PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 555 



Desert) on the 19 th, are interesting as evidence that this general stratum was ap- 

 parently no warmer in that spring than in the corresponding month of 1920, although 

 the upper 40 meters of water was considerably so. Thus, as the depth increases, 

 annual variations, like seasonal and regional variations, tend to diminish until a level 

 is reached below which the temperature is governed chiefly by pulses in the bottom 

 drift flowing in from the edge of the continent. 



The bottom water at and below 200 meters was fractionally cooler in the eastern 

 arm of the basin in April, 1920, than it had been in March, and fractionally wai'mer 

 off the northern slope of Georges Bank and off Cape Ann (station 20115, 6.36° at 

 200 meters), with the deepest readings ranging only from 4.73° to 5.28° at 200 to 

 290 meters in the basin, rising to 6.07° in the Eastern Channel (station 20107). No 

 observations were taken as deep as this on the continental slope in April, but a read- 

 ing of 6.47° at 150 meters off the southeast face of Georges Bank on the 16th 

 (station 20109) shows a rise of about 1.6° since March 12 (station 20068). 



In March, 1920, it will be recalled (p. 541), the trough of the Eastern Channel be- 

 low 100 meters was filled with water warmer than 6°, though no temperatures as high 

 as this were encountered anywhere within the gulf. By mid-April, however, still 

 warmer water (7.45° at 170 meters, fig. 26) had penetrated the channel, its effect 

 (6 to 6.39°) spreading inward to the western side of the basin off Cape Ann (station 

 20115) as a thin stratum at 180 to 260 meters, but with slightly cooler (4.92°) water 

 below it.^* 



Again, on March 5, 1921, there was a thin, warm stratum (6° to 6.4°) at 160 to 

 210 meters off Cape Ann. Evidently, therefore, temperatures as high as 6° may be 

 expected below about 175 to 200 meters in the western arm of the basin of the gulf 

 at anyftime from March to April (in summer, also), though not invariably. This 

 warm stratum, when it occurs, may either be sandwiched in between lower tempera- 

 tures in the bottom of the trough below, as well as above, or may extend right down 

 to the bottom, with the vertical distribution of temperature following the curves 

 shown in the accompanying graphs (figs. 3 and 5). 



Temperature and salinity combined establish the Eastern Channel as the source 

 of this indraft into the bottom of the gulf. Its course across the latter (unfortu- 

 nately not chartable in detail from the data yet on hand) is discussed in a later 

 chapter (p. 921). There is strong evidence that it takes the form of intermittent 

 pulses, the 6°-water encountered off Cape Ann in April, 1920 (station 20115), being 

 the result of such a pulse; for it seems to have been entirely cut off from the still 

 warmer source in the Eastern Channel at the time by fractionally lower temperatures 

 in the southeastern bowl of the gulf (stations 20112 and 20113). 



These pulses are so important in the general circulatory system of the Gulf of 

 Maine that an April profile along the arc of the banks (fig. 26) is introduced here 

 for comparison with that of the preceding month (fig. 19). The most important 

 seasonal alteration is the rise in temperature at 150 to 200 meters in the channel 

 just mentioned, which could only result from the actual introduction of water of 

 still higher temperature from offshore. On the other hand, vernal warming from 

 above and a delay in the westward flood of Nova Scotian water until later in the 



" No readings so high were obtained anywhere in the southern or eastern parts of the basin that April, the maxinia being 

 respectively, 5.28°, 5.14°, 5.28°, and 5.16° in depths of 210, 225, 175, and 165 to 230 meters at stations 20098, 20100, 20107, 20112, and' 

 20113. 



