PHYSICAL OOEANOGEAPHY OF THE GULF OF MAINE 963 



Arctic ice, with subsequent studies of currents at lightships and analyses of wind and 

 drift at localities widely separated in the Baltic, North Atlanti6, Mediterranean, North 

 Pacific, and Adriatic unite in proving that the wind drift does, in fact, average to 

 the right of the wind in the northern hemisphere, to the left of it in the southern, as 

 theory demands. 



According to Ekman's (1905) more recent mathematical analysis, the surface drift 

 in a free ocean of unlimited depth will be deflected 45° to the right of the direction 

 of the wind in the Northern Hemisphere, more and more to the right with increasing 

 depth, but decreasing correspondingly in velocity until a level (the so-called " fric- 

 tional depth" is reached where the drift is opposite the wind but at only about one 

 twenty-third the strength of the surface current. The depth of this level depends 

 on the strength of the wind and on the latitude; theoretic calculation for homogene- 

 ous water of a specific gravity (1.025) approximating that of the shoaler water of the 

 Gulf of Maine (Smith, 1926, p. 47, Table 14) locates it at 45 to 90 meters for the 

 latitude of the Gulf of Maine, with winds ranging in strength from 15 to 20 nautical 

 miles per hour (Beaufort scale, 3 to 4). 



The Gulf of Maine lies within the belt of variable winds, frequently reversing 

 in direction. The length of time required for the full development of a wind current 

 is therefore important. This is affected by many factors; but Ekman's mathemati- 

 cal study with the measurements of wind and currents, which have been made at 

 lightships in various seas, makes it almost certain that only a few days are required 

 at the latitude of the Gulf of Maine. It is therefore reasonable to assume that 

 winds prevailing from a given quadrant of the compass for 50 to 70 per cent of the 

 time, such as actually blow over our gulf, are sufficiently constant in direction to 

 play a major r61e in governing the circulation of at least the upper stratum of water, 

 if not of the deeper levels. 



If, then, the water of the gulf were homogeneous, free to move in any direction, 

 and considerably deeper than the "frictional depth," moderate winds, blowing com- 

 paratively steadily from one general direction for a few days, should set the whole 

 upper 45-90 meters in spiral. Actually, however, the vertical stability and generally 

 stratified state of the water of the gulf tend greatly to limit the depth to which 

 wind currents may be expected to penetrate downward. 



The angular deviation of the wind current from the direction of the wind may 

 also differ widely at sea from the theoretic expectation. If the depth of water be 

 less than the frictional depth, the angle will be less; and while this limitation does 

 not affect the development of wind currents in the basin of the gulf, it does affect 

 the coastal belt out, say, to the 40 to 50 meter contour. The vicinity of the coast 

 line, with the contour of the bottom, also governs the directions which surface drifts, 

 set in motion by the wind, must actually follow. The effects of these influences 

 have also been attacked mathematically by Ekman (1905); but, as Kriimmel (1911, 

 p. 469) has emphasized, so many variables, which can not be exactly measured, 

 enter in that the surface currents which the wind has actually been found to set up 

 in other coastwise localities, in comparable latitudes, still afford the best available 

 indication of what is to be expected in the Gulf of Maine. 



