440 



General Theory of Ocean Currents in a Homegeneous Sea 



According to the observational data the wind function F (w) can be approximately 

 given the form 



F{w) = b^/w, 



so that the wind stress T = bw^'"^. By this the results of Palmen are brought in mind 

 because they are in a way similar. The coefficients a and b thus like k and c characterize 

 the strength of the ice resistance and the effect of the wind. 



The seasonal changes in the relationship between wind and ice drift fit in well with 

 the above considerations. Table 129 shows these changes, together with the calculated 

 variations in the resistance coefficient and in the wind effect. Over the North Siberian 

 Shelf both the relative drift velocity and the angle of deflection show a pronounced 

 minimum in spring and a maximum in summer. This is partly due to the change in 

 the resistance coefficient k and partly due to the wind-effect c. 



The value of k increases gradually from a summer minimum until the first half of 

 the winter and then rises rapidly to a maximum at the end of the winter in order to 

 fall off again just as rapidly to the summer minimum. These variations can very well 

 be explained by the state of the ice cover during the year. In summer the ice resistance 

 is small due to the numerous open spots ("Wacken") and consequently greater free- 

 dom of movements for the ice. In autumn and at the beginning of winter these open 



Table 129. Seasonal changes in wind factor, angle of deflection, resistance coefficient 

 and the wind-effect on the ice drift 



* Minimum; f Maximum. 



stretches are covered with fresh ice, and the ice pressure increases the resistance until 

 a maximum resistance is reached at the end of the winter when the ice-cover is strongest 

 and most solid. The annual variation of the wind effect c is more complex. Sverdrup 

 was, however, able to show that it was in full agreement with the turbulent state of 

 the air movement over the ice. In the Weddell Sea also the seasonal changes in a and 

 b are completely analogous. The ice resistance shows, in general, an increase during 

 winter and spring, but the changes from month to month are more pronounced and 

 irregular because of the stronger changes in ice conditions of this broken cover. The 

 coefficient of the wind effect h follows a regular course with the lowest values around 

 the middle of winter and with an almost steady increase towards the end of winter. 



