General Theory of Ocean Currents in a Homogeneous Sea 



417 



density gradient during the summer gives rise to a strong differentiation in the magni- 

 tude of the frictional coefficients in a vertical direction v^hereby the angle of deflection 

 is reduced. 



Even more penetrating investigations have been made of the deflection angle in 

 shallow seas (lightship observations). These values have, however, mostly been made 

 in coastal areas or over large banks where disturbances can be expected but these 

 can be eliminated by special grouping of the data. According to the Ekman theory 

 there will be no strong deep currents in any largely enclosed sea (see p. 428). A com- 

 parison between theory and observation can then be made in such a case. For a shallow 

 sea (depth d) the theory requires the deflection to be smaller the smaller the ratio d.D. 

 On the other hand, the thickness D of the drift current will increase with increasing 

 wind strength. It can thus be expected that in a shallow e?iclosed sea, the angle of deflec- 

 tion will become smaller as the wind increases. From data on currents recorded by 

 Finnish light-ships, Witting (1909) found that the angle of deflection was always 

 cum sole and that it could be expressed by the relation 



a = 34° - 7-5 Vw, 

 where u' is the strength of the wind in m/sec. The strong ellipticity of the current ellipses 

 at the different lightships indicates a preferred current direction caused along the 

 longer axis of the sea which certainly affects the results. Qualitatively, however, it 

 corresponds fully to the requirements of the theory. Also Dinklage (1888) obtained 

 similar results from observations made at the Adlergrund light-ship (Baltic). 



The question of testing the Ekman theory has been discussed in detail by Palmen 

 (1930 b, 1931) in connection with an evaluation of the currents in the northern part 

 of the Baltic. This was based principally on observations made at the rather openly 

 situated Swedish lightship "Finngrundet" (60-0° N. 18-5° E. at the southern end of the 

 Gulf of Bothnia) for the period 1923-27. Tables 122 and 123 show clearly the relation- 

 ship between wind and current on the one hand for different wind strengths and on 

 the other hand for different wind directions. These correspond rather well to the 

 requirements of the theory. Especially the confirmation of the turn of the current 

 direction with increasing depth deserves our attention because only few observations 

 of that kind are available. After elimination of non-significant disturbances the 

 following corrected values are obtained for wind strengths of 4-5 Beaufort: 

 Vo = 9-2 cm/sec, ao = 35°, KgoiKo = 0-76; 



K,o = 7-0 cm/sec, ajo = 54°, Aa = 19°. 



Table 122. Currents at different wind strengths at the lightship ''Finngrundel'' (Gulf of 

 Bothnia, 1923-27) (according to Palmen) 



