94 



in the main, directions and rates of drift agreeing with the dynamic 

 topographic charts. The cases of discrepancy, some with regard to 

 rate and some with regard to direction, point to the possibiUty of 

 improving the current maps if our knowledge of the shape and location 

 of the motionless surface can be increased sufficiently. 



In 1935 the post-season cruise in the southern part of the Labrador 

 Sea included subsurface measurements from surface to bottom at all 

 stations. On the assumption that the isobaric surface in which the 

 velocity was most nearly uniform was also the surface in which the 

 velocity was most nearly zero, an investigation of the horizontal 

 gradients of anomaly of specific volume indicated that in this region 

 the most nearly motionless isobaric surface lay at a depth of about 

 2,000 meters.^ With 2,000 decibars as the reference surface, deep- 

 level dynamic topographic charts were constructed and proved to be 

 in agreement with the circulation of the deep and bottom water 

 deduced from charts of the bottom temperature made from this same 

 survey. 



Further evidence regarding the location and shape of the motion- 

 less surface in an adjacent area to the south, east of the Grand Banks, 

 developed from the 1937 post-season cruise in this vicinity. The 

 horizontal distribution of anomaly of specific volume, deep-level 

 dynamic topographic charts, a volume transport balance above and 

 below the assumed level of the motionless surface, and temperature- 

 salinity correlations combined with the phenomenon of cabbelmg, 

 all agreed in pointing to the existance of a most nearly motionless 

 surface at about 2,000 meters.* 



Because it was indicated, on the assumption that the 2,000-decibar 

 surface was the most nearly motionless, that in the 1937 survey the 

 most rapid motion of the deep water occurred in the 3,500-decibar 

 surface, the dynamic topography of the latter surface referred to the 

 2,000-decibar surface has been shown in figure 27 in comparison with 

 a chart showing the distribution of potential temperature in the 

 isentropic surface of at^=27.90. This potential density surface was 

 selected for isentropic analysis because its depth is approximately the 

 same as that of the 3,500-decibar surface. The axis of minimum 

 temperature is the most strikhig feature of figure 27 and has been 

 shown as a heavy broken line. It indicates the flow of the cold deep 

 water from the Labrador Sea southward beneath the Atlantic Cur- 

 rent. This is the same general picture of the circulation of the deep 

 water that is given by the topographic chart at the left. The simi- 

 larity of the two parts of figure 27 is considered to be a strong con- 

 firmation of the approximation to zero velocity at the 2,000-decibar 



' Soule, Floyd M. International Ice Observation and Ice Patrol Service in the North Atlantic Ocean- 

 Season of 1936. Oceanography, pp. 46-50, U. S. Coast Guard Bull. No. 26. Washington, 1938. 



* Soule, Floyd M. International Ice Observation and Ice Patrol Service in the North Atlantic Ocean- 

 Season of 1937. Oceanography, pp. 83-92, U. S. Coast Guard Bull. No. 27. Washington, 1938. . 



