83 



to the 4-year average. Such a boundary line could not be drawn 

 satisfactorily for the post-season cruise as it was evidently outside 

 of the area surveyed except for a few stations at about latitude 44° N., 

 near the eastern edge of the survey. This may be looked upon as 

 indicating either an error in standardization of salinities, or a break- 

 down of the criterion of the boundary, or the presence of an abnormal 

 quantity of mixed water which derived its negative anomaly from the 

 Labrador Current. That the last possibility is probable is supported 

 by other evidence discussed in connection with the post-season cruise 

 results. 



POST SEASON CRUISE. 1937 



As was mentioned previously in this report a post-season cruise 

 was made in the boundary region between the Labrador and Atlantic 

 Currents and was laid out with a view toward the occupation of 

 several sections which would cross the major portion of these streams 

 and their common boundary. It was also anticipated that, at least 

 in a portion of the area, the sections would be spaced sufficiently close 

 together so that a current map showing the main features of circula- 

 tion might be derived from the results of the survey. 



Prior to the construction of any maps the data were studied for the 

 selection of a reference surface. The horizontal distribution of anom- 

 aly of specific volume was examined by means of the construction of 

 horizontal sections for several of the deeper levels. Inspection of 

 these sections indicated that the smallest horizontal gradients of 

 anomaly of specific volume, and therefore probably the most nearly 

 motionless level, occurred at 2,000 or 2,500 meters depth. Using this 

 criterion the 2,000-decibar surface was selected for reference in the 

 computation of dynamic heights. 



Figure 40 shows the surface current situation found to exist during 

 this survey. Because of the previously mentioned disposition of 

 stations in the form of sections rather than in a close network, there 

 was a greater than usual uncertainty regarding the exact course of the 

 dynamic isobaths in the southeastern portion of the area surveyed. 

 Consequently the isobaths have been shown as broken lines in the 

 regions of greatest uncertainty. After careful consideration a tenta- 

 tive pattern of isobaths was selected and tested, where possible, by 

 other criteria than density distribution. Thus the map is in accord 

 with the direction of drift indicated by the wire angle at stations 

 where light wind or calm weather prevailed, and with the set indicated 

 by astronomical observations on runs between stations. The depth 

 of the 10° isotherm, which Iselin ^ has found so convenient an indi- 

 cator of the circulation in the western North Atlantic, was examined 



• Iselin, C. O'D. A Study of the Circulation of the Western North Atlantic. Papers in Phys. Oceanoe. 

 and Met. M. I. T. and W. H. O. I., pp. 69-72, Vol. IV, No. 4, Cambridge, Mass., 1936. 



