PHYSICAL OCEANOGEAPHY OF THE GULP OF MAINE 



955 



possible errors caused by the considerable period of time over which each survey 

 extended. The rapidity with which the density of the upper stratum may be 

 increased, if the surface be chilled by vertical circulation of any kind, makes it 

 unsafe ever to lay any stress on small regional differences where tidal currents cause 

 as much overturning of the water as they do in parts of the Gulf of Maine. 



The accompanying dynamic chart for the summer of 1914 (fig. 203) shows 

 the dynamic tendency toward circulation at the surface of the inner parts of the 

 gulf and of the waters off Marthas Vineyard for August and of the Georges Bank- 

 Browns Bank region for that July. Unfortunately, these two divisions of the pic- 

 ture are not strictly comparable because solar warming had been responsible for 



r .^si 



Fig. 202.— Density at 100 meters, July to August, 1914. Corrected for compression 



some shght decrease in the density of the surface stratum from the one month to 

 the next, and for a very considerable decrease close to Cape Sable, where stations 

 situated close together but occupied 17 days apart differed by 0.4 in density. Never- 

 theless, the general dynamic gradient proved so consistent for the gulf as a whole 

 for the two months that it has seemed justifiable to neglect the time interval in draw- 

 ing the contour lines; the more so since the heaviest centers for July and August 

 proved almost exactly equal in dynamic height. 



If the chart, so combined, be indeed typical of the season (as seems likely from 

 general knowledge of the temperature and salinity of the region) , two centers of high 

 density (indicated as "low" on the dynamic chart) are now to be expected— the one 



