PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 547 



March in the years 1913 and 1921, as just noted (p. 524). In 1925, too, the super- 

 ficial 10 meters of Massachusetts Bay did not become definitely and consistently 

 warmer than the underlying water untU the end of March (locally even later) ; and 

 although the whole column had been warming slowly at all the stations there since 

 the middle of February (p. 660), this change was at first so slow that the mean sur- 

 face temperature of the southern side of the bay was only about 0.3° higher on 

 March 10 (2° at stations 2, 10, 13a, 15, and 18a) than it had been on February 24 

 to 28, the mean bottom temperature for these same stations remaining virtually 

 unchanged. This probably applies also to the whole area of Massachusetts Bay, 

 for the surface had warmed by only about 0.56° just outside Gloucester Harbor, and 

 not at all within the latter. 



In Ipswich Bay, however, the surface had become definitely warmer than the 

 underlying water by the first week of March, and this was the case over the gulf 

 as a whole in 1920, as just described. 



From early March onward the progressive warming from above lowers the cold- 

 est plane in the western side of the basin to a depth of about 100 meters by the 

 middle or end of April. At the same time warming by slope water from below raises 

 the coldest plane in the northeastern part of the basin (the latter itself now slightly 

 warmer than in March) to within 15 to 20 meters of the surface. In the southeast- 

 ern part of the basin, however, the temperature was lowest at the 100-meter level on 

 April 17 (station 20112), instead of at 20 to 40 meters, as it had been on March 11 

 (station 20064). The minimum temperatures were recorded at about the same depth 

 (20 to 40 meters) for the two months in the Northern Channel, the Eastern Chan- 

 nel, and on the southeastern continental slope of Georges Bank. On Browns Bank, 

 however, where the upper 20 meters had been considerably coldest on March 13 

 (station 20072), the bottom (80 meters) was slightly coldest on April 16 (station 

 20106), and the whole column, top to bottom, had become nearly homogeneous in 

 temperature during the interval. 



Vernal warming, the normal event in boreal seas, is retarded — may even be 

 reversed temporarily — in the eastern side of the Gulf of Maine when the intermittent 

 Nova Scotian current floods past Cape Sable, as described in a later chapter (p. 832). 

 The cold water from this source affects a greater displacement of the isotherms 

 within the gulf and produces lower temperatures there in some springs than in others, 

 depending on the volume and temperature of the flow past the cape, on the date at 

 which this reaches its maximum, and on the duration of the period during which 

 this Nova Scotian water enters the gulf in amount sufficient to appreciably affect 

 the temperature of the latter. 



In describing the spring cycle vernal warming must be carried along hand in 

 hand with this chilling from the east. In 1913 the vernal warming of Massachusetts 

 Bay and of the Isles of Shoals-Boon Island region to the north was at first most rapid on 

 the bottom. Thus, the 82-meter temperature rose from 3 . 1 1 ° off Gloucester on Febru- 

 ary 13 (station 10053) to 3.61° on March 4 (station 10054), whereas the two surface 

 readings were less than 0.1° apart (both 2.83° to 2.89°). Mr. Welsh found the sur- 

 face still continuing fractionally colder (3.6°) than the deeper levels near Boon Island 

 on the 29th of the month also, although, judging by the date, the superficial stratum 

 almost certainly had experienced some increase in temperature by then. 



