As the slicks and bands of discolored water passed the 

 tower, the depth of the thermocline was recorded with 

 isotherm followers. One distant sea-surface slick that con- 

 tained foam was found to be oriented parallel to the crest of 

 an internal wave and on the descending side of the thermocline. 

 This location (area C) is the usual one for surface convergence 

 associated with internal waves. ^ The clearest near-surface 

 water was over the maximum depression in the thermocline, 

 that is, in the internal wave trough. In this convergence- 

 type circulation, the water above the thermocline was moving 

 toward the slick and sinking there. Since the upper 1 or 2 

 feet of the water column was clearer, the accumulated surface 

 water gave a clearer appearance to the water over the trough 

 (area D), since the phytoplankton were at a greater depth in 

 this location. Where the thermocline was near the surface 

 just ahead of the slick (area B), the organisms causing the 

 turbidity were close to the surface and more visible. As 

 the internal waves moved shoreward at a speed of about 0. 3 

 knot, the bands of discolored water also moved shoreward. 



This behavior is an example of those short-period 

 changes in the vertical distribution of turbidity that are con- 

 trolled by internal waves. In the Mission Beach area, in- 

 ternal waves have an average period of 5-6 minutes, and 

 thus turbidity fluctuations at the surface and at thermocline 

 depth have similar periods. Furthermore, the orientation 

 parallel to internal waves continues as a group of internal 

 waves moves shoreward, thus tending to create a band 

 distribution of clear and turbid water easily distinguishable 

 by eye. 



ACOUSTIC SCATTERING FROM TURBID LAYERS 



Plankton and other organisms are capable of causing 

 echoes when high-frequency sound is directed through the 

 water. Since some of the organisms that produce turbidity 

 may be the same as those that are responsible for the 

 acoustic scattering, another approach to the study of short- 

 period changes and patchiness in turbidity is by way of 

 acoustics. 



One such study was made at the NEL tower by placing 

 an NK-7 echo-sounder on the ocean floor with the sound 

 beam directed upward toward the surface.'^ The instrument 

 was operated at 21 kc/s with a beamwidth of 20 degrees at 

 the 6-db down points. The purpose was to determine the 

 distribution, the behavior, and if possible, the nature of 

 the sound scatterers. 



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