176 MARION EXPEDITION TO DAVIS STRAIT AND BAFFIN BAY 



Gulf Stream (immediately north of the United States-Europe 

 tracks), is able to intelligently follow the devious movements of 

 icebergs in this critical area and thereby more effectively protects 

 approaching steamships. The critical area shown on Figure 112, for 

 tlie duration of the ice season, is the one always under the ice 

 patrol's most watchful guard. When the southern terminus of the 

 icebergs has been thoroughly searched by the ice patrol, or during 

 a light ice year, then, and only then, the patrol ship is free to scout 

 northward to find the next southernmost threatening ice. 



The current maps obtained by dynamic surveys have been found 

 to remain reliable for a period of 7 to 15 days around the Grand 

 Bank. Minor fluctuations of short duration have often been ob- 

 served, however, especially along the boundary of mixed waters and 

 Gulf Stream. Such swirls or vortices appear to be secondary super- 

 ficial tongues 5 to 10 miles in width and sometimes several times that 

 in length and the tracks of icebergs, especially the small shallow- 

 draft ones, are often modified by such departures. In this connec- 

 tion, Ricketts (1930, p. 94) calls attention to the value of surface 

 isotherms in predicting the behavior of icebergs. 



Summarizing, the two methods now employed to safeguard North 

 Atlantic commerce from the great dangers incident to Arctic ice 

 are: (a) Prescribed tracks laid south of the normal ice zone; and 

 (b) a continuous ship patrol south of Newfoundland during the 

 ice season. 



Seasonal Character of the Ice Invasions to the North Atlantic 



The four principal factors controlling the seasonal incursions of 

 ice into the North Atlantic (plus several minor ones) are wind, 

 current, pack ice, and winter temperature. They show an inter- 

 relationship so complex that each must be considered in connection 

 with all the others. 



In ice regions where steep atmospheric gradients prevail for a 

 month or more at a time, the resulting steady winds may pile up 

 large masses of water against shore lines, or against other 

 hindrances, and thereby establish a deep, slope current. '° A change 

 in the character of the winds, due sometimes to the succession of 

 winter and summer, may result, therefore, in seasonal variations in 

 the size and velocity of the related ocean current. 



A study of atmospheric conditions the year round shows that 

 during the colder months when high pressure spreads over North 

 America the northwestern North Atlantic is dominated by a strong 

 northwesterly air stream. The exchange of temperature character 

 l)etween the land and the ocean, accompanying the increase of insola- 

 tion in spring, calls forth a similar exchange of atmospheric pressure, 

 and the northwesterly winds are replaced in spring by those from the 

 southwest as well as variable ones. Thus the northwesterly winds 

 from December to March amass great quantities of the surface water 

 of Davis Strait against the 1,000-mile continental slope, from Baf- 

 fin Land to the Grand Bank. Although the direction of the wind 

 trends slightly off the coast, the deflecting effect of the earth's rota- 



■" An excellent exposition of currents established by winds is contained in Eknian (1928), 

 to wliich the reader is referred. 



