PHYSICAL OCEAN OGKAPH Y OF THE GULF OF MAINE 
57 3 
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 surface temperatures (12.1° to 13. 3°), right across atthemouth 
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 wanned 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 
8951—28 37 
