36 



BJ0RN HELLAND-HANSEN 



[REP. OF THE "MICHAEL SARS" NORTH 



It does not seem worth while making these variations 

 the subject of a closer analysis, it would be necessary 

 to develop the values in question in series with many 

 terms and the results would be very problematic, as the 

 observations are not sufficiently continuous. Important 

 investigations might evidently be made by means of ther- 

 mographs registering at different levels. 



28. Causes of the Variations. 



It has been repeatedly emphasized in the preceding 

 sections that our observations are too incomplete for a 

 satisfactory study of short-period variations in the sea. 

 But although the discussion has to be hypothetical the 

 observations and the calculations described in this chapter 

 may be utilized for a preliminary judgment of the prob- 

 able causes of such variations. The present lengthy treat- 

 ment is justified only from the point of view of trying 

 to attain a rational working hypothesis in a case where 

 satisfactory observations do not exist at all. The only 

 certain fact is that important variations in the vertical 

 distribution of various elements sometimes do occur in the 

 sea within short intervals of time. 



As far as our observations and calculations go we 

 may, then, make the following statements: 



1. The variations in temperature are, to a great extent, 

 of really periodic, diurnal and semi-diurnal, character. 

 The diurnal variations below the surface in the deep 

 sea are probably of a greater magnitude in proportion 

 to the semi-diurnal, than they are in the rise and fall 

 of the tides at the surface along the neighbouring 

 coasts. 



2. The phase of the variations is more or less variable 

 from level to level at a fixed station. 



3. The phase is more or less variable in a horizontal 

 direction, even within fairly short distances. 



4. The amplitudes of the variations are upon the whole 

 independent of the vertical gradient of temperature 

 (or the stability). 



If correct, these statements lead to important conclu- 

 sions with regard to the dynamics of the oscillations 

 creating the variations. The oscillations may be caused 

 by variations in the currents as well as by waves of dif- 

 ferent kinds. Before proceeding to a discussion of the 

 various possibilities we must remember the following 

 facts, already touched upon above (section 20): 



In a sea with no currents, but with vertical variations 

 in the physical properties, the isopycnal surfaces will form 

 level surfaces, and the same thing will practically always 

 be true of the isothermal and isohaline surfaces. This static 



equilibrium is disturbed when a current appears, and the 

 more so the stronger the current is. Owing to the rota- 

 tion of the earth (a Coriolis' force) the isopycnal, isother- 

 mal and isohaline surfaces then assume a slanting position, 

 whereby a field of force (a ''solenoid field") is created. 

 The maximum obliquity of the said surfaces will appear 

 in a section transversely to the direction of the current. 

 On the other hand, a solenoid field formed primarily (for 

 instance by local heating, or supply of river water) will 

 create a current. It will take some time, but probably 

 only a short period of some hours for a dynamic equili- 

 brium to be established between the Coriolis' force and 

 the solenoid forces. The latter will in some cases accele- 

 rate and in others retard variations in the currents. 



It. Variations in the currents. 



We shall here consider the convection (or gradient) 

 currents and the tidal currents only, disregarding the pure 

 wind-currents in the surface layers. To begin with we 

 shall also leave out of account possible formations of 

 internal waves as a result of vertical variations in the 

 velocities of the currents, and will now only discuss the 

 primary effect of the currents. 



The convection currents may have different velocities 

 and directions in different layers or at different stations, 

 but if this state of things remains unaltered and is not 

 subject to temporal variations, the horizontal and vertical 

 distribution of densities (temperatures and salinities) will 

 also remain constant except for other motions or agencies. 

 Assuming that the convection currents are practically con- 

 stant within a reasonable interval of time, we shall find 

 no short-period oscillations created by them. The currents 

 and the solenoid fields are subject to seasonal and annual 

 changes, but the resulting horizontal and vertical variati- 

 ons in the physical elements have a much longer period 

 than the variations now dealt with. 



Our knowledge of the tidal currents has been greatly 

 advanced by the observations of Professor Sverdrup in 

 the Polar Sea and the subsequent theoretical investigations 

 by himself and Dr. J. E. Fjeldstad. Sverdrup has shown 

 [1926] that the tidal currents at some distance from the 

 shore must, on account of the rotation of the earth, be 

 rotatory even if we have no interference of two or more 

 crossing tidal waves. Quite close to the shore the tidal 

 currents will go to and fro along a straight line, but fur- 

 ther out these currents will go in ellipses with a decreasing 

 excentricity as we proceed seawards. On account of the 

 friction against the bottom the direction of the major 

 axis of the ellipse as well as the maximum velocity and 

 the time when the latter occurs (the phase) will vary from 

 the surface to the bottom, in other words, the tidal cur- 



