622 



The Tropospheric Circulation 



to this concept it is the residual pressure field even though it is weak that provides the 

 driving forces. This is the basic idea of the Rossby theory. It is undoubtedly attractive 

 but whether it actually corresponds to reality is impossible to say. In any case it 

 deserves considerable attention. 



The further phenomena that occur when the medium in which the free jet is formed, 

 is stratified, can be fairly readily dealt with. If there are two layers in the medium the 

 velocity of the upper layer will affect the sea surface slope and also the position of the 

 internal boundary surface between the two water masses. The sea surface slope and 

 the internal boundary slope are given by equations (XIV. 6 and 7) (p. 455). If the lower 

 layer is assumed to be motionless then the velocity of the free jet gives the mass distri- 

 bution in a transverse section. This is shown schematically in Fig. 292. In the current the 



Motionless 1 Jet current 

 500 



E 



a 1000 



Q 



1500 



Motionless 



ibb'^>.^ 



Fig. 292. Cross-section through a jet (Freistrahl) current in a two-layered ocean. 



boundary surface will slope downwards, in the Northern Hemisphere from left to 

 right, and the thickness of the free jet will therefore vary across the current. The total 

 mass transport through a cross section will be 



M = lpu{D, + Ci + y dy. 



(XIX. 15) 



where Dq is the mean thickness of the top layer and ^^ and i^ are the deviations of the 

 physical sea level and the boundary surface from their positions when the system is at 

 rest. Evaluation of this integral gives the result that the difference. Drigiu — ^left 

 between the two sides of the current must increase downstream as long as the mass 

 transport increases. This has several effects on the course of the current. The inflow 

 of water from the surroundings into the free jet will be asymmetric because of the 

 asymmetry of the system. On the left there will be only a shallow water layer available, 

 but on the right the water can be drawn in from greater depths. Under steady conditions 

 the transverse velocity must therefore be greater on the left-hand side than on the 

 right. 



The surrounding water masses can be assumed to be stationary, but this state can 

 hardly be expected to persist under the given conditions. At some distance from the 

 current boundary the thickness of the layer D in the motionless water will be somewhat 

 greater than Dieit and Dri^ht at the left- and right-hand edges. On the way from motion- 

 less water towards the boundary and into the interior of the free jet the water columns 

 drawn into the current will undergo deformations, which will be associated with 

 hydrodynamic vortex formation at the current boundary. The theoretical form for a 

 cross-section through a free jet of this type is that shown in Fig. 293. This requires 



