108 



BJ0RN HELLAND HANSEN 



[REP. OF THE "MICHAEL SARS" NORTH 



result first obtained corresponds to tiie swing of the ship 

 early in the morning (30. April) but it is possible that 

 the diurnal variations have a decisive influence upon 

 the direction of the turn. 



If the average variation of the current in 12 lunar 

 hours (represented by the broken lines in Fig. 42) is due 

 to diurnal tidal variations these must be very important 

 and able of themselves to cause a current with a maximum 

 velocity of at least 43 cm. /sec, but probably more. In this 

 connection I may refer to the results arrived at in section 26. 



coast and the outlet of the fjord, thus temporarily hindering 

 the outward movement of the surface water of the fjord. 

 Similarly, variations in the air-pressure and winds on both 

 sides of the Straits of Gibraltar must cause variations of 

 the currents in the Straits. I do not possess the necessary 

 observations for a further study of the variations of these 

 currents and cannot therefore decide how far the above- 

 mentioned increase of the surface current in the Straits 

 during the time of observation is caused by meteorological 

 changes or diurnal tidal variations. 



W^ 



Fig. 43. The current at Stat. 18, 9 metres. 



The average variation in question may, however, also 

 be caused by changes in the meteorological conditions. 

 The author has found that some great variations in the 

 surface current usually running seawards in the Sogne- 

 fjord seem to agree with variations in the mean air- 

 pressure gradient over the North Sea outside the fjord. 

 The wind blows nearly at a right angle cum sole from 

 the direction of the pressure gradient. The wind, on its 

 part, creates a wind current which causes an average trans- 

 port of the upper water-layers at a right angle cum sole 

 from the wind, /. e. in a direction contrary to that of the 

 air-pressure gradient. Current measurements were made 

 in the Sognefjord during 60 hours in July 1929, and on 

 that occasion the surface current happened to slacken very 

 markedly in the course of 20 — 30 hours which could not 

 be explained by the local wind conditions. Shortly before, 

 however, the mean air-pressure gradient over the northern 

 North Sea veered from north to west, whereby the surface 

 water was probably driven more and more towards the 



We have seen that occasionally the upper current may 

 turn and run westwards owing to the tidal streams. Our 

 observations show no turning of the under-current at, for 

 instance, 200 metres, but great variations in its velocity. 

 The surface current (or the upper current) and the under- 

 current are naturally defined in accordance with the rest- 

 current at different levels. We are not able to calculate 

 the rest-current except at 9 metres, and even there only 

 approximately. We may, however, use the vertical distribu- 

 tion of salinity as a criterion of the vertical extension of 

 the two currents. 



Vertical series of observations on salinity and tempera- 

 ture were made at four different times at Stat. 18 (Table 

 II, pp. 22* and 23*). The first series, 18A, which is the 

 most complete one, is used for the more detailed records 

 in Table 111 (p. 35*) and the curves reproduced on p. 66*. 

 The stability was very pronounced in the upper 75 metres, 

 and especially between 25 and 50 metres, where the verti- 

 cal variation of salinity was great. The salinity was a little 



