130 MARION AND GENERAL GREENE EXPEDITIONS 



The development and position of these vortices in the mixing zone 

 continually varies, but they may be easily recognized on many of 

 the dynamic topographic maps (figs. 102 to 121), and are also 

 reflected in the drifts of the icebergs. 



As the position of the two principal currents continually change 

 according to the described system (fig. 126), so also does the surface 

 velocity vary. The slope band of the Labrador Current over the 

 200-meter contour along the eastern slope of the Grand Banks often 

 may become constricted to a width of 6 miles when velocities in 

 the axis have attained 110 centimeters per second. In bands of the 

 Gulf Stream and the Atlantic Current, only the borders of which 

 lie within the region of the Grand Banks sector, velocities more 

 regularl}^ reach 80 to 100 centimeters per second. The Labrador 

 Current apparently is subject to greater fluctuations in the Grand 

 Banks sector than farther north or than the Atlantic Current. The 

 average surface velocity of the Labrador Current is estimated to 

 differ little from that found farther north in the American sector, 

 8.2 miles per day (18 centimeters per second). 



A departure in the course of the surface currents from that de- 

 scribed above, and which has particular significance for the Inter- 

 national Ice Patrol, has been indicated during the period 1900-30 

 by the phenomenal drifts of icebergs in the western North Atlantic 

 Ocean. (See Smith, 1931, pp. 160-166.) Such rare drifts appear 

 to originate between longitudes 49° and 46°-50' in the Grand Banks 

 sector, and thence proceed southerly and sometimes finally westerly. 

 But if this track be plotted (fig. 93) it does not coincide with the 

 streamlines of the Atlantic Current, south of the Grand Banks, nor 

 with the southern branch of the Atlantic Current which is commonly 

 believed to follow the trend of the 4,000-meter isobath (fig. 93) 

 southeasterly to about latitude 38° longitude 43° between which 

 position and the mid-Atlantic Ridge the current turns southwesterly. 



The Michael Sars observations, on the other hand, stations 64 to 

 70, June 24-30, 1910 (fig. 93), clearly indicate a southerly direction to 

 the Atlantic Current south of the Grand Banks. The current was 

 easterly between stations 70 and 68 with a volume of 40.3 million 

 cubic meters per second, but between stations 68 and 64 it had 

 westerly direction and a net volume of approximately 2 million 

 cubic meters per second. Reference to the respective dynamic 

 heights of the latter pair of stations shows that from the surface 

 to a depth of about 550 meters the Atlantic Current ran westerly 

 but below that easterly. It probably closely paralleled the plane 

 of the stations. 



Reference to the back of the United States Hydrographic Office 

 Pilot Chart of the North Atlantic Ocean for the month of July 1935 

 indicates the general course of the southward branch of the Atlantic 

 Current on tlie sea surface, and the general trend and bounds have 

 been plotted on our figure 93. It will be seen that southwesterly 

 surface currents often prevail as far west as latitude 35° longitude 

 60°, the center of the great Atlantic eddy apparently lying north- 

 west of this position. The remains of an iceberg from the Grand 

 Banks was sighted by the steamer Baxtergate (and the report veri- 

 fied) June 5, 1926, latitude 30°-20', longitude 62°-32', near Bermuda. 



The foregoing strongly suggests, therefore, that portions of the 

 Atlantic Current to depths as great as 500 meters sometimes turn 



