178 



WATER MASSES AND CURRENTS OF THE OCEANS 



through the Strait of Gibraltar, which is noted for its strong currents. 

 The bottom topography of the Strait is complicated, because in its shal- 

 low portion two or possibly three ridges are present with sill depths of 

 about 320 m. The character of the water masses passing in and out 

 through the Strait of Gibraltar is illustrated in fig. 46, which shows iso- 

 halines and isotherms in a longitudinal section through the Strait in the 

 months of May, June, and July. 



In the upper layers, Atlantic water of a salinity somewhat higher 

 than 36.00°/oo and of a temperature higher than 13° flows in, whereas. 



Fig. 46. 

 Gibraltar. 



Temperature and salinity in a vertical section through the Strait of 



along the bottom, water of a salinity higher than 37.00°/oo and of a tem- 

 perature about 13° flows out. The deeper water in the Mediterranean Sea 

 inside the Strait of Gibraltar has a salinity of about 38.40°/oo and a 

 temperature of 13°, but in the Strait intensive mixing takes place by which 

 the salinity of the outflowing water is greatly reduced and that of the 

 inflowing water is increased. In the Strait the inflowing water has a 

 thickness of approximately 125 m, but the boundary surface separating 

 the inflowing and outflowing layers lies deeper on the African side of the 

 Strait, and the greater inflow therefore takes place on the southern side. 

 This inclination of the boundary surface is due to the effect of the earth's 

 rotation. The average velocity at the surface in some localities is in 

 excess of 200 cm/sec (4 knots), and close to the African coast a narrow 

 countercurrent with velocities up to 100 cm/sec (2 knots) is often encoun- 

 tered. Superimposed on the currents carrying water in and out of the 

 Mediterranean Sea are strong tidal currents that greatly reduce the inflow 

 when they are directed from the Mediterranean Sea to the Atlantic Ocean, 

 and greatly increase the inflow when they are directed from the Atlantic 

 to the Mediterranean. The inclination of the boundary surface probably 

 varies with the speed of the inflowing current, and great vertical oscilla- 

 tions of tidal period therefore take place. 



The average velocity of the total inflow, according to measurements 

 on Danish expeditions, is approximately 100 cm/sec (2 knots). By 

 means of this value, Schott has computed the inflow to be approximately 



