Currents in a Strait 529 



south is due to the following facts. During outflow the upper water is not subject to 

 any resistance and may therefore spread out arbitrarily at the surface, while during 

 inflow the more saline water advances towards lighter water in front of it, and in this 

 case the bottom topography exerts great influence. On passing the rises in the south 

 the denser water sinks down to the bottom and the position of the front at the surface 

 remains fixed near the rise. In this way large amounts of highly saline water flow into 

 the basin of the Baltic thus renewing the stagnating deep and bottom water. Such 

 processes are necessarily connected with long periods of weather favourable for inflow, 

 which cause the front to remain in extreme southern position. 



More recently, Knudsen (Jacobsen, 1936) has organized detailed hydrographic 

 investigations in the area to the south of Denmark. This work has been devoted mainly 

 to the collection of accurate records for the sections between Gedser and Dars and 

 across the Fehmarn Belt, thus providing continuous surveillance of the water inter- 

 change between the Baltic and the Kattegat. 



(c) The Straits of Gibraltar and Bab el Mandeb 



In the strait of Gibraltar, instead of a single bottom rise there are three, all west of 

 Cape Tarifa. The first one extends in an arc from the Cabezos reef to Punta al Boassa 

 (maximum depth 320 m), the second one runs from Cape Trafalgar over "The Ridge" 

 (in places only 55 m deep) to Cape Spartel (maximum depth 366 m) and the third 

 one lies about 10-20 km west of the second with a maximum depth of over 300 m. 

 The water interchange between the Atlantic and the Mediterranean takes place in the 

 two channels, one to the north and one to the south of "The Ridge" and follows 

 exactly the same principles as those outlined above. Complete scientific use has been 

 made of the available observational data by Schott (1928 b). Longitudinal tempera- 

 ture and salinity sections are presented in Pt. I, p. 182, Fig. 83 for the transitional 

 period between spring and summer during which more or less mean current conditions 

 prevail. Seasonal variations in the velocity and extent of the upper current towards 

 the east and in the lower current towards the west are quite large. In the winter months 

 (including April) the thickness of the upper current is small, while that of the lower 

 current is rather large. During the summer months (to the end of October) the thick- 

 ness of the upper current increases by 80-100 m and that of the lower current is de- 

 creased correspondingly. During this part of the year the upper current must make up 

 the evaporation deficit in the Mediterranean. From the limiting position of the 

 boundary layer between the two water types it can be concluded that its annual varia- 

 tion west of the rise is of the order of 70-80 m, while east of the rise correspondingly 

 100 m or little more. The current boundaries also vary by similar amounts. The water 

 boundaries and the boundary between the currents do not coincide, but mixed 

 Mediterranean water is carried back with the upper current over almost the whole of 

 the area. Figure 245 gives a schematic representation of this. According to de Buen 

 (1926), Mediterranean water does not pass westward over the Gibraltar rise in the 

 deep layers, but is piled up on the eastern side and is carried backwards into the Medi- 

 terranean by the upper Atlantic current with an upward motion. Analysis of the ocean- 

 ographic series observations leaves no doubt of the incorrectness of this view of 

 de Buen. 



2M 



