Of the four cases described, Case IV is the only one that is always 

 unstable. Therefore it cannot persist in the ocean and will not be further 

 considered. Case I predominates beneath the central water masses of the 

 ocean. Case II predominates within the central water masses. Case III 

 is not conamon but is found in the Arctic above the core of Atlantic water 

 (Coachman and Barnes, 1963). 



Layering in Case I 



Woods (1968) observed step structure in the thermocline of Maltese 

 waters which have Case I conditions. He experimented with dye diffusion 

 and concluded that shear induced instability in a local region led to a break- 

 down of the stable "sheet" or "offset" which separates two homogeneous 

 layers. There is presumably high heat flux into the lower of the two layers 

 after the break down occurs since the scale of interaction motion is suddenly 

 driven into lower wave numbers by breaking of waves on the interface. 

 However, Woods was able to follow a given layer for 40 miles. 



Cooper and Stommel (1968) concluded that layers examined off Bermuda 

 at 600 to 700 m (also Case I) have horizontal dimensions of 400 to 1000 meters. 

 We have made one series of CTD casts from a drifting ship where Case I 

 prevails off Oregon. We observed several layers or lenses at a depth of 

 1100 meters (Fig. 2). Ship drift during our series of casts was about 2 

 nautical miles. In order to apply the spasmodic mixing hypothesis dis- 

 cussed by Stommel and Fedorov (1967) to the data in Figure 2, one must 

 account for the tapering of lens thickness to its lateral terminus. It seems 

 likely that the lenses shown here are old, having spread under the influence 

 of horizontal pressure gradients which remained after a spasmodic mixing of 

 two layers. Since such a spreading motion includes a friction layer of some 

 thickness at the upper and lower interfaces, the rate of spread would rapidly 

 diminish when these friction layers merged; moreover, such a terminus 

 would be distinguished in a temperature section as a point value within the 

 large temperature gradient separating the parent layers as seen in Figure 2. 



Layering in Case II 



Laboratory studies by Turner (1967) have shown layering in Case 11. 

 He started his experiments with homogeneous layers in a vessel; warm, 

 salty water was on top of colder fresh water. He sprayed a small anaount 

 of salt water on top. The salt fingering that resulted through the top did not 

 penetrate the whole layer. Salt fingering also occurred simultaneously across 

 the lower interface. In this marginally stable condition salt fingering trans- 

 ports both salt and heat downward. Once a salt column is created by a 



459 



