PHYSICAL OCEANOGEAPHY OF THE GULF OF MAINE 915 



ature (11.1°) at the surface, simply reflected active vertical mixing by tidal currents. 

 Any tendency for the water to move from west to east over Georges Bank would 

 necessarily be diverted by the considerable area shallower than 40 meters in which 

 the bank culminates. *' According to the rule general in the Northern Hemisphere, 

 this shoal might be expected to act as the vortex for a clockwise circulatory move- 

 ment, and the fact that the 40-meter salinity was somewhat lower on the eastern 

 side of the bank than on the western side at the time, with the transition from values 

 lower than 32 per mille to higher than 34.5 per mille most abrupt off its southeastern 

 slope, is evidence of such a drift eddying eastward and southward around the shoal 

 area. 



The dominant cu-culation of the gulf is most clearly reflected in salinity at the 

 time of year (spring and summer) when the regional variations in this respect are 

 widest. 



The progressive equalization of salinity that takes place during the autumn 

 (p. 799) makes it increasingly difficult to reconstruct the horizontal circulation, even 

 in its broadest aspects. In the midwinter of 1920-21 salinity yielded no definite 

 evidence of any indraft into the eastern side of the gulf, either at the surface or at 

 40 meters (p. 804). It is unfortunate that observations could not be taken off Cape 

 Sable during this midwinter cruise, for without such it is impossible to state whether 

 the low values (31.2 to 31.3 per mille) recorded near Yarmouth, Nova Scotia, on 

 January 4 (station 10501) reflected any movement of water past the cape from the 

 eastward or were simply the product of local drainage from the land. However, 

 it is certain that stiU lower salinity at the surface a few miles south of the Merrimac 

 River, across the gulf, a few days earlier (30.02 per mille at station 10492) had the 

 latter origin, and the rather abrupt transition appearing in both sides of the gulf on 

 the surface chart (fig. 163) between water of low salinity (<31.5 per mille) close in 

 to the land and considerably higher values (32.5 per mille) a few miles out at sea 

 is definite proof that this coast belt was (or had been) drifting parallel to the shore 

 line (if at all), not spreading inshore or offshore in either side of the gulf. However, 

 the fact that the surface belt less saline than 32.3 per miUe was much broader 

 abreast of Penobscot Bay than in the ofling of Casco Bay, on the one side, or off 

 Mount Desert, on the other, points to some slight tendency for the water to drift 

 out from the coast off the former, such as appears more definitely in the summer 

 isohalines for 1912 (p. 770). Some such eddying movement is also indicated by the 

 undulatory course of the isohalines off the mouth of the Bay of Fundy, suggesting a 

 movement of water of low salinity out of its northern side toward the southwest, 

 but no observations were taken close enough to the Nova Scotian side of the bay to 

 develop the inward drift to be expected there. 



The data for deeper levels were not distributed generally enough over the gulf 

 during the midwinter cruise for safe interpretation in terms of dominant drift. 



In early spring, when the discharges from the rivers increase, the courses of the 

 isohalines become much more instructive with respect to the dominant drift, because 

 they give a trustworthy clue to the lines of dispersal of the fresh water from the land. 

 One of the most interesting phenomena in the hydrographic cycle of the gulf is the 



3 Minimum depth about 6 meters. 



