CASE I 



1. Gravitationally stable 



2. Mean density fluxes co-directional 



3. Mixing dominated by velocity shear 



4. Layering structure observed in 



all oceans, but generally is 

 confused and irregular 



S T 



CASE II 



1. Marginal gravitational^ 



stability 



'instability 



2. Mean density fluxes oppose 



3. Salt fingering across interface 



4. Strong layering observed in 



Eastern Atlantic; layer 

 steps highly regular 



1. Marginal gravitational^? 



CASE III 



^stability 

 'instability 

 Mean density fluxes oppose 

 Thermal diffusion across interface, 

 with strong vertical convection 

 above and below interfaces 

 Strong layering observed in the 

 Arctic Ocean 



3. 



CASE IV 



Gravitational instability 

 Mean density fluxes co- 

 directional 

 Layering not observed 



Figure 1. Four cases of possible tennperature and salinity gradients. 

 Cases II and III can be either marginally stable or unstable. 



467 



