general decrease in density of the entire water 

 column over the eastern slopes of the Grand 

 Banks resulting from a general freshening due 

 to increased transport in the cold core of the 

 Labrador Current and mixing of the water of 

 the cold core with surrounding waters. During 

 the 20 June occupation of the station im- 

 mediately west of the trough (station 271), the 

 cold core was most pronounced (<1.0°C, 

 < 33.55°/ oo between 30 and 90 meters) and 

 the water column was generally fresher down 

 to about 350 meters than during the 24 May 

 occupation (fig. 42). Also, from 18 May 

 through 20 June a gradual increase in the dy- 

 namic height of the trough station was found 

 (fig. 37). This may be attributed to a decrease 

 in density of the water column brought about 

 by lateral mixing of the cold core or to a re- 

 circulation to the northeast of less dense slope 

 water (fig. 3). 



The dynamic heights of the trough station 

 and adjacent station during the 26 June oc- 

 cupation of A3 were substantially less than 

 previous values (fig. 37). The decline was 

 greatest for the adjacent station, resulting 

 in a decrease in cross-stream gradient of dy- 

 namic height and a sharp reduction in the 

 transport of the cold core and the total trans- 

 port of the Labrador Current (fig. 43). This 

 decrease in dynamic height between the 20 

 and 26 June occupations of the adjacent station 

 (stations 271 and 315) was primarily due to 

 an increase in the density of the water below 

 100 meters, resulting from higher salinity in 

 the water column (fig. 41). Once this influx of 

 more saline water began, the drop in surface 

 dynamic heights was accelerated by a reduction 

 in the transport of the cold core, causing a de- 

 crease in the creation of lighter surface waters 

 through mixing with the cold core, and a 

 further decrease in the surface dynamic height. 



It appears that the variation in the trans- 

 port of the Labrador Current observed during 

 May and June 1968 was caused by a sequence 

 of events beginning with a change in the 

 density of the water column below 200 meters 

 west of the dynamic trough. During May a 

 decrease in the density below 200 meters at 

 the adjacent station increased the surface dy- 

 namic height and the transport of the less 

 saline, negative-temperature water in the up- 

 per 100 meters, causing an increase in the rate 

 of mixing and rate of formation of lighter 



water in the surface layers. This increase in 

 the rate of formation of lighter surface water 

 west of the dynamic trough then reinforced 

 the increase in surface dynamic height 

 initiated by the decrease in the density of the 

 water column below 200 meters, further 

 strengthening the transport of the Labrador 

 Current. Morgan (1969) attributed an in- 

 tensification of the Labrador Current along the 

 eastern slopes of the Grand Banks in May 

 1967 to unusually low density water west of the 

 dynamic trough below 264 meters. Kollmeyer 

 et al. (1966) recognized the influence of the 

 advection of cold water from the north upon 

 dynamic heights on the Grand Banks, but their 

 hypothesis that accumulated arctic and sub- 

 arctic meltwater was the direct cause of an 

 initial increase in dynamic height on the Grand 

 Banks does not apply to the sudden increase 

 in dynamic height found west of the trough in 

 May 1968. In June 1968 an increase in the 

 salinity of the water column below 100 meters 

 west of the dynamic trough resulted in a de- 

 crease in the cross-stream gradient of dynamic 

 height. The accompanying deceleration of the 

 Labrador Current reduced the rate of forma- 

 tion of lighter water in the surface layers, 

 resulting in a further decrease in the cross- 

 stream gradient of dynamic height and the 

 transport of the current. 



While it must be emphasized that the 

 oceanographical and meterological conditions 

 in the Grand Banks region are so variable that 

 a hypothesis adequately explaining the ob- 

 served conditions during one Ice Patrol season 

 may be inadequate for other seasons, neverthe- 

 less the results of the 1968 Ice Patrol suggest 

 that increased attention should be given to 

 variation in the water characteristics in the 

 deeper layers of the water column. The varia- 

 tion in transport of the upper 200 meters of 

 the Labrador Current appears to be a mani- 

 festation of deep, underlying changes in the 

 water column. 



LABRADOR SEA SECTION 



The Labrador Current showed its charac- 

 teristic temperature distribution on the section 

 from South Wolf Island, Labrador to Cape 

 Fai-ewell, Greenland with a negative tempera- 

 ture core over the Labrador Shelf, a steep 

 horizontal temperature gradient over the con- 

 tinental slope and relatively warmer water 



