PHYSICAL OCEANOGEAPHY OF THE GULF OF MAINE 849 



evidences of the general tendency of the slope water to move westward from the 

 Nova Scotian slope. 



The slope water, moving westward, is forced against Browns Bank by the earth's 

 rotation. Consequently, with the Eastern Channel offering an open route for it to 

 the right, it is reasonable to think of a screwing motion as taking place into the 

 latter around the southerly and southwesterly slopes of Browns Bank so long as the 

 propulsive dynamic force resulting from regional inequalities of density persists over 

 the Nova Scotian slope to the eastward. 



Additional evidence that the bottom water does actually move inward through 

 the Eastern Channel is afforded by the inequalities of density within the basin of the 

 gulf, where the surfaces of equal density (approximately horizontal in the upper 

 50 to 60 meters) show a considerable slope from the channel inward at depths greater 

 than 80 to 100 meters. 



This density gradient in the deep water may be illustrated most graphically by 

 charting the depth to which it is necessary to sink in order to reach water of a 

 given value, choosing 1.027 as the most illustrative (figs. 171 and 172). The pre- 

 cise upper contour of this mass of heavy bottom water has varied from month to 

 month, as might be expected. Thus, in June, 1915, the slope was steepest near the 

 entrance to the channel, with the surfaces of equal density lying nearly horizontal 

 thence inward along the western arm of the basin. In July and August of 1914 the 

 the most abrupt slope, involving the whole column of water deeper than 50 meters 

 (fig. 171), was situated farther within the basin. A density gradient of the same 

 sort was again recorded in the eastern part of the basin in March, 1920, and a 

 weaker contrast (but one of the same order) between the channel, on the one hand, 

 and the inner parts of the basin, on the other, in April of that year, sufficient to 

 show it a permanent characteristic of the gulf. 



The implication of a density gradient of this sort is obvious. Only by the 

 introduction of heavy water into the gulf via the channel could it be maintained 

 against the action of the hydrostatic forces that are constantly tending to make hor- 

 izontal the surfaces of equal density. 



The inflowing bottom current, which maintains the high salinity (34.5 to 35 

 per mille) of the deeps of the gulf, thus corresponds, both in cause and in effect, to 

 the indraft of offshore water that has been recorded in many an estuary. The 

 gulf, in fact, is nearly as estuarine in this respect as it would be if the offshore banks 

 (Georges and Browns) were above water, and so actually inclosed it except for the 

 deep channels between. 



In the preceeding discussion I have spoken as though this inflowing current and 

 the gradients of density that give rise to it were comparatively constant. Actually, 

 however, our observations on temperatures and salinity have revealed considerable 

 fluctuations in the volume of water that enters the gulf via this route at various 

 seasons and in various years. 



It goes almost without saying that no sharp distinction can be drawn in salinity 

 between waters of different origins, especially where the water is stirred as actively 

 as it is in the Gulf of Maine; but the isohaline for 34 per mille may be taken as 

 roughly outlining the "slope water" that has recently entered the gulf or that has 

 continued little altered during its sojourn there, if the product of an earlier invasion. 



