1 00 The Three-dimensional Temperature Distribution and its Variation in Time 



of 19' and S of the upper layer hy. The density p^ of the second layer //o corresponds to 

 the isopycnal that passes through the points B and C. Since the density of the upper 

 layer must be equal to the density of the lower layer, as must be the case at the end 

 of the mixing by convection, then either the temperature must decrease hy AB = 

 —A'd'i if the salinity is constant, or the salinity must increase by AC — ASi if the 

 temperature is constant. A convenient connection of the point A with a point D on 

 the ispycnal p2, between B and C, gives the value of the disturbance for the tem- 

 perature and the salinity if both are present at the same time. The determination of 

 magnitude of the disturbances from the [r5]-diagram in this way offers little 

 difficulty, 



A simple schematic diagram gives a convenient representation of the results of 

 convective mixing. Fiaure 47 shows the normal vertical distributions of d and S and of 



%^^<=^ At AS 



Fig. 47. Change in the thermo-haline structure of the sea 

 produced by convection processes. 



the specific volume a; they represent the conditions before the mixing of the upper 

 layers by a convection extending only to a depth h. If the convectional disturbance is 

 entirely due to a reduction in & (by radiation) then the state of the upper layer at the 

 end of the convection process is characterized by the broken straight line; if, on the 

 other hand, the convection disturbance is entirely due to an increase in salinity the 

 dotted straight line shows the final state. It can be seen that the convection levels out 

 any differences in the vertical gradient for the different factors. 



(d) Dynamic Convection (forced vertical mixing) 



While thermo-haline convection is produced by external sources of disturbance 

 and continues as long as these disturbances remain, dynamic convection depends on the 

 forced mixing of superimposed layers of water embedded in a turbulent current. The 

 disordered eddying flow of larger quanta of water within such a current causes a con- 

 tinuous mixing of the water mass in both vertical and horizontal directions. This mixing 



