PHYSICAL OCEANOGRAPHY OF THE GUU' OF MAINE 679 



than it is off the western shores of the gulf in summer. The following exposition 

 may more graphically explain this general phase of the gulf temperatures : 



Let us assume two localities, both with an initial temperature of 2°, surface to 

 bottom, but with vernal heating in the first (a) uniformally propagated downward 

 through the whole column to a depth of 50 meters by active tidal stirring, but absorbed 

 in regularly increasing ratio, with increasing depth, at the second (&), to nil at the 

 bottom. If enough heat were received at the surface to warm the whole column at 

 a to a temperature of 10°, the same amount of heat entering the water at 6 would 

 warm the surface to 20° there, but not affect the temperature at all 50 meters down. 

 The ideal condition represented by a is most closely paralled in the Gulf of Maine 

 area by the most tide-swept parts of the Bay of Fundy region. An approximation 

 to the vertical distribution of temperature at 6 is to be found in the western side of 

 the basin off Cape Ann, where the surface warms from a winter minimum of about 

 3° in February to a summer maximum of about 19° to 20° in August, but where the 

 temperature of the 50-meter level rises by only about 1° during the same interval. 

 The relative rates at which heat is dispersed downward in these two parts of the 

 Gulf of Maine correspond directly to the relative activity of the tidal currents, which 

 are weaker in the deep water in the offing of Cape Ann than anywhere else in the 

 Gulf of Maine. 



THERMAL EFFECTS OF UPWELLINGS 



Upwellings of water from below have little effect on the temperature of the sur- 

 face stratum of the gulf in winter, because the whole column of water is then so 

 nearly homogeneous that the rising currents have about the same temperature as the 

 water which they replace. From April on, however, the upwellings that follow off- 

 shore winds in the western side of the gulf are reflected in a chilHng of the surface, 

 as described above (p. 550). This is not the case in the eastern side, however, or on 

 the banks, where tidal stirring keeps the water more nearly homogeneous, vertically, 

 throughout the warm season as well as the cold. The relationship between these 

 upwellings from small depths and the temperature of the surface water is sufficiently 

 described in connection with the midsummer state of the gulf (p. 588) . I need only 

 add that the thermal effect of vertical circulation of this sort along our New England 

 coast has long been appreciated and has recently been discussed by Brooks (1920). 



THERMAL EFFECTS OF HORIZONTAL CIRCULATION WITHIN THE 



GULF 



The effects of the transference of cold water by the Nova Scotian current is dis- 

 cussed below (p. 680) . A word is also in order as to the opposite process. The trans- 

 ference of heat, from the tropics to high latitudes, by the great ocean currents, is 

 reflected on a very small scale in the Gulf of Maine in summer by the drift of surface 

 water, warmed in the western side, across to Nova Scotia by the dominant anti- 

 clockwise drift. The outflow from the eastern end of Nantucket Sound, now reason- 

 ably established (p. 886), must similarly tend to raise the temperature of the water 

 over Nantucket Shoals. On the other hand, the westerly drift from the Bay of Fundy 

 combines with the active tidal stirring to maintain the low surface temperatures char- 

 acteristic along the eastern sector of the coast of Maine. 



