93 



slope. Incidentally, this admixture of Labrador Current water of 

 negative salinity anomaly, spreatling- horizontally below about 1,200 

 meters as indicated in figure 26, may be a contributing factor hi the 

 establishment and maintenance of the layer of maximum salinity 

 anomaly usually found off the eastern and southern slopes of the 

 Grand Banks in hitermediate depths of about 1000 meters. 



The current maps regularly supplied to the patrol cutters during 

 the ice- patrol season are maps of the dynamic topography of the 

 sea surface relative to the topography at the 1,000-decibar surface. 

 In the practical application and use of these maps it is assumed that 



Fjgure 20.— Salinity anomaly profile southwest of Grand Banks, May 1-2, 1939. 



they are absolute current maps. In other words, the assumption 

 is that the 1,000-decibar surface is geocentric and motionless. Inas- 

 much as there are many practical difficulties in the way of verifying 

 such an assumption by direct current measurements every efiort has 

 been made to examine indirect evidence bearing on the validity of 

 the assumption. In recent years various criteria for a motionless 

 reference surface have been proposed, that of the minimum oxygen 

 layer provoking the most discussion. The selection of the layer of 

 minimum oxygen as the motionless surface breaks down in the vicinity 

 of the so-called polar front because it is in this vicinity that the layer 

 of minimum oxygen reaches the sea surface. As much of the area in 

 which the International Ice Patrol is interested lies to the north of 

 this, the layer of minimum oxygen cannot be used. 



Probably the best indication that an isobaric surface of inter- 

 mediate depth is approximately motionless is the fact that year after 

 year the bergs, acting as drift bottles, have been observed to follow, 



