The Tropospheric Circulation 



623 



a counter current at the left-hand edge of the free jet and a current in the direction of the 

 main current at the right-hand edge. Due to the increased sea level difference between 

 the right- and left-hand sides, the counter current on the left-hand side will increase in 

 strength downstream, while on the contrary the other current will become weaker 

 on the right-hand side until it finally vanishes. The effect of the free jet and the counter 

 current must thus increase steadily downstream and therefore tend towards impossible 

 unstable conditions. The effect of the vortex formation will give rise to a water move- 

 ment through the main body of the current from right to left, and since the left-hand 



500 



1000 



1500 



Deep woter 

 (motionless) 



Fig. 293. Cross-section through a jet (Freistrahl) current in a two-layered ocean with a full 

 development of a counter current and compensation current in the adjacent water masses 



(according to Rossby). 



edge and the counter current are shallow there must also be a transverse current in 

 the lower part of the top layer in the opposite direction in order to compensate the 

 upper transport. This gives a cross-circulation as was assumed by Dietrich. In addition 

 to his earlier work, the processes occurring at the edges of a jet-form current penetra- 

 ting a motionless water body have been discussed in two later papers by Rossby 

 (1937, 1938). Thereby, he assumed that the initiation of the current from a state of 

 rest was due to a wind field whose action was restricted to a band-like oceanic region. 

 Particular attention was paid on the one hand to processes at the edges of the current, 

 on the other hand to oscillatory processes which occur while the current tends towards 

 a steady state. In such cases counter currents are formed on both sides of the basic 

 current; in homogeneous water they are broad and slow, but in a two-layered sea 

 narrow and intense. The zones between the basic current and the counter current are 

 dynamically unstable and show a tendency to break up into large horizontal vortices. 

 The depth to which a surface disturbance may penetrate into the lower layer down to 

 the sea bottom, and the time required for the restoration of stationary conditions, are 

 of particular interest and are especially important in dynamic oceanography (see 

 Chap. XXI. 4). 



Without question the theory has applications to the Gulf Stream between the Florida 

 Strait and the Newfoundland Banks, and several theoretical consequences are un- 

 doubtedly realized in the actual behaviour of the Gulf Stream. The criticism on this 

 theory expressed by Ekman is concerned not so much with the theoretical fundaments, 

 but more with the question of the extent to which the Gulf Stream actually keeps the 

 character of a free jet and contains the energy (momentum of motion) required by the 



