968 BULLETIN OF THE BUREAU OF FISHERIES 



currents to a stratum much shoaler than the theoretic " frictional depth " as calculated 

 by Smith for homogeneous water at corresponding latitudes (p. 963). 



With an average wind strength of 3 to 4, by the Beaufort scale (a fair average 

 from the gulf in summer), this depth is set by him as about 43 to 70 meters at 

 latitudes 40° to 50°. It is not likely, however, that the wind ever sets water as 

 stable as that of the western side of the gulf in motion half so deep as this during 

 the brief periods when it blows steadily from any given direction at a strength as 

 great as 4, on the Beaufort scale (about 20 nautical miles per hour), during the 

 summer months. With the more usual summer breezes no stronger than 10 to 15 

 miles per hour (2 to 3 on Beaufort scale), the frictional depth must be even smaller. 

 Frequent reversals of the wind direction, with periods of calm, also further hinder 

 the propagation of wind currents downward into the underlying water. On the 

 whole, then, it is unlikely that wind currents are effective deeper than 10 to 20 

 meters in the gulf in summer, except perhaps during brief periods of windy weather. 

 Even if this limitation be too small it leads to the important conclusion that what- 

 ever currents may be set up in the gulf in summer by the wind are confined to a 

 very thin superficial stratum, and that the dominant anticlockwise and estuarine 

 circulation of the deep water below the 40 to 50 meter level is caused by hydro- 

 static forces and by the tidal oscillations (p. 970). 



The pulses of slope water into the gulf via the trough of the Eastern Channel 

 are equally independent of the wind. 



In winter the winds of the gulf of Maine area blow stronger (average about 3 to 

 5 on the Beaufort scale), and the prevailing quarter is northwest (p. 966). Winds of 

 this character tend, theoretically, to drive the surface water of the whole gulf out to 

 the southward, toward the open sea. Probably it is this prevalence of strong off- 

 shore winds all along the North American seaboard, from Chesapeake Bay to the 

 Gulf of Maine, during the cold season, which is primarily responsible for the reces- 

 sion of the tropical water from the edge of the continent during autumn and winter, 

 their cessation allowing its inshore movement in summer. The prevailing north- 

 west winds of winter tend, therefore, to strengthen the dominant southerly drift 

 along the western side of the gulf. With the coast line trending north and south, 

 the deflective effect of the earth's rotation gives a long-shore character to currents 

 caused by winds from this quarter, except so close in to the land that the whole 

 depth of water is less than the frictional depth. Under these last conditions (by 

 Ekman's calculation) the wind current will set more nearly with the wind than in 

 deeper water offshore.'' 



Consequently, the prevailing winter winds from the northwest quadrant do not 

 tend to cause any general or constant upwelling along the coast sector from Cape 

 Ann to Cape Elizabeth except within 2 to 3 miles or so of the land, where the water 

 is shoaler than one-fourth the assumed frictional depth of 50 meters. This is cor- 

 roborated by our station data, but upwellings, such as are actually recorded (p. 588), 

 necessarily tend to follow these same west to north winds along the north shore of 

 Massachusetts Bay. This same tendency for water to well up from below must 

 operate spasmodically throughout the winter all along the coast of Maine, where 



"Theoretically, 21.6° to the right of the wind, i( the depth ot water be one-fourth the frictional depth. 



