PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 
555 
Desert) on the 19th, 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 warmer 
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. 14 
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 any time 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 
14 No readings so high were obtained anywhere in the southern or eastern parts of the basin that April, the maxima being 
respectively, 5.28°, 5.14° ,5 28° .and 5.18° in depths of 210,2 25, 175, and 165 to 230 meters at stations 20098, 20100, 20107. 20112, and 
20113. 
