Chapter XVI 



Currents in a Strait 



1. Water Stratification and Water Movements in Sea Straits 



Sea straits connect the open ocean with mediterranean and adjacent seas. By means of 

 the water flux through the connecting straits directed towards the open ocean a medi- 

 terranean sea can often produce considerable effects on the oceanographic conditions 

 in the open ocean. This influence is sometimes so powerful as to involve entire parts 

 of an ocean, changing drastically the oceanic conditions in these parts. Present 

 knowledge of oceanographic conditions in sea straits is only partly satisfactory. The 

 main outlines and the typical features are known but much remains to be explained 

 especially in the details, that will require systematically arranged observations and 

 measurements. 



The continuous interchange of water between mediterranean and adjacent seas 

 which are completely surrounded by land and the open ocean is controlled very 

 largely by two factors : 



(1) by the proportion between fresh- water inflow (precipitation and run off (river 

 water and other water)) and evaporation in the mediterranean sea, and 



(2) by the depth and width of the passage to the open ocean, that is, the morphology 

 of the sea strait. 



The currents in a sea strait are a consequence of the difference in vertical thermo- 

 haline stratification between the water masses in the adjacent sea and that of the open 

 ocean off the entrance to the strait. 



Sea straits can be divided on the basis of the currents flowing in them into two 

 groups : 



(1) Those in which the adjacent sea is surrounded by arid land masses. Here 

 evaporation exceeds precipitation (E — P) > 0. The loss of water due to this excess 

 must be replaced from the open ocean through the strait. 



(2) If the entire oceanic area lies in a humid climate (E — P < 0), then the excess 

 of precipitation over run-off will flow out into the ocean through the connecting 

 strait. 



To the first group belong — inside the area of the Eastern Hemisphere rich in 

 evaporation and with little precipitation — the Strait of Gibraltar, connecting the 

 Atlantic with the high-salinity European Mediterranean; the Strait of Bab el Mandeb, 

 connecting the Indian Ocean (Gulf of Aden) with the highly saline Red Sea and the 

 Strait of Hormuz between the Arabian Sea (Gulf of Oman) and the Persian Gulf, 



To the second group belong — in the northern humid region — ^the weakly saline 

 Baltic Sea which is connected by way o^tiarrow belts and Sounds through the Kattegat 

 and the shelf-like North Sea with the open ocean; the predominantly humid Black 

 Sea connected with the arid Mediterranean through the Bosporus and the Dardanelles; 



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