PHYSICAL OCEANOGRAPHY OF THE GULP OP MAINE 921 



CIRCULATION IN THE DEEP STRATA AS INDICATED BY TEMPERATURE 



AND SALINITY 



Dawson's (1905) observations made it known that the tidal currents of the 

 eastern side of the gulf run about as strongly down to a depth of 55 meters as they 

 do at the surface, and measurements taken at 5 stations by the Grampus in the 

 summer of 1912 showed bottom currents varying in velocity from 0.1 to 0.25 knot 

 per hour in depths of 100 to 265 meters (Bigelow, 1914, p. 86). Evidently, then, 

 the basin of the guK is constantly in a state of active circulation right down to the 

 bottom, its whole mass of water oscillating to and fro with the tides, though with 

 velocities somewhat lower in the deep water than at the surface. 



Up to the present time no attempt has been made to determine the nontidal 

 movement of the bottom water of the guK with current meters or by the use of deep 

 drift bottles, such as have proved so instructive in the North Sea, but the regional 

 differences in temperature and salinity outline the major movements over the bottom. 



At depths greater than 100 meters the gulf of Maine is an inclosed basin with 

 the narrow Eastern and Northern Channels as the only possible entrances or exits 

 through which water can flow in or out of its basin. It follows from this that any 

 deep current into the gulf can enter only in its eastern side. Such entrance might be 

 via either of the two channels or through both, so far as the contour of the bottom 

 is concerned. Actually, however, salinity and temperature show that the indraft of 

 slope water over the bottom is restricted to the Eastern Channel, the abrupt west- 

 east transition in salinity and in temperature, which characterizes the Northern 

 Channel, being incompatible with any large transference of bottom water through 

 the latter in either dhection. 



The dominant drift in the eastern side of the Eastern Channel is clearly northerly 

 (into the gulf) at aU times of year, but a considerable difference between high values 

 of temperature and salinity in the eastern side of the channel and lower values in its 

 western side in March, April, and July (pp. 770, 789) point to an outflowing current 

 via the latter, continuing southward and westward around the slope of Georges 

 Bank. 



Slope water is betrayed in the deep strata of the gulf by its high salinity 

 (33.5-34 per mille, p. 849) and moderately high temperature (4.5° to 8°). At the 100- 

 meter level the isotherms and isohalines show the Luflowing current hugging the eastern 

 slope of the basin in March as a rather definite tongue of high temperature and 

 salinity (figs. 13 and 94), veering westward around the northern side of the basin, 

 with a countermovement of cooler and less saline water setting southward and 

 eastward around the southern side of the basin. In fact, physical evidence could 

 hardly be clearer that the general Gulf of Maine eddy was effective to a depth of at 

 least 100 meters in this particular month, though complicated by an indraft through 

 the Eastern Channel in the deeper levels, which did not directly affect the surface 

 (p. 704). 



An anticlockwise circulation is also indicated on the 100-meter charts for April 

 (figs. 25 and 116), though less clearly, by concentration of the highest salinities and 

 temperatures in the eastern and northern parts of the basin, the lowest in the west- 

 ern and southern parts. In this case, however, the westerly component involved a 

 broader and less definite band off the coast of Maine than in March, and the easterly 



