PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 605 



Such, for example, was the case near the northern edge of the bank on July 23, 

 1914 (station 10224), v/hen surface and bottom temperatures (11.11° and 10.78°) 

 differed by less than 0.5° in 55 meters depth. This same state prevailed at a station 

 on the western end of the bank (10059) on July 9, 1913 (surface 13.3°; bottom 12.6°), 

 and again on July 23, 1916.^° In August, 1896, Doctor Kendall foimd a maximum 

 difference of only about 1° between surface and 18-meter readings at many localities 

 along its northern and northwestern sides. 



On the parts of the bank where the water is more than 50 to 60 meters deep, 

 and where tidal currents do not run so strong, the surface warms more rapidly during 

 the progress of summer, the bottom less so; witness readings of 14.8° to 17.8° at the 

 surface and 6° to 9° on bottom in 60 to 70 meters on the northern and eastern parts in 

 August, 1926 (stations 20203 to 20208). The temperature gradient likewise differs 

 widely from place to place in the Nantucket Shoals region in the late summer, 

 depending on the topography of the bottom, with the water most nearly homogeneous 

 over the shoal banks and ridges. Thus, the temperature of the entire column of 

 water was 10° to 10.5° in 30 meters at a station 12 miles ESE. from Eound Shoal 

 buoy on July 15, 1924 (station 10655) ; and in August, 1925, when a greater number 

 of serials was taken, the surface was invariably less than 1° warmer than the bottom 

 on Rose and Crown Shoal, Round Shoal, and Great Rip in depths ranging from 20 

 to 30 meters, the actual temperatures ranging from 11.5° to 15° from station to 

 station (p. 595). 



The surface temperature rises liigh above that of the bottom water by the end 

 of the summer over the smoother bottom to the south of the shoals, a regional 

 contrast illustrated by two Grampus stations for July 25 a,nd 26, 1916. One of these, 

 located on the southern edge of the shoals (station 10355), was only about 1° warmer 

 (11.95°) at the surface than at the bottom (10.97° in 30 meters). The other, in 

 deeper water 23 miles to the southeast (station 10354), was 5° warmer at the surface 

 (13.6°) than at the 30-meter level, and 7.6° warmer than on bottom at a depth of 

 70 meters. Readings of 16.1° at the surface, 14.1° at 18 meters, and 10.2° at 46 

 meters, near by, show about this same vertical range on July 9, 1913 (station 10060). 

 A steep temperature gradient also develops to the west of the shoals by the end of 

 August, illustrated by Grampus stations 10258, 10259, and 10263 (p. 987), and by the 

 many serials taken off southern New England by Libbey (1891) in 1889. 



TEMPERATURE GRADIENT ALONG THE CONTINENTAL EDGE 



Sudden fluctuations in temperature are to be expected along the edge of the 

 continent where the conflict between warm oceanic and cool coastal waters is con- 

 stant. The station data do, in fact, show wide variations in the upper 100 meters 

 along this zone (fig. 51). The one extreme, which may fairly be described as 

 subtropical, is exemplified by stations 10218, southwest of Georges Bank, July 21, 

 1919, and station 10261, in the offing of Marthas Vineyard, August 26, 1914. These 

 chill, with increasing depth, from a very warm (20° to 24°) surface stratum to 7° 

 to 9° at 400 meters and to about 5.25° to 6° at 500 meters. These contrast with 

 stations showing a well-marked cold stratum at 40 to 80 meters, as south of Cape 



J» station 10347, surface n.39°, bottom 9.61° in 80 meters; station 1034S, surface 11.67°, bottom 11.26° in 51 meters. 

 ,377fl5— 27 7 



