PHYSICAL OCEANOGRAPHY OF THE GULP OP MAINE 



959 



The dynamic state is not so clear for the southwestern part of the banks area 

 in summer, where the rise in temperature during the time interval between the two 

 cruises of 1914 (July 20 to 21; August 25 to 26) may have been more than counter- 

 balanced by some encroachment of water of high salinity inward over the shelf. 

 Consequently, the dynamic values for the offing of Marthas Vineyard for that Au- 

 gust are not directly comparable with those taken farther east during the month pre- 

 ceding. However, no gradient is suggested sufficient to account for the repeated 

 drifts of bottles westward around Nantucket Shoals from the vicinity of Cape Cod. 



The dynamic relationship along the continental slope in the offing of Marthas 

 Vineyard and eastward about to longitude 68° for July and August, 1914 (fig. 203), 

 recalls the March state (p. 939; fig. 188) so closely that a low or dynamic trough, 

 with the gradient rising to seaward as well as shoreward, may be taken as typical of 



Fig. 204.— Dynamic gradient along tlie continental slope, bottom to 100 decibars, July to August, 1914. Contours for every 



dynamic centimeter 



this belt. Its circulatory implication has already been discussed (p. 939). At the 

 date of our August profile for 1914 the calculated velocity of the easterly or "Gulf 

 Steam" drift along the offshore edge of this low, and relative to the latter, was at 

 least half a knot off Marthas Vineyard, or about the same as in March, 1920 (p. 939),*^ 

 which corresponds very well with the average velocities reported ra this sector of the 

 so-called "inner edge of the Gulf Stream" by passing ships ta summer. 



The dynamic contours at 100 decibars for that July and August (fig. 204) show 

 the easterly set actually washing the continental slope to the west of longitude 68° 

 then swinging offshore. We have here a ready explanation for the fact that 

 water of high temperature and high salinity — the "warm zone" — usually bathes the 

 slope along this western section but is separated from the slope farther eastward by 

 the colder counter drift out of the Eastern Channel. 



" For ttie reasons stated above (p. 939), the calculation of velocity in this region can be taken only as a rough approximation. 



