mixing joins with frictional, and where heat, released to the atmosphere from the sea in winter is 

 greater than in the open sea, the isotherms are completely vertical, which is always a sign of 

 convective mixing. 



UTEEATURE: 47, 62, 65, 77. 



Section 31. The Index of Freezing 



In figure 17 the point of Intersection of curves S-^ and S - attracts our attention. This point 

 is significant in that vertical circulation may proceed to that level where this point is located, due 

 solely to a drop in the temperature of surface layers of the sea. For vertical circulation to pene- 

 trate deeper than this point, the surface layers must increase in salinity and accordingly, ice must 

 form. On the diagram, curves S^ and S intersect at a depth of 18 m. For this depth 



5m = 5. = 30.70»/oo, 



tm =8°.6, 



T = — 1°.7. 



From this we get 



im-r =10°. 3, 



qr = 0.\ {t^ — T)p^ = 18.5 KZ-KQAJCM'^, 



where p is the depth to which vertical circulation may proceed without the formation of ice, (in 

 the given example 18 m); I have called this depth the critical depth of vertical circulation, and q 

 is the amount of heat which must be released to the atmosphere during the process; 1 have called 

 this value the index of freezing of the sea. 



In the preceding paragraph we gave the complete procedure for computing vertical winter 

 circulation. Naturally, even with such a calculation, the computations should be made only to that 

 level at which the amount of heat released to the atmosphere during vertical circulation does not 

 exceed the total amount of heat released by the sea to the atmosphere in a given region during the 

 winter. 



If we limit ourselves only to a determination of the indices of freezing, it would be unneces- 

 sary to compute the elements of convective mixing from the surface of the sea all the way to the 

 bottom, but only to the level at which S-jn and S^ are equal. 



We have assumed that the vertical distribution of temperature and salinity at the oceano- 

 graphic station whose data has been given in table 23 changes only as a result of vertical winter cir- 

 culation. But let us assume that before the very start of this circulation, the layers from the sea 

 surface to a depth of 25 m, vnll be mixed by the wind. 



From table 23 it is easily seen that in this case the overall temperature of the mixed layers 

 will be 7.2° and the overall salinity 31. 13 o/oo, with a corresponding specific volume of 76.21. In 

 order to cool this 25 m layer to -1.7°, i. e. , to the freezing point, the sea must release to the at- 

 mosphere 22.3 kg-cal/cm . 



78 



