average vertical specific 

 volume above a reference level located below the thermocline will change and 

 cause a considerable difference in the calculation of current. Short-period 

 variations and the depth of the thermocline are difficult to treat, though long- 

 tidal-period oscillations have been considered for serial stations off the California 

 coast (Defant, 1950). 



A tidal influence on the computation of relative currents was also suspected 

 from an examination of mid-Pacific temperature data taken near Bikini 

 Atoll (LaFond, 1949). Here a large, transitional layer separates the relatively 

 light surface water from the heavier, deeper water. An internal wave causing a 

 change in the depth of the transitional layer would materially change the 

 vertical mass field. 



Internal waves are not wholly random, but appear to fall in a cyclic pattern. 

 The general trend is shown by dotted lines on the temperature curves (Fig. 10). 

 The high phases fall about 12 h apart and have nearly the same period as the 

 tide. The greatest changes in temperature, as well as in salinity, occur at 700 ft 

 below the surface ; above 400 ft, the changes are small. At 900 ft below, the 

 changes in temperature, indicative of vertical fluctuations, are somewhat 



1 Mission Beach, La JoUa and San Diego Bay. 



