2. The Coriolis force, directed perpendicular to the drift and proportional to the drift speed 

 and the mass of the ice. 



3. The friction between ice and water. 



4. The internal resistance of the ice caused by the collision of individual ice floes moving 

 differently. Sverdrup assumes that this force is proportional to the drift and acts in a direction 

 opposite that of the drift. 



Two approaches may be used to solve the problem. 



First, the ice may be regarded as a thin film which moves together with the surface waters. 

 Consequently, the mass of the ice and therefore the Coriolis force acting on the ice can be ignored. 

 In this case, the problem is solved by determining the elements which cause the water-ice friction. 

 This force may be calculated from the wind speed, because in the case of steady motion the three 

 forces, air-ice friction, ice- water friction and the internal resistance of the ice should balance. 

 Sverdrup used this approach to analyze Brennecke's observations in the Weddell Sea on the drift 

 of thin scattered ice . 



Using the second approach, one may neglect the force of the ice-water friction, in other 

 words one may neglect the mass of the wind current layer. Sverdrup used this method to analyze 

 his observations made on the Plaud expedition for the drift of the close-packed and relatively thick 

 ice of the East Siberian Sea. Here the internal resistance of the ice was great. It reduced the 

 drift and rendered the ice- water friction negligible. 



Sverdrup introduced the internal resistance of the ice into the examination to explain why the 

 drift angle of the ice was smaller, according to his observations, than that required by Ekman's 

 theory. However, introduction of the resistance force should also have involved a reduction of 

 the speed of the wind drift or, in other words, of the wind factor, but the observations showed 

 otherwise. 



Figure 142 shows the results of Brennecke's observations of the wind drift of relatively thin 

 (about one meter thick) and scattered ice fields in the Weddell Sea. These observations show the 

 connection between the wind factor (dashed line), the drift angle (solid line) and the wind speed. 

 From the figure it is evident that the drift angle decreases with increasing wind speed, while the 

 wind factor remains nearly constant with increasing wind speed. 



Let us also note here that Brennecke's observations show that the wind current affected only 

 a very thin layer of underwater ice. At a depth of just 2 m, the current deviated 19° from the ice 

 movement, and the velocity of the wind current was only 58 per cent that of the ice drift. At a 

 depth of only 25 m the wind drift was practically 0. 



0.04 



0.03 



0.02 



0.01 



0.00 





40" 

 30' 



20° 



10 



o Figure 142. 



10 



The relationship between the wind speed in 

 m/sec (lower scale), the drift angle of the 

 ice (scale on the right) and the wind factor 

 (scale on the left) in the Weddell Sea. 



387 



