Currents in a Strait 



517 



A special phenomenon found in the current systems in irregular shaped straits is 

 the occurrence of stationary vortices with vertical axes. They are used with considerable 

 advantage in navigation. In the Bosphorus and in the Dardanelles they are well 

 developed in both surface and bottom currents (Fig. 261). In some cases two stationary 

 vortices are formed side by side with an opposite sense of rotation so that side branches 

 ("neer" currents) develop returning later into the direction of the main current. 



Table 142 gives an idea of the very large amounts of water passing through major 

 sea straits. In broad and deep straits, such as the Straits of Gibraltar and Bab el 

 Mandeb, the transport may be 5 to 20 times greater than in narrow shallow straits. 

 Expressing the amount of inflow or outflow by means of water-level change (in mm) 

 of the total Mediterranean, a particularly clear idea of the great difference between 

 the humid and the arid, semi-arid areas, respectively, is obtained. 



2. Theory of Currents in Sea Straits 



The dynamic cause of currents in sea straits lies in the density difference between the 

 open ocean and the enclosed sea, or more exactly, in the density difference at the 

 level of the bottom of the strait between the entrance to the strait and its outlet. The 

 thermodynamic mechanism inherent in this circulation can be demonstrated by a 

 simple model (Defant, 1955) (Fig. 238). In a strait with a depth h limited at ad and 



-i-h- 



FiG. 238. Cross-section through a rectangular sea strait and its current system. 



be at either end by two water columns belonging respectively to the ocean and the 

 enclosed sea, the system will be made up of horizontal layers of water 1 and 3 between 

 ah and cd, respectively, and vertical columns 2 and 4 between be and ad, respectively. 

 We assume a stationary state and neglect — because of the narrow strait — the effect 

 of the Coriolis force. The currents which adjust under stationary conditions must be 



Table 139. Water transport through sea straits {according to Moller) 

 ( + , current from the adjacent sea; — , current into the adjacent sea) 



