TABLE 95. PHASE ANGLES OF THE PRINCIPLE SEMI-DIURNAL TIDES 



Thus, a preliminary computation of all elements of the tide requires a separate determina- 

 tion of the harmonic constants for a sea free of ice as well as for a sea covered with ice. 



LITERATURE: 54, 77, 102. 



Section 121. Influence of Tides on Freezing and Break-up of Sea 



The tides are distinguished from the permanent currents by their periodicity in respect to 

 speed and direction and by their great velocity gradients and the resulting extreme turbulence. 

 The latter, in certain regions such as narrow straits, results in a complete mixing of the sea 

 waters from the surface to the bottom which makes the tidal currents similar to river currents. 



But we have seen that the larger the vertical gradients of density in the surface layers, the 

 faster surface ice begins to form. From this it follows that the tides, by mixing the water masses, 

 as a rule retard the start of ice formation. However, the tides subsequently continue to break up" 

 the ice cover and thereby an intensified ice formation goes on in the open expanses of clear water. 

 Due to this the total quantity of ice is greater by comparison with the quantity of ice which would 

 form in the given region in the absence of tides . 



Great variations of tidal level are obsejrved in Mezenski Bay of the White Sea and especially 

 in the mouth of the Mezen River. For example, in the mouth of the Semzha River, a tributary of 

 the Mezen, the greatest amplitude of tide is 11.7 m and the smallest is 2.9 m. Due to this, ice 

 never forms on the Mezen below Mys Tolstik. With every flood tide the ice breaks up and hum- 

 mocks toward the mouth of the river; with every ebb tide the ice is carried into the sea where it 

 piles up on the coastal banks and shoals . 



Fast ice does not form around Morzhovets Island due to the tidal current entering Mozenski 

 Bay and rounding this island to the south and north. Branches of the current, meeting along the 

 eastern side of the island, create there a strong tide rip in the summer and considerable hum- 

 mocking and formation of stamukhi in the winter. In the straits of Matochkin Shar near Mys Uzki, 

 where the depth is25to20m and the speed of tidal current reaches 5 knots, a polynya situated in 

 the middle of the straits is quite typical. The polynya even lasts through the winter in particularly 

 warm years. In normal winters it closes over with ice and opens up again in the spring. 



Thus, in the period of ice formation the tides 



1. Retard formation of initial ice forms until the whole water layer which is mixed together 

 by the tides is cooled to the freezing point. 



2. Hinder formation of fast ice and large ice fields. 



330 



