are approximately equal to the surface temperatures shovn along the upper 

 margin of the figure. For purposes of further outlining the internal -wave 

 forms, the 55-degree isotherm (broken line) in the upper part of the ther- 

 mocline and the 52-degree isotherm (dotted line) in the lower part of the 

 thermocline were plotted. The BT's used in this figure were taken from a 

 drifting ship approximately 100 nautical miles south of OWS CHARLIE. 



Short-period internal waves at the top and bottom of the thermocline 

 are shown in Figure 6 which is constructed from BT's taken every 3 to h- 

 minutes. Temperature values are given at the surface and 350-foot depth. 

 Periods of the internal waves range from about 5 to 9 minutes; amplitudes 

 are about 5 feet. These short-period waves are superimposed on tidal 

 and longer period internal waves. 



Strong eddies produced by tidal currents meeting at right angles may 

 considerably reduce or even destroy surface waves. Heavy rain may also 

 reduce surface waves. This reduction is apparently due to turbulence 

 created in a limited surface layer. In like manner, turbulence at the 

 interface of the mixed layer and thermocline may also be expected to 

 hamper internal waves. Evidence in direct support of this assumption is 

 not available; however, there is a general impression that active mechani- 

 cal mixing diminishes the oscillations of the interface during periods of 

 increasing thermocline depth. 



THEORY 



Mixing 



Mass exchange between the sea surface and the top of the thermocline 

 forms the mixed layer. Water particles must move vertically through the 

 entire mixed layer, either in one continuous motion or by intermittent 

 steps from one level to another. The latter is more likely to occur in 

 all types of mixing. 



The main types of mixing considered decisive in the formation of the 

 mixed layer are: (l) instability mixing produced by sinking of dense 

 water; (2) mixing due to rising or sinking of water caused by divergence 

 or convergence; and (3) mechanical mixing, a turbulent transfer of momen- 

 tum from one level to another by combined action of wind waves and associ- 

 ated wind currents. All three types of mixing often occur simultaneously, 

 especially in autumn, when mixing action is more effective, that is, 

 faster and deeper. 



Instability Mixing Due to Advection 



In areas where vertical boundaries exist, tongue-like advections of 

 warm water beneath cold water masses or cold water flowing over warm water 

 create unstable situations. If the density of the overlying cold water is 

 greater, instability mixing takes place quite rapidly either with or with- 

 out the help of mechanical mixing. Such situations occur frequently at 

 station DELTA. An example is shown in Figure 7« The cold mixed layer 



10 



