700 BULLETIN OF THE BUKEAU OF FISHERIES 



is its general temperature made more than 2° to 3° lower than would be the case if 

 the gulf were entirely barred to currents, cold or warm; but the chilling effect of the 

 Nova Scotian current is more important than this bald statement suggests, for it 

 counteracts, by several degrees, the effect of the warm sources just mentioned. 



Autumnal and winter chilhng, so conspicuous a feature of the gulf, results 

 primarily from the loss of heat from the surface by radiation, after the date when 

 the mean temperature of the air falls below that of the water; neither cold currents 

 from the north nor upweULng from the oceanic abyss have any major part in it. 



Snow falling and melting on the surface is also a cooling agency of some effi- 

 cacy; so, locally, is melting ice in Cape Cod Bay and among the islands along the 

 coast of Maine. River drainage, by its low temperature in early spring, also tends 

 to retard vernal warming. Evaporation from the surface also tends to chill the water 

 throughout the year, accounting for a probable cooling of the mean temperature of 

 the upper 50 meters by 5° to 6°. 



The temperature of the superficial 100 meters of water is governed chiefly by 

 these climatic (including solar) influences from above, by the thermal effect of the 

 inflows into the eastern side of the gulf, and by the chilling effect of evaporation 

 from the surface. 



The cold layer that persists in the basin throughout the summer at a depth of 

 100 to 150 meters in most years is simply reminiscent of the lowest temperature to 

 which this level chilled during the preceeding winter — not of an Arctic current. 

 This layer is colder than the deeper water in most summers because the temperature 

 of the latter is determined chiefly, not by seasonal climatic influences, but by the 

 volume of the warmer slope water flowing in through the eastern channel, and by 

 the course that this current follows inward along the two branches of the trough of 

 the gulf. If the inflow of slope water is smaller than usual, or cooler, the summer 

 temperature of the inner part of the basin is virtually uniform, vertically, from 

 about 100 to 150 meters down to the bottom, as was the case in 1912. 



It is not yet possible to estimate, quantitatively, what thermal effect the slope 

 water has on the upper layers of water as it is gradually incorporated into the Gulf 

 of Maine complex. Any increment from this source will tend to cool the surface 

 stratum in the summer but to warm it in winter and early spring. 



The chilling effects of the rigorous winter climate of the land mass to the w'est 

 and of the Nova Scotian current, balanced against solar warming plus the warming 

 effect of the slope water and of the surface indrafts from the Browns Bank-Cape 

 Sable deadwater region, maintain a comparatively constant state in the gulf from 

 year to year; but it is easy to see how any one of them, if more or less effective 

 than usual, might profoundly influence its waters. In attempting to determine the 

 causes of such fluctuations as have been recorded, the evidence of salinity, as well 

 as of temperatiu-e, must be weighed. 



Unusually high summer temperatures, with normal salinity, might result either 

 from a mild winter preceding, from unusually rapid solar warming during the 

 spring, or from a smaller increment from the Nova Scotian current than normal. 

 High temperature, with very high salinity, would point either to an unusual inflow 

 of slope water during the preceding winter or to one of the rare overflows of tropic 

 water (p. 836). Abnormally low summer temperatures, with normal salinity, would 



