550 BULLETIN OF THE BUKEAU OF FISHERIES 



The temperature followed a similar cycle in 1913, when the surface warmed to 

 5.56° near Gloucester by April 14, though no appreciable change had taken place 

 at 25 meters during the preceding two weeks (about 4° to 4.1°; stations 10055 and 

 10056). 



In 1923, following a very severe winter, the surface of the central part of the bay 

 had warmed to only 2.8° by April 18, with 1.6° at 40 meters and 0.4° at the bottom 

 in 80 meters. The bay continued nearly as cold as this until the end of April in 1920 

 (also following a cold winter) with readings of 3.6° at the surface, 2.87° at 20 meters, 

 1.58° at 40 meters, and 1.78° at 90 meters in its central part on the 20th (station 

 20119), but with the regional distribution (warmest, 4.4° in Cape Cod Bay, station 

 20118) essentially the same as in 1925. Probably the records for 1925, on the one 

 hand, and 1920 and 1923, on the other, cover the extremes to be expected in the 

 bay in April, except in very exceptional years. 



Seasonal progression in the coastwise belt north of Cape Ann is illustrated for a 

 warm year by serial observations taken by W. W. Welsh near the Isles of shoals 

 and near Boon Island at intervals during the spring of 1913 (p. 980) . Here the winter 

 state prevailed until the end of March (fig. 23) . On April 5 the temperature was 

 equalized, surface to bottom, and after the middle of the month the surface was 

 warmer than the underlying layers, warming progressively thereafter as illustrated by 

 the graph (see also Bigelow, 1914a, p. 394). 



The rate at which the surface warms along this part of the shore during April is 

 irregular, often interrupted or even temporarily reversed by climatic conditions. 

 During the winter, when the column of water is of nearly uniform temperature from 

 the surface downward, the upwellings that follow offshore winds have little effect on 

 the surface temperature; but as soon as the surface becomes appreciably warmer 

 than the underlying water, any upwelling of the latter, or vertical mixing, is at once 

 made evident by a decided, if temporary, chilling of the surface. Northwest winds 

 are a frequent cause of such upwellings along the western shores of the gulf in early 

 spring, and a blow from any quarter causes a more or less active stirring of the upper- 

 most stratum by wave action. 



During the spring of 1913 a northwesterly gale cooled the surface from 5° near the 

 Isles of Shoals on April 13 to 4.6° on the 14th and 15th. The water then warmed 

 to 7.9° by April 26, under the influence of unseasonably warm weather, when a north- 

 easterly gale, with rain, followed by high northwest winds, once more chilled the 

 surface to 6.7°. This was followed by another rise in surface temperature to 9.78° 

 by May 6, when a third northwest gale, of several days duration, once more reduced 

 it to about 7.2°. The wind then changed to the south, and by the 14th of May, 

 when the latest observation was made, the surface temperature had risen to 8.11°.'° 

 Temporary upwellings of this sort are as clearly evidenced by a rise in salinity (p. 729) 

 as by a fall in temperature. 



APRIL 



It is necessary to turn to the station data for 1920, combined with odd records for 

 1913 (p. 980) and 1925 (p. 1012), for a general picture of the temperature of the off- 

 shore waters of the gulf in April, remembering that after a mild winter readings 1° 

 to 2° higher than those pictured (fig. 24) are to be expected in the coastal belt. 



'» For further details see Bigelow, 1914a, p. 395, fig. 7. 



