STANDARD SECTION SURVEYS ON 

 THE GRAND BANKS 



The general surface circulation south and 

 east of the Grand Banks of Newfoundland dur- 

 ing the spring can be inferred from the monthly 

 average dynamic topography of the sea surface 

 relative to the 1000-decibar surface during the 

 months of April, May, and June (figs. 1, 2, and 

 3; Soule, 1964). These charts are based on 22 

 years of data with equal weight being given 

 to each year represented. Broken-line contours 

 have been used in areas where the data base is 

 6 years or less. The charts show the Labrador 

 Current as a southward flow positioned between 

 the 100- fathom and 1000-fathom contours 

 on the eastern slope of the Grand Banks. To 

 the east of the Labrador Current is a dynamic 

 trough region of water of low specific volume 

 resulting from the mixing of Labrador Current 

 water and North Atlantic Current water. The 

 dynamic trough is a region of current reversal, 

 with the Labrador Current flowing south on the 

 western side of the trough and the North At- 

 lantic Current flowing north on the eastern 

 side of the trough. Along the southern and 

 southeastern edges of the contoured region, an 

 intrusion of the North Atlantic Current is 

 evidenced by a pair of meanders at about 41 °N 

 50°W and 43°N 46°W. 



During the 1968 Ice Patrol season, the prac- 

 tice of serially reoccupying three standai-d sec- 

 tions (figs. 4-11) rather than conducting a 

 broad areal survey was continued. This practice 

 was instituted in 1966 (Wolford, 1969) because 

 experience had shown that large, short-term 

 (1-2 weeks) fluctuations occurred in the vol- 

 ume flow of the Labrador Current, and there- 

 fore broad areal surveys covering a time span 

 comparable to the period of the short-term 

 fluctuations would not be suflficiently synoptic. 

 It was hoped that rapid occupations of the three 

 standard sections would provide synoptic in- 

 formation which would be adequate, when used 

 with the average monthly surface dynamic 

 topographies, to draw conclusions about the 

 circulation in the Grand Banks region. While 

 the rapid occupation of three standard sections 

 improved synopticity, the increased distance 

 between sections inherent in this method made 

 contouring the dynamic topography more un- 

 certain. Accordingly, the charts of dynamic 

 topography (figs. 4-11) have been contoured 

 between standard sections only when additional 



stations were occupied between sections A3 and 

 A4 (three times during the season) to obtain 

 better definition of a meander in the North 

 Atlantic Current. 



During the first survey of the season, the 

 meander of the North Atlantic Current was 

 farther to the northwest than usual. This was 

 readily apparent from comparison of the posi- 

 tion of the 971.1 dynamic-meter contour as ob- 

 served during the 2-10 April survey (fig. 4) 

 with the position of this contour on the April 

 normal dynamic topography chart (fig. 1). Ad- 

 ditional evidence of this excursion of the 

 meander to the northwest exists in the high 

 dynamic heights found immediately east of 

 the trough during the first occupation of A3 

 (fig. 34) and the incursion of water warmer 

 than 12°C and saltier than 35.5°/oo which ex- 

 tended westward in the upper 200 meters to 

 about 48°30'W (fig. 13). During the month of 

 April, the meander, moved seaward from the 

 Grand Banks to a more offshore position where 

 it remained the rest of the Ice Patrol season. 



In April a portion of the Labrador Current 

 (temperature <0°C) was observed to flow 

 westward around the Tail-of-the-Banks during 

 every occupation of A4 (figs. 14, 17, and 20). 

 No flow of Labrador Current Water around 

 the Tail-of-the-Banks was detected during the 

 remainder of the Ice Patrol season (May- 

 June) . 



During four of the occupations of A3 (figs. 

 13, 16, 19, 27), horizontal gradients of tem- 

 perature sufficiently strong to be considered 

 frontal were observed at or near the sea 

 surface. Whenever such strong horizontal tem- 

 perature gradients were encountered, the low- 

 est value in dynamic height was associated 

 with the station nearest the front on the cold 

 side. (figs. 34 and 35). 



TIME-VARIATION OF THE LABRADOR 

 CURRENT 



The time-variation of the Labrador Current 

 has been a subject of considerable interest to 

 the International Ice Patrol because this cur- 

 rent is the chief agency for transporting ice- 

 bergs to the Grand Banks. Because most of the 

 oceanographic survey eff'ort of the Ice Patrol 

 has been expended on the eastern slopes of the 

 Grand Banks during April through June, the 

 months when the Iceberg hazard on the Grand 

 Banks is greatest, the annual cycle of the 



