sovnd channel axis; No. U5 has tvo strong gradients in very re- 

 stricted layers which define an inversion in the soxond velocity of 

 some four meters per second over 17O meters. These features are 

 in the nat\are of distortions to the velocity profile, and it will 

 next be suggested that they define a zone of transition between 

 changes in the sound velocity profile across the ridge crest. 



Geographic Classification of Sound Velocity Profiles 



The velocity profiles in the immediate neighborhood of the 

 ridge axis can be systematically divided into three groups. The 

 four stations furthest from the axis can, more or less, be brought 

 into the discussion, but they introduce differences due to their 

 relative separation. 



In Figure 3, two groupings of profiles are apparent in a super- 

 position of seven lowerings. They are designated as east and west 

 of the axis because that is their predominant geographical distri- 

 bution, as seen in Figure h. Two other stations which can be 

 classified by their profiles are also included. The difference be- 

 tween the profiles of each group is the resxilt of a relatively saline 

 core of Msditerranean water (diffused to 35.5 0/00 from about 36.2 

 0/00 after the initial mixing at Gibraltar) which is present to the 

 east and absent to the west. The velocity profiles diverge because 

 the horizontal gradient in salinity across the ridge axis is balanc- 

 ed by an inverse temperature gradient, as the isopycnic surfaces 

 tend to stay horizontal. 



A third group of profiles is transitional between the main east 

 and west groups, both in geography and in velocity profile. In 

 Figure 5 they are compared at intermediate depths on an individual 

 basis. The transitional profiles are of the two types described 

 earlier, and they are separated accordingly. Though only one clear 

 example of an iso-velocity layer at the channel axis is shown. No. 

 3l|- was in many ways similar. In the plot of their locations. Figure 

 h, the intermediate position of these transitional profiles is strong- 

 ly suggested. 



With only sound velocity information it would be difficxilt to 

 offer any explanation for what has been presented. Examining the 

 salinity data identified the source of the changes as a difference 

 in the appearance of Jfediterranean water to either side of the 

 ridge axis. A comparison of temperatvire- salinity diagrams will 

 STiggest how transitional profiles are formed. 



Inferred Dynamics of the Transition Zone 



The temperature -salinity diagram has been the most useful tech- 

 nique of the oceanographer in inferring the circulation of ocean 



416 



