636 



BULLETIN OF THE BUREAU OF FISHERIES 



Amiual differences in temperature — Continued 

 WEST SIDE OF EASTERN BASIN 



AUTUMNAL COOLING 



SURFACE 



The surface is at its warmest at some time during August in all those parts of the 

 Gulf of Maine where the surface temperature rises much above that of the deep 

 water in sununer.^' This includes the whole open area, except for the northeastern 

 part, and the sites of active tidal mixing on the banks, the precise date of maximum 

 surface temperature for any given summer depending on the prevailing weather. Our 

 recent studies have not been sufficiently intensive precisely to locate this critical 

 date for any one year or for any given locality in the gulf, but the records collected 

 by Rathbun (1887) for the years 1881 to 1885 show that it may fall at any time 

 between the first and last of August for the western and northern shores of the 

 gulf between Nantucket Shoals and Penobscot Bay. After the first of September 

 the surface of this subdivision cools as the autumn advances. 



Experience in the summers of 1912, 1913, and 1914 suggests that the temper- 

 ature of the upper layers of the western and deeper parts of the gulf generally (i. e., 

 where vertical circulation is only moderately active) probably had passed its mid- 

 summer maximum, and that autumnal cooling had commenced there by the date of 

 our late August and early September cruise of 1915. Thus, the highest reading 

 recorded on August 31 and September 2 of that year, on the run eastward from 

 Gloucester toward Cape Sable, was only 17.6°, contrasting with a probable maxi- 

 mum of about 19° to 20° over the western side of the basin during mid August. 

 The seasonal schedule seems to have been about the same in 1925, also, when the 

 Halcyon had surface readings of 16.6° a few miles north of Cape Ann, 15.2° on Platts 

 Bank, and 14.7 between the latter and Portland on September 3. 



<'Tbe temperature ot Inclosed harbors is highest in July, mirroring the summer maximum for the air (p. 585). 



