PHYSICAL OOEANOGKAPHY OF THE GULF OF MAINE 969 



prevailing winds (and the strongest winds), between west and north, drive the 

 surface water offshore to the southward. 



By this reasoning wind currents go far to explain the very interesting fact that 

 in April the freshening effect of the spring freshets is so much more evident (in low- 

 ered salinity at the surface) along the coast sector west and south of the Kennebec 

 than it is off Penobscot Bay (fig. 101 ). The discharges from the former,from the Saco, 

 and from the Merrimac, driven southward by the prevailing northwesterly winds of 

 March and April, parallel the trend of the coast and so preserve the identity of the 

 coastwise belt of low salinity. Off Penobscot Bay, however, the more or less active 

 upwelling that must follow this same southerly drift off this west-east coast line, 

 combined with tidal stirring, tends to prevent the development of so fresh a band 

 next the land, but at the same time to carry the least saline water farther out from 

 the land. The distribution of salinity at the surface for March and April, 1920 

 (figs. 91 and 101), is of this sort. 



It is probable that the development of a tail of very low salinity from the St. 

 John River southward across the Bay of Fundy in April (p. 808) similarly reflects a 

 southerly set caused by the northwest winds, which often blow strong there during 

 the first month of spring, though their average direction veers through west to 

 southwest during April. 



The pool of low-surface salinity spreading out to the southwest from Nova 

 Scotia, which appears on the surface chart for March, 1920 (p. 703; fig. 91), like- 

 wise finds plausible explanation as a wind-driven drift out from the bays south of 

 Yarmouth, where northerly winds prevail in February (p. 966). 



The effects of the winter winds are more puzzhng in the eastern side of the basin 

 of the gulf, where prevailing west-north winds tend to produce a southeasterly or 

 southerly drift at the surface, but where the evidence of salinity and temperature 

 points to a movement in just the opposite direction — i. e., northerly toward the Bay 

 of Fundy in winter as well as in. summer (p. 910). 



It is evident here that although strong northerly winds may and no doubt do 

 temporarily drive the surface water southward, the general dominant drift is caused 

 not by the wind but by other forces (p. 976) strong enough to overcome the wind 

 effect in the long run. Consideration of the depth to which wind currents may be 

 set in motion corroborates this conclusion, because the frictional depth of the average 

 winter wind of about 4, on the Beaufort scale, is theoretically only about 67 meters. 

 Actually, the water of the eastern side of the gulf not being homogeneous, the depth 

 of the wind current will be something less than this — perhaps 50 meters with the 

 state of stability prevailing in winter. The thickness of the stratum which the wind 

 can set ia motion at an appreciable rate is still less. 



According to the long series of observations on wind and current that have been 

 carried out by the United States Coast and Geodetic Survey, the velocity of the wind 

 current is 1.5 to 2 per cent that of the wind — say, about 0.4 knot, with a wind of 4 

 (Beaufort scale, 20 nautical miles per hour). Smith's table of theoretic velocities 

 (Smith, 1926, p. 46, Table 8), applied to a current of this strength with assumed 

 frictional depth of 50 meters, gives a residual current of only 0.2 knot at a depth of 

 10 meters, about 0.15 knot at 20 meters, and 0.07 knot at 30 meters. Theoretically 

 (in a free ocean), in the example just stated the current at 10 meters should set 36° 



