692 BULLETIN OF THE BUEEAU OF FISHEKIES 



Further evidence that slope water is of little importance in the thermal cycle of 

 the Bay of Fundy results from the fact that we found the 200-meter level 1° colder 

 (4.3°) mthin the latter than just outside (5.4°) in March, 1920 (stations 20079 and 

 20081), with a corresponding difference in salinity. A reading of 1.71° reported by 

 Mavor (1923) at 175 meters in the bay on April 9, 1917, is colder than the coldest 

 reading so far obtained anywhere in the open basin of the gulf at this depth. 



The deep readings for different times of year warrant the following generaliza- 

 tions: At depths greater than 150 meters the temperature is most nearly uniform 

 through the year in those parts of the gulf which the slope water reaches in greatest 

 volume, and shows its widest seasonal fluctuation in the partially inclosed bowls that 

 receive least water from this source. Were it not for this deep current flowing in, the 

 floor of the gulf would be several degrees (perhaps 3° to 4°) cooler in winter than is 

 actually the case, and its mean for the year slightly lower. The bowl off Gloucester 

 and the trough west of Jeffreys Ledge show the nearest approach to the thermal 

 state that would prevail in the gulf were it neither open to the inflowing bottom 

 current nor stirred by such strong tides as those that disturb its eastern side. 



The thickness of the bottom stratum where temperature is governed by the volume 

 and precise physical characters of the slope water is of interest. Its upper boundary 

 in the inner part of the basin of the gulf may be set tentatively at about the 150- 

 meter level, rising to within 80 to 100 meters of the surface in the southeastern part 

 at the entrance to the Eastern Channel. On the other hand, the deep temperature 

 is most influenced from above where tidal or other convectional stirring is most active. 



WINTER CHILLING 



Abyssal upwelling, as I have shown (p. 853), is barred out as a possible source of 

 autumnal cooling in the Gulf of Maine. It is equally certain that the Nova Scotian 

 current usually serves as a cooling agent in the gulf only in the spring, because none of 

 our observations for autumn or winter suggest that progression of cooling from east 

 to west across the gulf, which would reflect any inflow of cold water past Cape Sable 

 at that season. We must therefore credit the very rapid loss of heat which the Gulf 

 of Maine suffers in autumn and winter entirely to local causes, chiefly to the radia- 

 tion of heat out from the surface to and through the colder air above it; to evapo- 

 ration; in less degree to the melting of the snow that falls on the sea; and, locally, 

 to the melting of ice. 



The warming effect of the sun's rays is combatted the year round by local 

 influences tending to reduce the temperature of the water or as least to retard ver- 

 nal warming. Evaporation from the surface, for one thing, uses up heat, thus cool- 

 ing the water (p. 680) . Furthermore, the heated surface radiates heat out into the 

 air whenever the temperature of the latter drops below that of the water, even in 

 spring and summer. 



The solar energy absorbed by the water is more than enough to offset these 

 forces up to mid or late August; consequently the temperature of the surface of all 

 parts of the gulf continues to rise. However, the amount of solar heat daily absorbed 

 by the water, at its maximum when the sun is at its highest declination, is constantly 

 decreasing after June 22 to 23; and after a certain date toward the end of summer 

 or early in autumn, a date that varies regionally, as. described in an earlier chapter 



