■^■W 



+JZ i-M 



■I -zr 



Figure 121. Semidiurnal tidal currents in the Laptev Sea at 75° 26' north, 

 122° 20' east, 7 to 9 February 1938. 



3 . Tidal movenaents of ice and tidal currents at a depth under the ice do not coincide in 

 direction and sometimes may be directly opposite. 



LITERATURE: 77, 120. 



Section 125. Pressing Together and Thinning Out of Ice 

 Due to Tides 



As has already been noted, tidal movements of ice cause pressures forcing the floes together 

 or driving them apart. 



Nansen notes in his diary, on 13 October 1893, that arctic explorers have repeatedly empha- 

 sized the connection between compression of ice and the tides. He states that according to obser- 

 vations of the Fram the tidal compression was especially noticeable in the spring tides and was 

 greater at new moon than at full moon. Ehiring the course of 24 hours the ice was twice pulled 

 apart and twice pushed together. In the neap tides the tidal pressures were nearly imperceptible. 

 In addition, Nansen shows that the tidal pressures attained their greatest force and regularity at the 

 start of the drift, near the open sea to the north of Siberia, and at the end of the drift when the 

 Fram. was approaching the Greenland Sea. In the Arctic Basin proper these phenomena were 

 imperceptible. Exactly similar occurrences were noted on the Sedou during its drift across the 

 Arctic Ocean. 



It must be noted that the increased tidal pressures at the start of the drift of the Fram and 

 Sedou are properly explained not so much by their proximity at that time to a sea free of ice, as 

 by the fact that they were then over the continental slope, where the tides are more sharply defined 

 (due to sharp change of depth) than they are over the level depths of the ocean. * 



Figure 122 shows the movements of particles along the vertical and the outer form of a 

 regular tidal wave. From examination of the figure it may be seen that at point a , where the 



*Tidal pressures in regions of strong tides present a considerable danger to navigation. For 

 example, on 7 February 1944 the steamship Mosta , leaving Nagayevo Bay accompanied by a line 

 icebreaker, was crushed by the ice at 58° 26' north, 151° 26' east. The winds at this time were 

 weak but there were spring tides. It is well known that the amplitudes of the tide are very great 

 along the northern shore of the Okhotsk Sea. For example, in Nagayevo Bay the amplitude of the 

 tide reaches 4.3 m and in Penzhinskaya Guba it increases to 11.3 m. 



338 



