Currents in a Strait 523 



However, the stratification does not appear to be of decisive importance to the prin- 

 cipal phenomena of the water interchange and therefore the simple case of two 

 homogeneous water types gives the essential outlines. 



3. Ocean Currents in Individual Sea Straits 



{d) Bosphorus and Dardanelles 



Due to the investigations of Merz and Moller (1921, 1938, with Atlas) these are 

 the straits in which conditions are best known. Systematic surveys along cross-sections 

 and longitudinal sections have given a good understanding of the three-dimensional 

 thermo-haline structure of the water masses and the corresponding currents in both 

 straits and some insight into the detailed mechanism of the processes involved. Over 

 the whole area of water interchange between the Aegean and the Black Sea there is a 

 characteristic stratification with a sharp density transition layer. From a depth of 

 200-1 50 m in the Black Sea it rises at the entrance into the Bosphorus to less than 1 50 m 

 and in the narrow part it rises rapidly to 20-15 m at Istanbul. It remains at this depth 

 throughout the Marmara Sea until it rises again in the Dardanelles, at first very 

 slowly, then more rapidly in the straits between Nagara and Tschanak to 10 m. 

 At the southern entrance to the Dardanelles it reaches almost to the surface. Figure 241 

 presents the density distribution in two longitudinal sections along both straits. 



The wedge-form of the upper water shows clearly in both straits ; in the lower water 

 it is present only in the Dardanelles, since the sea bed in the Bosphorus slopes down- 

 wards towards north as much as the internal boundary surface. At the entrance to the 

 Bosphorus the salinity of the upper water is 16-18%o and at the outlet from the Dar- 

 danelles into the Aegean it is 26-28%o. Of this increase 2%o occurs in the Bosphorus, 

 5%o in the Sea of Marmara and 3%o in the Dardanelles. Mixing in the straits thus can- 

 not be very effective ; this is also shown by the maintenance of the temperature in- 

 version which is still partly present in the Dardanelles (see Fig. 237). 



The upper current runs through the channels as a narrow band within limits set by 

 the projections of the coast. In several coastal bays on both sides of the straits numerous 

 standing vortices occur. The current profile shows that the velocity is greatest at the 

 sea surface and decreases rapidly with depth. Due to the wedge-form of the current it 

 increases from north to south; under average conditions it is 40-50 cm/sec at the 

 entrance to the straits and 1 50 cm/sec or more at the other end. 



The lower current follows the windings of the channel more closely than the upper 

 current and the stream lines of the two currents are therefore not always super- 

 imposed. The lower current is strongest in the central parts of the lower water (in the 

 Bosphorus about 16 m and in the Dardanelles about 45 m above the bottom). The 

 velocity is 100-150 cm/sec in the Bosphorus and decreases from 25 to 10 cm/sec in the 

 Dardanelles. 



In the straits the boundary surfaces between different currents and between different 

 water types do not coincide ; the first rises from north to south more slowly than the 

 thermo-haline transition layer and they intersect at the narrowest part of the straits. 

 Thus in the northern part of both straits upper water flows with the lower current and 

 in the southern parts lower water returns with the upper current. The changes in the 

 currents due to variations in wind and atmospheric pressure are pronounced. During 

 strong north-east wind the surface current is accelerated, the current core thereby 



