Labrador Current is poorly defined. The sea- 

 sonal characteristics of the Labrador Current 

 are further obscured by its interaction with the 

 North Atlantic Current and with the region's 

 bathymetry. 



With these difficulties in mind, the data ac- 

 quired during the 1968 Ice Patrol were ex- 

 amined for time-variation in the flow of the 

 Labrador Current. For the purposes of this 

 report, the Labrador Current is defined to be 

 the total southward flow through the east-west 

 leg of Section A2, the total southward flow 

 west of the dynamic trough through Section 

 A3, and the total westward flow north of the 

 North Atlantic Current through Section A4 

 (fig. 4). Particular attention was focused on 

 Section A3 because this section transects the 

 Labrador Current, the dynamic trough east of 

 the Labrador Current, and the North Atlantic 

 Current. In addition, the oceanographic condi- 

 tions along this section give an indication of 

 the probability of icebergs reaching the 

 shipping lanes near the Tail-of-the-Banks. 



To determine whether variation in the 

 Labrador Current during the 1968 season was 

 due primarily to changes in dynamic height 

 on the Grand Banks, changes in the dynamic 

 height in the trough between the Labrador 

 Current and the North Atlantic Current, or a 

 combination of these factors, profiles of sea- 

 surface dynamic height along Section A3 (figs. 

 34-36) were plotted. In addition, dynamic 

 heights were plotted versus time for the station 

 at the eastern extremity of the steep gradient 

 of dynamic height above the slope of the Grand 

 Banks (the trough station), the next station 

 immediately to the west (the adjacent station), 

 and the westernmost station occupied during 

 each transit of A3 (fig. 37). It was recognized 

 that relative distances between these stations 

 varied from one occupation of Section A3 to 

 the next, but this spatial variation was not 

 sufficient to bias the observed trend of dynamic 

 heights. 



Variation in the transport of the Labrador 

 Current during the 1968 Ice Patrol season was 

 due to changes in the dynamic heights of both 

 the trough station and the adjacent station. 

 The great increase in transport observed be- 

 ginning 24-25 May and the sudden decrease in 

 transport observed during 25-26 June (fig. 43) 

 were due primarily to larger changes in dy- 



namic height at the adjacent station than at 

 the trough station. 



During April both the trough station and the 

 adjacent station undenvent an increase in sur- 

 face dynamic height that caused only a slight 

 change in the gradient of surface dynamic 

 height between the two stations, but in May, 

 the relative difference in dynamic height be- 

 tween the two stations increased sharply. The 

 survey of A3 during 18-19 May disclosed a 

 minimum surface dynamic height for both the 

 trough station and the adjacent station. By 

 the 24-25 May occupation of A3, the surface 

 dynamic height of the adjacent station had 

 increased dramatically, and the greatest dif- 

 ference, during the season, in dynamic height 

 between the trough station and the adjacent 

 station was encountered. The surface dynamic 

 height of the adjacent station then remained 

 at about the same level while the surface dy- 

 namic height of the trough station showed a 

 gradual increase through the 20 June occupa- 

 tion of A3, after which the dynamic heights 

 of both stations fell precipitously to the lowest 

 observed levels of the season (25-26 June 

 occupation of A3). 



Level for level, the trough stations possessed 

 water slightly wanner and slightly saltier (fig. 

 38) than the stations west of them. The dy- 

 namic heights of the trough stations and the 

 adjacent stations were observed to diverge 

 from the i-eference level (1000 meters) to the 

 sea surface (fig. 39), with the divergence being 

 especially large from 24 May until 20 June 

 (stations 224 and 225, 247 and 248, and 271 

 and 272). The rate of divergence shows a 

 significant increase between 300 meters and 

 the surface which is attributed to the presence 

 of Labrador Current Water west of the dy- 

 namic trough. Below 300 meters the more 

 gradual divergence of the dynamic heights of 

 the trough stations and the adjacent stations 

 is an indication of the effective mixing of 

 Labrador Current Water and North Atlantic 

 Current Water occurring on the eastern slopes 

 of the Grand Banks. This is not unexpected in 

 view of the great horizontal and vertical shears 

 in current velocity observed in this region. 



To determine whether the Labrador Current 

 in the Grand Banks area has the charac- 

 teristics of a spring freshet, variation along 

 section A3 of temperature-salinity charac- 

 teristics and cross-stream dynamic height were 



4 



