516 



Currents in a Strait 



current direction (level of no-horizontal motion). The latter surface can be found 

 either above or below the boundary layer between the two water bodies, but the 

 height-difference is never large. This phenomenon has been observed in the Bosphorus, 

 the Dardanelles and in the Strait of Gibraltar. As has been shown theoretically 

 (NoMiTSU, 1927) the two layers can coincide only when the water bodies are com- 

 pletely homogeneous. Deviations from such a state are due to mixing processes 

 occurring at the boundary surface between the two oppositely moving currents, 



/, "C S, %o i^, cm/^ec 



10° 12° 14° 16 24 32 40 20 ^ 60 100 140 



Fig. 237. Vertical curves of temperature, of salinity and of the current for always a single 

 station in the northern and southern Bosporus. Full lines: station 110 in the section 

 Karibdian Burnu-Porias Burnu, 12 May 1918. Dashed lines: station 123 in the section Orta 

 Koi-Istawros, 23 May 1918. At the current curves the arrows in the current are situated 

 such that: west, towards left; east, towards right. 



The currents within a strait are completely known only in the Bosphorus and in 

 the Dardanelles where accurate current profiles have been obtained by Merz (Moller, 

 1928). For other sea straits current measurements have been made for short time 

 intervals only and at few stations. In general, the greatest velocity in the upper current 

 is found close to the sea surface. In a cross-section the current distribution corresponds 

 to that of a river in which the lines of equal velocity (isotachs) follow approximately 

 the river bed. Due to the wedge-form of the upper water body the velocity increases 

 in the direction of the current (in the Bosphorus and in the Dardanelles from 50 to 

 150 cm/sec). The transition from the upper to the lov/er current does not occur dis- 

 continuously but increases in sharpness as the density jump becomes greater. This 

 phenomenon is also due to the turbulence of the current, which is strongly suppressed 

 when there is a large density discontinuity in the vertical. When the depth of the water 

 is not too great the lower current follows the bottom topography of the sea strait, 

 and therefore the cores of the upper and the lower current need not lie exactly above 

 each other. The vertical current distribution in the lower current shows a maximum in 

 its central core situated about half way between the current reversal layer and the 

 bottom. For continuity reasons a decrease in velocity occurs in the lower current if 

 the depth increases along the strait; however, if the depth decreases there will be a 

 corresponding increase in velocity. At the bottom the velocity may be so intense that 

 it causes considerable erosion. The occurrence of rolls in a depression of the sea bottom 

 (Koike, deep hole) may simulate a decrease of the velocity to zero near the bottom 

 (see p. 390). 



