that the actual distribution of salinity is the result of diffusion of 

 salts in the water and that vertical mixing predominates over hori- 

 zontal mixing, whence he deduced that the second derivative of 

 salinity with respect to depth is ecjual to zero at the level of no motion. 

 He concluded that in the area investigated the depth of the motionless 

 surface varied between 900 and 1,400 meters, and that in the northern 

 part of the area it was between 1,000 and 1,100 meters with indications 

 that it sank to deeper levels north of 20° X. 



Since the condition that the second derivative of salinity with 

 respect to depth is equal to zero occurs at points of inflection on a 

 vertical distribution curve of salinity, the curves for the stations 

 taken eastward of the Grand Banks in 1937 were inspected for such 

 points of inflection. However, in the levels which might reasonably 

 be expected to contain a motionless surface, the salinity gradients are 

 so small that the method is not sufficiently sensitive and usually more 

 than one point of inflection was found. 



From currents deduced from dead reckoning and sliip positions from 

 Loran fixes, von Arx and others have determined proportionality 

 factors for the von Arx current meter when it is operated in the 

 currents ot the North Atlantic eddy. As the current meter indica- 

 tions are dependent on shear, or vertical gradient of velocity, the 

 proportionality factors so derived can be used to deduce the depth 

 to which appreciable shear extends and hence the depths of a motion- 

 less surface. Depths determined in this manner turn out to be some- 

 what in excess of 1,500 meters. 



In consideration of the foregoing, 2,000 decibars has been selected 

 as the reference surface for the construction of a velocity profile 

 along section W which in the June survey was extended southward 

 to latitude 38° N., and probably comes close to completely crossing 

 the eastward ff owing part of the North Atlantic eddy in this longi- 

 tude. This profile is shown in figure 21 . The section shows a volume 

 of ffow of 68.4 million cubic meters per second, a mean temperature 

 of 13.39° C and a resulting heat transport of 915.7 million cubic 

 meter °C per second eastward. While the section did not completely 

 cross the eastward flowing water it seems very nearl}^ to have done 

 so and an estimate has been made that an additional 3.5 million cubic 

 meters per second move eastward just south of the section shown in 

 figure 21. 



The temperature-salinity relationships of the water masses found 

 in the Grand Banks region have been reported upon in earlier bulle- 

 tins of this series. As might be expected, the Labrador Current water 

 and the Atlantic Current water have been found to have the uniformity 

 of T-S relationship characteristic of water m.asses. UsuaUy the mixed 

 water from these water masses has been distinguished by a degree of 

 uniformity which has led to its consideration as a virtual water mass. 

 Surface fluctuations produce departures from the characteristic 



75 



