CONCURRENT STANDARD SECTION TIME SERIES 



During the April 1965 Ice Patrol cruise, it was 

 observed that large changes in the volume trans- 

 port of the Labrador Current occurred during the 

 nine days separating successive occupations of 

 section IT (approximately the same as the present 

 standard section A3) (Kollmeyer et al., 1966). 

 Data from three occupations of standard section 

 A3 in 1966, where reoccupations occurred within 

 36 hours, showed volume transport variations 

 that were highly irregular (Wolford, 1966). 

 Large variations in the volume transport over 

 short time periods at standard section A3 were 

 also observed in 1967 (Morgan, 1969), 1968 

 (Andersen and Moynihan, 1971), 1969 (Andersen 

 and Moynihan, 1971a), and in 1970 (Ettle and 

 Wolford, 1973). 



Earlier efforts, such as those reported by Smith 

 (1931) and Soule and Challender (1949), at- 

 tempted to relate changes in transport of icebergs 

 to changes in surface atmospheric pressure using 

 monthly mean pressure maps. Based on the 

 rapid variation in current volume across section 

 U over a ten day period in 1965, Day (1966) 

 tried to show that short term meteorological 

 effects immediately prior to section occupation 

 might have a significant effect on volume trans- 

 port. For 90 measurements of volume transport 

 of the Labrador Current made between 19.50 and 

 1965, it was evident that, in many cases, local 

 winds had a strong effect on the volume trans- 

 port. The mechanism of wind-induced set-up 

 along the Newfoundland coast had a direct and 

 nearly immediate effect on the flow through 

 down-stream sections. There were, however, too 

 many cases where Day's theory did not appear 

 to be valid. 



An attempt was made in 1970 to show that 

 intensification of the Labrador Current occurred 

 when local winds induced a mass transport of 

 surface waters toward the eastern continental 

 slope of the Grand Banks at standard section A3 

 (Ettle and Wolford, 1973). The piling of water 

 along the shelf break produced downwelling, 

 resulting in a strong intensification of the Lab- 



rador Current. Ettle and Wolford also showed 

 a possible second example of downwelling based 

 on data from the 13-14 June 1964 occupation of 

 standard section A3. 



To test the hypothesis that localized surface 

 winds along the continental slope may produce 

 upwelling and downwelling resulting in rapid 

 changes in volume transport, a concurrent time 

 series of observations were conducted at standard 

 sections A2 and A3 during May 1971. 



The continental shelf break occurs at a depth 

 of about 100 meters at both standard section A2 

 and A3, with the bottom dropping off rapidly 

 to depths in excess of 1000 meters to effectively 

 form a boundary for the ocean at the western 

 end of both sections. Each section was occupied 

 nearly concurrently on three occasions. A2 by 

 EVERGREEN and A3 by ROCKAWAY. The 

 sections were occupied at three to four day inter- 

 vals between 20 and 27 May 1971. Observed 

 weather conditions near both standard sections 

 between the first and second occupations showed 

 a mean wind from the south at approximately 

 5 knots at A2. Mean winds at A3, 130 miles 

 south of A2, were from 145° true at 18 knots. 



The work of Ekman (1905) provides a basis 

 for understanding the effect of wind stress on 

 ocean circulation. Due to the effect of the earth's 

 rotation and frictional forces, the wind-induced 

 transport is 90 degrees to the right of the wind. 

 Thus at both sections between 20 and 24 May 

 1971, the net movement of the surface water 

 layers was offshore, nearly parallel to the sec- 

 tions. However, the effect of the wind at A3 

 was much greater as a result of the higher mean 

 wind. An approximate value of wind stress on 

 the sea surface can be calculated from the 

 formula : _ 



T = CdPa|Ua|Ua 



where: T=Wind stress (dynes/cm^) 



Pa = Density of the air (gm/cm=) 

 Ua = Wind velocity (cm/s) 

 Cd=Dimensionless drag coefficient 



