Currents in a Strait 515 



In Fig. 236 which shows a cross-section through the strait, u^ (upper current) is 

 positive and Mg (lower current) is negative, and as a consequence the sea surface rises 

 to the right while the boundary surface between the two water types slopes downward 

 to the right. This latter inclination is considerably steeper than the first. For a certain 

 definite velocity ii^ the water mass of the upper current may be too small to cover, 



JIv 





Fig. 236. A model in order to study the thermo-haline circulation in sea straits. 



during its displacement to the right, the whole of the lower water mass. Choosing in 

 equation (XVI. 1) 



^ = 4^ = - ^«i, (XVI.2) 



/= 10-^ g = 1000 and u^ = 100 cm/sec, then Ah = 10-^ X L, where Ah is the 

 elevation of the water level for a given width L of the strait. For the Dardanelles 

 L = 5 km and therefore Ah is 5 cm; in the Strait of Gibraltar L = 20 km and Ah is 

 20 cm. The latter value is already quite large. For quite a large width of the strait the 

 inclination may be so steep that in a narrow strip along the coast the opposite moving 

 heavier water may rise to the surface, so that in the strait at the sea surface there will 

 be a front with currents flowing in opposite directions on either side. In narrower 

 straits transverse slopes of this sort will be barely detectable. 



The internal structure of both water bodies is usually stratified; however, this 

 stratification is only slight in salinity, but at time it may be pronounced in the tempera- 

 ture. In all cases in low-salinity seas where the access depth to the strait is deeper than 

 the discontinuity layer, due to increased radiation in summer, a temperature inversion 

 will be formed within the upper current with a minimum above the boundary surface ; 

 below this in the lower water a secondary maximum appears and then the temperature 

 will decrease again to the bottom value. Figure 237 shows two vertical temperature and 

 salinity curves of this type for the northern parts of the Bosporus. The temperature 

 minima always decrease in the direction of the surface current due to the effect of 

 mixing. 



The discontinuity layer in the salinity remains fairly sharp along the total length of 

 the strait, though in each of the water bodies the absolute values will change somewhat 

 due to mixing: in the upper water body the salinity will therefore usually increase and 

 in the lower it will decrease. The changes in the Bosphorus and the Dardanelles are 

 thus over 300 km about 10%o. Similar values have been found in the Danish sounds 

 (Belts). The greatest changes are, of course, usually found where there are large 

 irregularities in the bottom topography where eddies and vortices are generated. 



As a further characteristic phenomenon found in sea straits the boundary layer 

 between the water bodies often does not coincide with the level of reversal of the 



