ABSTRACT 



Three Ice Patrol oiuises from April to July 1973 were conducted to the 

 Grand Banks to determine sea surface currents for input to a computer model 

 for the prediction of iceberjr drift. During a three week periotl in .June-July 

 three Intensive Dynamic Surveys were performed in an approximately 1° 

 latitude square within the Tee Patrol survey area to provide data sets to test a 

 developmental numerical model for predictinjr dynamic ciian<res of the Labra- 

 dor Current on the eastern slope of the Grand Banks. Analysis of the sea sur- 

 face {leostrophy and the water mass properties revealed that the Xorth Atlantic 

 Current was to be found approximately 74 km farther north than its usual 

 position and by impinpinir on tlie Tail-of-the-Bank forced a portion of tiie 

 Labrador Current up onto the Bank. The May cruise data disclosed the for- 

 mation of a large cyclonic meander in the North Atlantic Current, which was 

 believed to produce the cyclonic rinjr found to the south in the warm water 

 gradient of the North Atlantic Current. This may be a mechanism for trans- 

 porting icebergs across the North Atlantic Current and would account for 

 the occasional sightings of icebergs southeast of the warm current. Near 

 bottom direct current measurements made on the eastern slope of the Grand 

 Banks had mean speeds of 2.0 cm S"^ in April-May and 5.3 cm s"' in May-June, 

 and the direction was mainly southwest. The only identifiable periodic com- 

 ponent was the lunar semidiurnal tide which was intermittently tletected. 

 Comparison of the direct current measurements with the geostrophic deter- 

 mination of the current at the current meter depth indicates that the geo- 

 strophic method imderestimates the current by 10% to 30% most probably 

 attributable to some residual baroclinic flow at the assumed "level of no 

 motion" (1000 decibars). Geostrophic current and volume transport analysis 

 of the Intensive Dynamic Surveys demonstrated that the present method for 

 computing dynamic heights in water shallower than the reference level, bj' 

 extending temperature and salinity contours horiztmtally into the slope, may 

 introduce an erroneously large cross slope flow by carrying the slope of the 

 between sections isosteric surfaces into tiic Bank where intuition dictates that 

 these surfaces be parallel to a zero geopotential surface. A method for esti- 

 mating and correcting this error in the operational use of geostrophic cur- 

 rents is given. A detailed comparison of the vohune transports from the 

 Intensive Dynamic Surveys revealed a period wlien the transport of the 

 Labrador Current and North Atlantic Current increased simultaneously, 

 forcing a portion of the Labrador Current up onto the Grand Bank. The 

 progression of the crest of a North Atlantic Current meander through the 

 Dynamic Survey area is believed to be the source of the observed variations 

 in volume transports. 



Editor's Note: Referenco to a product or eonimetit with respect to it in this publica- 

 tion does not indicate, or ])frmit any person to hold out l).v repulilication in wliole, or in 

 part or otherwise, that tlie product lias lieen endorsed, authorized, or approved l)y the 

 Coast Guard. 



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