528 Currents in a Strait 



Table 142. Mean currents in the Oresund and in the Great Belt (cm/sec) 

 ( + , directed towards the North Sea; — , directed towards the Baltic Sea) 



The lightship "Lappegrunden" hes in the most northern part of the sound, the 

 lightship "Drogden" lies in the central part and the Schultz Grund lightship lies in 

 the southern part of the Kattegat at the entrance to the Great Belt. For larger depth 

 the mean upper current is directed towards the North Sea and the lower current towards 

 the Baltic. The current profile corresponds rather well to that deduced theoretically. 

 In the shallow waters of the Oresund ("Drogden") the entire current from the surface 

 down to the bottom is directed towards the North Sea. As shown by Jacobsen, the 

 internal field of force is very weak here and the great width of the channel permits 

 cross-circulations to play an important part. 



This steady water interchange produced by the internal field offeree is [superimposed 

 on the strong currents] almost always present in this area; which are produced by 

 differences in level between the Kattegat and the southern part of the Baltic due to the 

 piling up of water by the wind and due to differences in atmospheric pressure. These 

 are also antitriptic slope currents and give rise to displacements of the internal front 

 between the water bodies which here are situated side by side (Skagerrak and Baltic 

 water) (see Pt. I, p. 182, Fig. 85). Wattenberg (1941) has made a detailed investigation 

 dynamics of the displacement of these fronts and of the duration of the movements, 

 and has given a basis for the estimation of mixing in the Belts and of the flow of North 

 Sea water into the Baltic. There is a close correlation between the flow through the 

 Great Belt (computed by means of lightship current measurements) and the changes in 

 salinity. A rather close connection exists also with the meridional pressure gradient. 

 The duration of inflow and outflow periods changes, of course, according to the varia- 

 bility in the all-over weather situation over wide limits; the long-period variations are 

 well shown by cyclic variations in the salinity, since the inertia of the water masses 

 weakens or even completely suppresses the smaller shorter-period disturbances. 

 Extreme positions of the internal fronts are due to prolonged inflow and outflow 

 periods (Fig. 244). In the north the front may reach out from the Belt Sea as far as 

 into the middle of the Kattegat. Towards the south under reversed conditions the 

 front may in extreme cases reach the Darsser and the Drogden rises separating the 

 Baltic from the Danish sounds. This difference in behaviour to the north and to the 



