PHYSICAL OCEANOGEAPHY OF THE GULF OF MAINE 975 



In some summers, if not in all, the westerly drift just mentioned involves the 

 surface water across the whole breadth of the continental shelf in the offing of 

 Marthas Vineyard and Nantucket. This, however, can not be regarded as a direct 

 continuation of the outdraft from the gulf around the eastern end of Georges Bank 

 On the contrary, the latter probably swings offshore to join in the easterly move- 

 ment of the so-called "inner edge of the Gulf Stream." 



The evidences of temperature, salinity, and of dynamic gradient unite to show 

 this "Gulf Stream" current departing from the edge of the continent as it crosses 

 the mouth of the gulf from west to east, so that while it may be encountered within 

 15 miles of the 200-meter contour line at longitude 69° to 70°, it is usually at least 

 40 to 50 miles out at longitude 66°. Farther east, however, it again approaches the 

 slope, at least in some summers. 



Our recent cruises have afforded no evidence of any movement across Georges 

 Bank from south to north, though the surface water not infrequently drifts northward 

 from the edge of the continent to the west of Nantucket Shoals during the late 

 summer. 



The chief seasonal variations from the circulatory scheme just outlined result 

 during the autumn and winter from a shift in the heavy ("low ") center of anticlock- 

 wise circulation to the Eastern Channel, from a speeding up of the coastwise drift 

 around the northern and western shores in spring, and from the brief overflow of 

 the Nova Scotian current into the eastern side of the gulf at that same season. 



As a result we find the circulation centering chiefly around the Eastern Channel 

 in March with velocities greatest as it drifts inward along the eastern side and out- 

 ward along the western side of the latter. From March to April, however, the 

 center of circulation shifts northward across the basin; the movement slackens in 

 the southeastern part of the area, and the coastwise drift gathers strength. Shortly 

 thereafter, when the water of the Nova Scotian current floods into the gulf from 

 the east, the heavy center is shifted southwestward right across the gulf. At the 

 same time (in May) the northeast-southwest drift around the northern and western 

 coasts attains its highest velocity and its most definitely long-shore character, and 

 is most definitely continued southward past Cape Cod. It also involves Massa- 

 chusetts Bay, not only crossing the mouth of the latter, but also skirting its coast- 

 line from north to south, and so out again past Cape Cod. Under these circumstances 

 flotsam of any kind (buoyant fish eggs, for instance, or the larvae hatched therefrom) 

 that may drift from the north into the northern side of Massachusetts Bay, or 

 that may be produced there, tends to drift out of its southern side. 



This long-shore movement (involving Massachusetts Bay) may continue, little 

 altered, into the summer; but some time between May and July the heavy center 

 again shifts eastward, and in some years, at least, this center becomes divided into 

 the two lows recorded for the summe!: of 1914 — the one in the offing of the Bay 

 of Fundy, the other in the region of the Eastern Channel. This completes the 

 yearly cycle. 



On the bottom the water moves inward along the eastern side of the Eastern 

 Channel during the early spring, and at other times of year in pulses not yet under- 

 stood, usually outward along the western side. At depths of 150 meters, or deeper, 

 the general tendency within the basin is northward along the eastern (Nova Scotian) 



