534 Currents in a Strait 



disturbances due to atmospheric pressure and wind variations (velocities of up to 

 50 cm/sec). The current transport in the strait can be estimated from the Knudsen 

 relations (see Chap. XII. 5). For cross-sections at the narrowest point (Punta Pezzo- 

 Ganzirri) and at the rise of Punta Pellaro to the south of this, the mean salinities are : 



s = 37-9, z = 38-4 and s' = 38-5, z' = 38-75%o. 



The equations (XIII. 19) then give as an approximation / = u and /' = m' = 2/. Under 

 stationary conditions the transports will be the same in upper and lower currents, but 

 the transport through the southern cross-section is twice as large as that over the rise. 

 Thus only about half of the water of the lower current entering the southern part of 

 the channel flows over the sill to the north, the other half is carried back in the upper 

 current mixed with Tyrrhenian water. A corresponding calculation for a cross-section 

 to the north of the sill shows that part of the Tyrrhenian water entering the strait from 

 the north mixes with the lower current and is carried back into the Tyrrhenian Sea. 

 There must therefore be large turbulent mixing processes within the strait (see Vol. II). 



4. External Influences (Bottom Topography, Tides) on the Oceanographic Conditions 

 in Sea Straits 



The normal steady current conditions in sea straits may be modified by external 

 circumstances. It has already been mentioned that the atmospheric pressure and winds 

 have considerable influence. Some idea about these influences can be obtained by 

 simple numerical calculations. Besides these there are also other effects, especially 

 that of the bottom topography of the strait and also those of tides, which penetrate 

 from both sides from the open ocean into the sea strait and give rise to special current 

 phenomena there. These latter phenomena will be discussed later in Vol. II, but it 

 seems to be of advantage to mention these processes in connection with the funda- 

 mental phenomenon of water interchange between two seas already here. 



(a) Disturbances Due to the Sea Bottom Configuration 



The influence of a wave-form bottom topography on a horizontally flowing current 

 can be understood quite easily by means of theoretical computation, provided the 

 bottom relief can be expressed in the simple form 



>'=-/? + y cos KX, (XVI. 14) 



where k = 2irjL is determined by the known wavelength of the bottom waves. The 

 current with a velocity c over such a bottom will also take a wave-form, i.e. all layers 

 from the bottom to the surface will follow the bottom topography but with an ampli- • 

 tude decreasing with distance from the bottom. The sea surface itself will be a stream 

 line and its profile is determined from 



^ ^ cosh Kh{\ - (glKC^) tanh k/j) ' (XVI. 15) 



The denominator will be positive or negative according to whether 



c ^ {(glK) tanh KhY'K 



This expression is, however, the velocity of propagation of a wave in motion/ess water 

 of constant depth h. If the dimensions of the bottom waves are large compared with 



