Occurrence of Slicks 



From the foregoing it appears that the occurrence of visible slicks depends on 

 several factors. In addition to proper wind, lighting, and sufficient organic matter 

 on water, slick formation depends on the nature of the internal waves. The con- 

 centration of the surface film depends on the interrelation of internal wave height 

 and period. In addition the average depth of the internal wave and its relation 

 to the depth of water will also influence the type of circulation and thus the for- 

 mation of slicks. 



Position of Slicks with Respect to Internal Waves 



Sometimes, the surface slick was over the trough of the depression in the ther- 

 mocline; at other times, the slick wandered to a position nearly over the crest of 

 the wave. However, for 85 out of 105 cases shown in figures 3 to 14, the slick was 

 on the descending thermocline, somewhere between the crest and following 

 trough. The cause of this relationship is believed to be the water circulation cre- 

 ated by internal waves. 



Water Motion 



The circulation in slicks may be deduced by several means: (1) The concen- 

 tration in lines or patches of film at the surface indicates a motion towards the 

 patch from one or both sides; otherwise, the film would spread out and break up. 

 (2) From the vertical structure, the lowering of the thermocline is indicative of 

 downward motion. Also, maintenance of a depressed thermocline requires a force 

 towards this zone. (3) The accumulation of turbid material in zones implies either 

 a downward motion from the surface, where turbid material frequently collects, 

 or an upward motion from the thermocline, which is frequently another source 

 of turbidity. It is possible that in regions of upward motion, some organic ma- 

 terial is brought to the surface where the action of air or change in the proper- 

 ties of the organism will lower its density and cause it to resist the downward 

 circulation under the slicks. 



Current drags, on the other hand, sometimes drifted through a slick; on other 

 occasions, they moved towards a slick and aligned themselves in it. This variation 

 is indicative of different water movements associated with slicks. 



Distortion of a straight line made on the water surface by sea marker dye 

 showed that, in addition to vertical motion normal to the slick, there is a flow in 

 or parallel to the longer axis of some slicks. 



Vertical movement was once investigated by means of dye marker. Dropping 

 the dye marker vertically and observing the horizontal distortion showed that a 

 shearing stress existed in the thermocline. 



From time-lapse films of the sea surface taken at Mission Beach, La Jolla, and 

 San Diego Bay, the motion of the surface was studied. In all cases, the sea slicks 

 moved on-shore at speeds of 0.11 to 0.6 knot, corresponding to the average of 

 0.31 knot measured from the ship with range markers. The shape of slicks varied 

 as they moved shoreward and the slicks appeared to refract as they moved into 

 shallower water. In the presence of currents in San Diego Bay, slicks moved with 

 the direction of flow and were predominant at water type boundaries. 



In the sea, internal waves are believed to take the form of progressive waves. 

 The nature of progressive waves between two liquids of diff^erent densities is given 

 by Lamb.' 



25 



