240 - 



G t = 0.092 

 87 OBS 



3.0At(°F) 



^^^M 



0.16 0.20 0.24 0.28 0.32G t (°F) 



FIGURE 14 FREQUENCY DISTRIBUTIONS AND MEANS OF At AND TEMPERATURE GRADIENT (G t ) 

 PER FOOT IN THE THERMOCLINE BASED ON BT'S TAKEN EVERY 5 MINUTES FOR 



8 ONE-HOUR INTERVALS 



In both cases At values are more evenly distributed about the mean 

 than are gradient values. Use of the 500-foot reference level for At instead 

 of the 400-foot level -would certainly show a considerably narrower band of 

 oscillation. In either case, application of the temperature gradient rather 

 than temperature difference would not be more advantageous from the point 

 of view of constancy. 



Temperature gradient oscillations in the thermocline can be expected 

 to increase the mixing rate when the gradient is small and to decrease it 

 when the gradient is large. Oscillations in the upper part of the thermo- 

 cline do not occur evenly over a large area but follow the spectral dis- 

 tribution of the internal wave systems at the top and bottom of the thermo- 

 cline. The mixing process may be very effective in some small areas because 

 of decreased stability in the thermocline and may be arrested in other areas 

 by increased stability. The overall situation is constantly changing; 

 however, this unevenly distributed mixing process may affect the spectral 

 distribution of the internal waves at the interface between the mixed layer 

 and the thermocline. 



The mean temperature gradient in the thermocline depends on the mean 

 stability index At and on the thickness of the thermocline which, to a 

 certain extent, is inversely proportional to the mixed-layer thickness. 

 Increase of the mixed-layer thickness decreases the thermocline thickness 

 and increases the temperature gradient in the thermocline, if At is nearly 

 constant. However, decrease of the thermocline thickness with increasing 

 mixed-layer thickness is not linear, because the bottom of the seasonal 

 thermocline tends to move downward as the top of the thermocline moves 

 downward. Downward movement of the bottom is considerably smaller and 

 possesses a certain time lag in relation to downward movement of the top. 

 When the top of the thermocline rises, the mixed-layer thickness decreases, 

 and the bottom of the thermocline also rises. 



28 



