PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 573 



chiefly, it would seem, on differences in the extent to which the water is stirred by 

 the tides and on the freedom of interchange of water between the coastal zone and 

 offshore — perhaps to some degree on upwellings. 



In midwinter the Plymouth shore and Cape Cod Bay to the southward see winter 

 chilling more rapid than in any other part of the Massachusetts Bay region (fig. 81). 

 With the advance of spring, however, the regional relationship is reversed, so that by 

 May we find the surface water warmest in Cape Cod Bay (p. 557, fig. 28). During 

 the last week of that month, however, and the first half of June, the western side of 

 Massachusetts Bay had caught up with Cape Cod Bay in the progression of temper- 

 ature, so that all this area (inclosed by the isotherm for 15° on fig. 39) was now 

 nearly uniform (15 to 15.2°) in surface temperature, except for one station off Plym- 

 outh Harbor, where vertical circulation of some sort was responsible for a slightly 

 lower reading (14.43°). 



Considerably lower surf ace temperatures (12.1° to 13.3°), right across at the mouth 

 of the bay, show that the offshore waters had lagged behind the coastal belt in 

 warming; and still lower readings (12° to 13°), along the north shore of the bay 

 deserve emphasis because the 20-meter level was warmest here, coldest at the mouth 

 of the bay, and with a rather surprisingly wide range in temperature (12.03° to 4.56°) 

 from station to station. Active vertical stirring is clearly responsible by bringing 

 the upper 20 meters within the immediate effect of the sun's rays, to warm nearly 

 uniformly along the northern shore. At the same time it is probable that the 

 warming of the upper stratum in this particular region is forwarded during June by 

 a more or less constant drift of the surface water — already warmed to 12° to 14 

 temperature — around Cape Ann and westward into the bay. Consequently, a some- 

 what higher mean temperature for the upper 20 meters may be expected to prevail 

 along its northern shore than in its central parts in June, just as was actually 

 recorded in that month in 1925 (Fish Hawk cruise 14, stations 35 to 37), instead of a 

 lower mean temperature, as is the case later in the summer. 



More rapid warming of the surface along the Plymouth shore and in Cape Cod 

 Bay, but a slower rise in temperature at 20 meters, points to a less active overturning 

 by the tides; and the fact that the surface and 20-meter readings both averaged 2° 

 to 3° higher there than over the deep sink off Gloucester {Fish Hawk station 31) is 

 evidence that the interchange of water between the open basin of the gulf, on the 

 one hand, and the western and southern parts of Massachusetts and Cape Cod Bays, 

 on the other, had been so slow for some weeks previous that the latter had acted as a 

 more or less isolated center of local warming. On the other hand, the low temperatures 

 (5 to 6°) at the 20-meter level along the eastern side of Stellwagen Bank, at the 

 mouth of the bay, point to a certain amount of upwelling over the slope of the latter, 

 bringing up cold water from greater depths offshore. 



These regional differences in the June temperatures for 1925 are smoothed out 

 over the Massachusetts Bay region with increasing depths. At 40 meters, for example, 

 the extreme range of temperature was then only from about 3.5° to about 6.1°, with 

 the mouth of the bay uniformly 4° to 4.5°, and the 40-meter temperature (about 3.6°) 

 off Gloucester for the 6th of the month, for 1924 (station 10653), falls within this 

 range. At 75 to 94 meters the temperatures of Massachusetts Bay were also about 

 37755—27 5 



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