of this with a minimum in the SW. and a maximum in the NE. In 1962 the 

 gradients characteristic of the sound velocity high in the southwest were 

 much higher than any found in 1964. Differences over the period of one 

 month as mentioned above cannot obliterate those features and so we feel 

 justified in these comparisons of regional features. 



GEOGRAPHICAL VARIATIONS 



The area can be divided into three regions depending upon the shape 

 of the sound velocity profiles above 500 m. Figure 6 illustrates typical 

 profile shapes in those three regions. In Profilers an 18 °C isothermal 

 region uetween 200 m. and 500 m. causes the sound velocity gradient to 

 be positive in this interval. The central profiles have gradients close 

 to zero in this depth interval but the southern profile has a negative 

 gradient from the surface to the SOFAR axis. Thus Profilers has a definite 

 secondary sound channel in addition to the SOFAR channel. The others do 

 not. Figure 7 shows the boundaries of the three regions considering all 

 the profiles. These boundaries occur between two adjacent profiles of the 

 1964 data. Near Profile 47 and 48 the boundary apparently moved north 

 of Profile 48, a distance of at least 30 miles, during the four days which 

 elapsed between Profile 34 and Profiles 47 and 48. 



The 196 2 data show that the northernmost profile, whose position is the 

 same as Profile 3 in 1964, has a shape similar to the profiles in the 

 central section. It might be inferred from this that in 1962 the 18 °C water 

 lay north of 30°N. 



Below the SOFAR velocity minimum there is little variation in sound 

 velocity over the area at any given depth; e. g. , at 5000 m. the variation 

 is less 1 1/2 "^/sec. 



DISCUSSION 



These are only two studies made in the same season two years apart 

 and we cannot determine from them the periods of the large scale changes 

 described. We can, however, put a lower limit of two months on them 

 since it seems reasonable from the results that the regional structure did 

 not change radically over the time required to collect the data. The 

 similarities and changes between the two years and the remarkably 

 consistent geographic pattern in one year suggest either an oscillatory 

 phenomenon such as internal waves or some very long period transient 

 in the circulation of the North Atlantic. Time series observations over 

 several years will probably be required to determine the true scales of the 



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