Currents in a Strait 527 



precipitation over evaporation (Witting, 1918). This inflow of fresh water disturbs 

 the equilibrium between the North Sea and the Baltic and gives rise to a water inter- 

 change with an upper current flowing towards the North Sea and a lower current flowing 

 into the Baltic. Knudsen's relations (see Chap. XII. 5) aff'ord an estimate of the water 

 interchange balance. It appears from this that the inflow due to the lower current over 

 the rise on the west side of the Arkona basin is equal to the inflow of fresh water into 

 the Baltic and that the outflow in the upper current is twice as great. Detailed data 

 indicate that the decrease in the water amount being carried by the lower current 

 between the Skagerrak and the Baltic is opposed by a corresponding increase in water 

 amount carried by the upper current. Therefore important mixing processes must 

 always act within the sea straits. 



Calculation of the proportion of water with a salinity of 33''/oo '" ^^^ lower current (see Table 

 above) shows that until the Fomas section, not less than 67% of the water entering the Kattegat 

 has mixed the water of the upper current and that almost ?0% of the remainder mixes with the upper 

 water before reaching the Arkona basin. Thus only 1° „ of the water of 33°/oo salinity entering the 

 Kattegat in the lower current finally enters the Baltic. The remaining 93 % mix with the upper water 

 and return to the Skagerrak. In the same way a large part of the upper water mixes with the lower 

 current and is carried again towards the Baltic. About a third of the water leaving the Baltic in the 

 upper current w est of the Arkona basin returns to the Baltic and not less than two-thirds of the water 

 in the under current flowing into the Baltic over the rises has come from the Baltic itself, and only 

 one-third is the water witha salinity of 33700 that flows into the Kattegat in the lower current (Schulz, 

 1930). 



This applies only for the annual means. For the investigation of the water interchange in individual 

 months the assumption of a constant water amount in the Baltic is no longer valid, since the water 

 level shows an annual variation and other shorter oscillations. In some months the outflow from the 

 Baltic is stronger and in others less. Investigations by Witting for the period 1 898 to 1912 indicate that 

 there are pronounced maxima in fresh-water outflow from February to June, as well as in September. 

 A detailed treatment of the data on currents in the Oresund and the Belts recorded between 1910 and 

 1916 by Danish and Swedish light-ships has been made by JACOBSEN(1925),who foundforthe period in 

 question good agreement with the annual variation in water outflow from the Baltic found by Witting. 



In water interchange processes two phenomena must be distinguished. The first is 

 the orderly steady water interchange that takes place in a strait connecting two seas of 

 different thermo-haline structure. This interchange is associated with the two currents 

 which are essentially antitryptic flowing along an inclined boundary surface. In addition 

 to this continuous steady water interchange there is a second phenomenon, the total 

 displacement in both directions of the entire water mass of the strait by the wind or 

 due to differences in atmospheric pressure. In the Bosphorus and the Dardanelles 

 these meteorological influences are of minor importance in comparison with the regular 

 thermo-haline water equalization, but in the connecting straits between the Baltic and 

 the North Sea conditions are reversed. Here the piling up of water by the wind 

 (" Windstau ") and by atmospheric pressure differences is so strong that the regular 

 steady interchange currents are almost completely masked. The main phenom^enon is 

 thus an irregularly occurring, occasional transport of the whole water mass in its 

 total vertical extent more or less in the same direction in spite of its pronounced 

 vertical stratification. The regular steady interchange can only be obtained by elimina- 

 tion of these irregular movements which can be achieved by taking mean values over 

 long periods. Strong tidal effects are also present and must be eliminated by a harmonic 

 analysis. Mean values have been calculated by Jacobsen (1909, 1912, 1913), and the 

 mean structure at four different stations is shown in Table 144. 



