603 



FIGURE 7. The intrusion of dyed salt solu- 

 tion into a salinity gradient at its own 

 density level. The distorted dye streaks 

 show that the fluid in the environment be- 

 gins to flow well ahead of the advancing 

 fluid. (The region shown is about 400iran. 

 wide. ) 



source. The process of vertical convection continues, 

 and further layers appear as the layers first formed 

 extend away from the source . The total voliome of 

 fluid affected by mixing is many times that of the 

 input, showing that the intrusions are overtaking 

 and incorporating the environment, rather than 

 just displacing it as in the experiment of Figure 

 7. The implication for the ocean is, of course, 

 that large scale intrusions will tend to break up 

 into thinner noses and layers, as is indeed observed. 



Each individual nose as it spreads contains an 

 excess of S relative to its environment, so that 

 conditions are favourable for the formation of a 

 diffusive interface above and fingers below, as can 

 be seen in Figure 8. This also implies that there 

 will be a local decrease with depth or an inversion 

 of T through each layer, and that the density 

 gradient above such an intrusion will be greater 

 than that below. These features have been demon- 

 strated in oceanic data by Howe and Tait (1972) , 

 Gregg (1975), and Gargett (1976). 



Note too the slight upward tilt of each layer 

 as it extends, which can be interpreted as follows. 

 Above and below an intrusion, the net density 

 differences are small and the double-diffusive 

 fluxes therefore large. The one-dimensional labo- 

 ratory observations indicate that the transports 

 across a finger interface (both in the sugar-salt 



and salt-heat case) are larger than those across 

 a comparable diffusive interface. Thus the flux 

 of positive buoyance through the fingers from below 

 can exceed the negative flux from above, so a layer 

 becomes lighter and rises across isopycnals as 

 it advances away from the source. There is also 

 a systematic shear flow associated with the inclined 

 layers, and both these features would seem worth 

 looking for when observations are made of oceanic 

 finestructure in the future. 



The interpretation of the layer slope in terms 

 of the differences in fluxes across the two inter- 

 faces is supported by experiments carried out in 

 the inverse sense. With a source of salt solution 

 (T) flowing at its own density level into a gradient 

 of sugar solution (S) , the behaviour is as shown 

 in Figure 9. Vertical convection near the source 

 is again followed by the spread of noses at various 

 levels, but now with diffusive interfaces below 

 and fingers above, corresponding to the excess of 

 T in the noses relative to their S environment. 

 There is a systematic downward tilt as the noses 

 advance , due again to the dominance of the buoyancy 

 flux at the finger interfaces, which now causes 

 the layers to become heavier as they extend. The 

 sense of the internal shear is also consistent 

 with this picture: the motion is inclined slightly 

 down and away from the source at the bottom of the 



FIGURE 8. The flow produced by releasing 

 sugar solution at its own density level into 

 a salinity gradient. Strong vertical convec- 

 tion occurs, followed by intrusion at several 

 levels. The density gradient and flow-rate, 

 and the scale of the photograph, are approxi- 

 mately the same as for Figure 7. 



