PHYSICAL OCEANOGKAPHY OF THE GULF OF MAINE 741 



a spread suggests that the bottom of the gulf had actually received much more 

 water via the charxnel in 1920 than in 1919 during the whole winter. 



No cause can yet be assigned to annual differences of this sort, except that they 

 do not result from local influences operative within the gulf, but from the state of 

 the reservoir outside the edge of the continent, which supplies the indraft (p. 848). 



SALINITY IN MAY 



SURFACE 



The salinity of the gulf is especially interesting during the first half of May, 

 because the two most important events in its vernal cycle — freshening of the surface 

 by land water in the western side, and by the Nova Scotian current in the eastern 

 side — culminate then. Unfortunately we have not been able to carry out a general 

 oceanographic survey of the whole area of the gulf in any one May, nor have obser- 

 vations been taken in its southeastern part during that month; but the data for 

 1913, 1915, 1919, 1920, and 1925 afford a composite picture, which may be taken as 

 representative for normal years because all are fairly consistent. 



In 1913 the surface salinity fell to its minimum (29.5 per mille) near the Isles 

 of Shoals about May 5, followed by an increase to 30.9 per mille in the middle of the 

 month; and while a northwest gale on the 10th, 11th, and 12th no doubt was 

 partly responsible for this increase by bringing up more saline water from below, the 

 spring influx of river water had evidently passed its peak by the first week of the 

 month, to be gradually absorbed into the general circulation of the gulf thereafter. 



Unfortunately, close comparison is not possible between the years 1913 and 1920, 

 for this region, because the locations of the stations do not coincide, which may cause 

 a very considerable difference in salinity where the precise value depends so much on 

 the proximity to the mouths of rivers. However, the surface again proved much 

 fresher south of the Isles of Shoals on May 7 to 8, 1920 (station 20122, 28.26 per 

 mille), than it had on April 9 (station 20092, 31.01 per mille) — a value even lower 

 than any recorded for 1913. 



In 1920, too, the salinity of the surface of the northern part of Massachusetts 

 Bay was almost as low as this on May 4 (stations 20120 and 20121, 29.1 to 29.16 per 

 mille) , but apparently this was close to the minimum for the month because followed 

 by a considerable increase at this same general locality to about 29.9 per mille during 

 the next 10 days (stations 20123 and 20124). 



In 1925 no observations were taken in Massachusetts Bay during the first 10 days 

 of May, when salinity was probably at its lowest there; and the values recorded there 

 on the 20th to the 22d (fxg 119) were so high*^ that some increase may be assumed 

 to have taken place during the second and third weeks of the month in that year, 

 as it certainly did in 1920. 



Whether or not the surface salinity of the northern part of Massachusetts Bay 

 fell below 30 per mille for a brief period in 1925, as April readings as low as 29 per 

 mille in Ipswich Bay (p. 725) suggest, water of relatively low salinity was certainly 

 drifting southward past Cape Ann as late as the third week of that May as a 

 tongue less saline than 31.5 per mille directed toward Cape Cod (fig. 119). The 



"31.1 to 31.9 per mille at the surface, averaging 31.6 per mille (Fish Bawk cruise 13). 

 37755—27 16 



