VARIABLENESS OF THE SOUND VELOCITY PROFILE 

 ABOUT THE MID-ATLANTIC RIDGE AXIS 



by 

 Eli Joel Katz 

 Associate Scientist 

 Woods Hole Oceanographic Institution 

 Woods Hole, Massachusetts, 02543 



ABSTRACT 



Mediterranean water, spreading across the North Atlantic Ocean, 

 is shown to cause a disturbance to the acoustic medium in the 

 vicinity of the Mid-Atlantic Ridge north of the Azores. In this 

 region, the ridge crests are sufficiently shallow so as to impede 

 the free movement of the intermediate waters. Warm and saline 

 layers, nearly two hundred meters thick, were repeatedly observed 

 in the lowering of continuous sampling sensors. These inversion 

 layers are shown, by reconstruction, to result from local water 

 movement across the ridge crest and are argued to be short lived 

 and spatially confined. As a direct consequence, the observed 

 depth of the sound -velocity minimum unpredictably fluctuated as 

 much as five hundred meters within an area of two hundred miles 

 square. The layers themselves result in secondary sound channels; 

 the inversions of both temperature and salinity combining to pro- 

 duce inversions of four meters per second deep in the thermocline. 



Introduction 



Throughout most of the world's oceans the water column consists 

 of a large bulk of cold, nearly isothermal water, topped by a 

 warmer surface layer. As the velocity of propagation of sound in 

 water increases with both depth (more precisely, pressure) and 

 temperature, velocities are highest at either end of the column 

 resulting in a widely spread sound channel in which sound can 

 travel, practically speaking, for unlimited ranges. In general, 

 the axis of the channel will be found at the bottom of the trans- 

 ition between the warmer and the colder water. The relative ef- 

 ficiency of the channel to propagate sound will be primarily 

 related to the total temperature contrast between the two and the 

 thicknesses of the intermediate and cold water layers. In the 

 open ocean the parameters defining the sound channel, being 

 products of the air-sea interchange and oceanic circulation, will 

 normally vary slowly by season and over distances measured in 

 hundreds of miles. Exceptions have to be explained in this same 

 context , 



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