PHYSICAL OCEANOGRAPHY OP THE GULF OP MAINE 771 



high surface saUnity (presumably about 32.5 per mille) is also to be expected over 

 the shoal part of the bank and near its northern edge. 



Very considerable fluctuations are to be expected in the salinity of the surface 

 along the edge of the continent abreast of the Gulf of Maine, as well as in its tem- 

 perature (p. 596), as the oceanic water of high salinity approaches the banks or 

 recedes from them. 



In the southwestern part of the area, in the offing of Marthas Vineyard, the 

 data for July, 1916, August, 1914, and for autumn (p. 801) make it reasonably certain 

 that surface water as saline as 33 per mille normally drifts in over the outer part of 

 the shelf during July and the first three weeks of August, but seldom (perhaps never) 

 approaches much nearer the shore than is represented on the chart for 1914 

 (fig. 136). 



Farther to the east the isohaline for 33 per mille may be expected to skirt the 

 southern edge of Georges Bank in July, lying a few miles farther in in some summers, 

 farther out in others, and crossing the oceanic triangle between Georges and Browns 

 Bank, but not, in our experience, encroaching at all over the latter. Still farther 

 eastward surface water as saline as 33 per mille overflows the edge of the continent 

 in July or August of some years, as in 1915, when Bjerkan (1919) had still higher 

 readings (34.27 per mille) at the 400-meter contour in the offing of Cape Sable on 

 July 22. In 1914, however, the surface water near by was only 31.22 per mille a 

 week later in the season (station 10233), though the difference in date would suggest 

 a difference in salinity of just the reverse order, evidence of considerable fluctuation 

 in this respect from summer to summer. 



It is doubtful whether sixrface water as salt as 34 per mille ever encroaches on 

 the edge of the continent abreast of the Gulf of Maine ; certainly we have no record 

 of such an event at any season, but the surface charts for the winter, spring, and 

 summer (figs. 93, 127, and 136) show that it is to be expected only a few miles out from 

 the 200-meter contour south of Marthas Vineyard and off the western end of Geor- 

 ges Bank by the first half of July in early seasons, but perhaps not until August in 

 late seasons. In some summers, as in 1914, water of this high salinity lies farther 

 out from the edge of the continent to the eastward. In other summers, however, 

 it evidently spreads shoreward over the slope off Shelburne as early in the season 

 as it does farther west — witness the records obtained by the Canadian Fisheries 

 Expedition in 1915, mentioned above (Bjerkan, 1919; Acadia station 41). 



None of our lines have run far enough out, abreast the gulf, to reach surface 

 water of full oceanic salinity (35 per mille and upwards) ; nor is it known how far 

 out from the edge of the continent water of 34 per miUe withdraws in winter and 

 spring. 



ANNUAL VARIATIONS IN SURFACE SALINITY IN SUMMER 



Passing reference has been made in the preceding pages to the variations that 

 have been observed in the salinity of the surface from summer to summer. The 

 most interesting fluctuation of this sort that has come to our attention is that surface 

 values averaged much lower in the southwestern part of the region in July, 1916, 

 than in that same month in 1912, 1914, or 1915; the surface of Massachussetts Bay, 

 for instance, was about 1 per mille less saline on July 19 to 20, 1916, than at about 



