514 



Currents in a Strait 



the White Sea and the Barents Sea with the so-called Gorlo and the Gulf of St Lawrence 

 connected with the Atlantic by the Cabot Strait and others. 



The interchange currents in all these sea straits occur characteristically on two 

 different levels; there are always two currents in the strait, one above the other. The 

 upper layer always flows toward the sea having greater density, the lower layer in the 

 opposite direction, and between them there is usually a well-developed discontinuity 

 layer in the density field (see Pt. I, pp. 133 and 182-184 (Figs. 56, 83-85) on the general 

 distribution of temperature and salinity in sea straits). Thus in straits of moderate 

 width there are always two water bodies one above the other with a boundary layer 

 between them sloping down from the sea with the greater density towards that with 

 the lesser. The wedge-form of these superimposed water layers along the strait is a 

 characteristic feature of the structure of the water masses in a sea strait. Table 1 38 gives 

 a summary of mean density in the upper and lower water layer and of the slope of the 

 boundary layer for some sea straits in different climatic regions (Vercelli, 1929; 

 MoLLER, 1931). The greater the slope the smaller the density difference, i.e., the 

 slower the interchange movements. In addition to this effect of the density differences 

 other circumstances also control the slope of the boundary layer, particularly the 

 bottom topography of the sea strait, because it affects the continuity requirement of a 

 complete balance between the mass transport in the upper and lower current under 

 stationary conditions. For example, in the Bosphorus, the slope of the boundary layer 

 is strongly dependent on the bottom inclination and because of this the wedge-form 

 character of the lower water is lost there. 



Table 138. Mean slopes of the boundary layer and mean densities of the 

 upper and lower water in several sea straits 



Besides this longitudinal slope there should also be a transverse slope of the boundary 

 layer due to the effect of the Coriolis force. The faster the currents and the wider the 

 strait the greater will this slope be. If the upper homogeneous water mass in the 

 strait has a velocity u^ and the lower one a velocity lu, and if the transverse inclination 

 of the sea surface is given by 8l,^jdy and that of the boundary surface between the upper 

 and the lower current by ^i^i^y^ then, under stationary conditions the equations 



g-^=-fih and 

 will be valid where (/= 2aj sin </•). 





f 



PoUo — piUi 

 P2 — Pi 



(XVI. 1) 



