DYNAMIC TOPOGRAPHY 



The general surface circulation along the east- 

 ern edge of the Grand Banks can be inferred 

 from the mean monthly dynamic tojjography 

 charts of the sea surface for April, May, and 

 June prepared by Soule (1964). Dynamic 

 heights for these charts were computed relative 

 to the 1000 decibar reference surface. 



The April 1971 survey was confined to the 

 eastern region of the Grand Banks between 

 standard section A'2 and special section A3B 

 (fig. 5). Because the major concentration of 

 icebergs during the first survey was located in 

 the area just east of Flemish Cap, a special sec- 

 tion, Si, was established for the April survey 

 between 46°N., 43.5°W. and 49°N., 43.5.°W to 

 investigate geostrophic currents in this region. 

 The dynamic topograjshy from the April survey 

 can be described as a "classical" circulation pat- 

 tern, which can be expected on the Grand Banks 

 during an average month of April. The circula- 

 tion was characterized by the Labrador Current 

 as a southward flowing stream generally confined 

 between the 200 and 2000 meter contours along 

 the eastern edge of the Grand Banks. The maxi- 

 mum geostrophic current speed of the Labrador 

 Current (92.1 cm/s) occurred between stations 

 10860 and 10861 of standard section A3. Maxi- 

 mum speeds across the other sections were be- 

 tween 37.8 and 57.9 cm/s. These current speeds 

 are typical of values from previous April sur- 

 veys. To the east of the Labrador Current was 

 a region of water having low specific volume as 

 the result of mixing of Labrador Current and 

 North Atlantic Current waters. This dynamic 

 trough is a region of current reversal with the 

 Labrador Current flowing south along the west- 

 ern edge of the trough and the North Atlantic 

 Current moving northeast along the eastern side. 



Except for an edge of the North Atlantic Cur- 

 rent moving eastward along the southern end of 

 special section Si, there was very little change 

 in dynamic height over the section. What geo- 

 strophic current that was present was flowing 

 toward the west, apparently contrary to the gen- 

 eral drift of the icebergs east of Flemish Cap. 



The dynamic topography contour chart devel- 

 oped from the May 1971 multiship survey (fig 6) 

 showed several significant differences from the 

 April survey. While the Labrador Current is 

 still well defined along the eastern slope of the 

 Grand Banks, geostrophic current speeds are 

 substantially less than those observed during 

 April. The maximum current speed of 61.2 

 cm/s occurred between station 10965 and 10966 

 of special station A2C. Maximum current speeds 

 across the other sections ranged from 9.6 to 44.6 

 cm/s. North of section A2B, the Labrador Cur- 

 rent was generally weak, with maximum current 

 speeds averaging about 21.3 cm/s. Near the 

 western end of section A2B, there were indications 

 of a strong current flowing off the Grand Banks 

 and joining the main stream of the Labrador 

 Current. The resulting average maximum values 

 of the Labrador Current south of section A2B 

 had increased to 42.8 cm/s. 



The bottom topography of the Grand Banks 

 region has a strong effect on the directions of 

 currents in the area. Normally, Flemish Cap 

 acts as a boundary forcing the North Atlantic 

 Current to bend eastward well south of it. Dur- 

 ing ]May it appeared that the location of the 

 current had shifted almost 100 miles north and 

 was flowing directly over the top of Flemish Cap. 

 The North Atlantic Current could also be seen 

 intruding deeply into the normal trough region 

 toward the continental shelf near 45°N., 47.5°W. 

 and 46°N., 46.5°W. 



