PHYSICAL OCEANOGRAPHY OF THE GULP OF MAINE 



929 



The courses of the curves for 1.5, 2, and 2.5 units of density on the chart (fig. 186) 

 give evidence that the shoal ground oflf Penobscot Bay and out to Cashes Ledge also 

 is the site of considerable vertical disturbance as the tidal currents are deflected 

 by it. 



As summer passes into autumn and the surface waters commence to cool, the 

 parts of the gulf that are most stable in summer become less and less so, with little 

 change in the eastern part, where the whole column of water loses heat more uni- 

 formly. The result is that vertical mixing is less and less opposed in the western 

 part of the gulf and regional differences decrease in this respect. 



The autumnal decrease in stability is illustrated for the southwestern part of the 

 gulf, generally, by the offing of Cape Ann, where the upper 40 to 50 meters lose sta- 

 bility most rapidly during the early autumn, then more slowly but constantly through 

 the winter. At depths greater than 100 meters no regular seasonal succession appears, 

 all the curves being roughly parallel, their differences attributable to annual fluctua- 

 tions in temperature and salinity. The seasonal succession is essentially of 

 this same kind in the deep water in the northeastern corner of the gulf, though, 

 thanks to strong tidal currents, the seasonal range of stabiUty in the upper 40 meters 

 (expressed in terms of density) is only about one-third as wide here as it is off Cape 

 Ann. 



Stabihty offers but little opposition to the free vertical circulation of water in 

 any part of the gulf after November ; less near the surface than at greater depths, 

 as appears from the following table for October and November of 1916: 



Vertical range in density for the superficial stratum and for the mid stratum 



The free mixing that takes place from that time on throughout the winter is illus- 

 trated by the uniformity with which the upper 50 to 100 meters cool off during 

 December, January, and the first half of February; evidently, water is constantly 

 being brought up to the surface from below, there to radiate its heat, and water 

 cooled at the surface is as constantly sinking. 



It is not necessary to follow in detail the changes in stability that take place in 

 winter in this connection. It is lowest over the gulf as a whole at the end of Feb- 

 ruary or first of March, when the difference in density between the surface and the 

 40-meter level has been only 0.1 to 0.33 for all our stations on the banks and within 

 the gulf, except at one off the Kennebec River (station 20058) . 



In fresh-water lakes, in high latitudes, autumnal cooling increases the density 

 of the surface until a dynamic overturning of the water regularly follows. Our first 

 winter's work in Massachusetts Bay (Bigelow, 1914a, p. 387) suggested that this 

 same process was partly responsible for the rapid chUling that takes place there; but 

 subsequent study, and especially the observations made in the bay from the Fish 

 Hawk in 1925, proves this earlier interpretation erroneous and make it unlikely that 



