568 PHYSICAL OCEANOGBAPHY OF THE GULF OF MAINE 



The spacial distribution of temperature in May may be illustrated in a more 

 connected way by three west-east profiles of the gulf — the first for April 28, 1919 

 (fig. 35), the second for May 4 to 7, 1915 (fig. 36), and the third for May 29 to 30, 

 1919 (fig. 37). 



The first of these is interesting chiefly as it outlines the extension of the cold 

 Nova Scotian current into the eastern side of the gulf, indenting like a shelf into the 

 warmer water of the basin (isotherm for 4°, fig. 35). Water almost equally cold, 

 washing the slope of Cape Cod at 60 to 120 meters in the opposite side of the profile, 

 is reminiscent of the previous winter's cooling in situ; and the definite separation of 

 these two cold masses by slightly higher temperatures in the central part of the basin 

 deserves emphasis. Unfortunately no readings were taken deep enough in the basin 

 to show what relationship the temperature of the bottom stratum bore to that of 

 the mid depths at the time. So far as they go, however, they point to a homogeneous 

 state at depths greater than 100 meters. 



Although the May profile for 1915 (fig. 36) was run only a week later in date, 

 the presence of a lenticular mass of 5° to 6° Vt^ater over the western part of the basin, 

 with maximum thickness of about 50 meters, illustrates a considerable advance in the 

 seasonal cycle, reflecting the penetration of solar heat downward from the surface 

 into the underlying water. Below it the cold coastal band that skirts the western 

 side of the gulf earher in the spring (the product of local chilling) is still represented 

 at the mouth of Massachusetts Bay by temperatures of 3.5° to 4° at depths greater 

 than 20 meters. 



Whether the cold water of Nova Scotian origin in the eastern side of the gulf 

 assumed a shelflike outline earlier in that particular spring, as it certainly did in 

 1919, is not known. If so, its tip had been eaten away by mixture with the sur- 

 rounding water until its limiting isotherm (4°) had come to assume the more nearly 

 vertical course shown on the profile (fig. 36). In actual temperature, however, this 

 cold water mass was very nearly the same in 1915 as the ice patrol found it in 1919, 

 one of the many illustrations that might be cited of the surprising constancy of the 

 gulf in temperature from year to year. The presence of appreciably warmer 

 (4° to 5°) water below it in both these years illustrates how strictly' the inflow past 

 Cape Sable into the gulf is confined to the upper stratum above the 100 to 120 meter 

 level, a phenomenon resulting from the distribution of density in this side of the gulf 

 (p. 946). As a consequence, the surface is the coldest level there in May, or at least 

 the lowest readings will be had only a few meters down. 



Figure 37 illustrates still a later stage in the thermal cycle, the Nova Scotian 

 current having slackened and the two cold water masses that hug the two sides of 

 the gulf earlier in the season having merged into the general stratum of minimum 

 temperature (4° to 5°) at the 50 to 120 meter level. Vernal warming is illustrated 

 further on this profile by a rise in the temperature of the upper 10 meters from about 

 5° at the end of April (5° to 6° on May 4 to 6, 1915) to 8° to 9°. In the deeps of 

 the gulf a rise in temperature from about 4.5° to 5.6° to 6° during the precedmg four 

 weeks (cf. fig. 37 with fig. 35) is evidence of a considerable movement of slope water 

 through the Eastern Channel into the gulf during the interval. However, the nearly 

 horizontal course of the isotherm for 5 degrees across the basin on May 28 (fig. 37), 



