658 



The Tropospheric Circulation 



extended more or less zonal boundary surface, but rather in form of individual mass 

 intrusions {quantum-like) at different places whereby as a consequence mixing is con- 

 siderably increased. The nature of the processes involved can be illustrated by putting 

 side by side successive stages of the oceanic state in a meridional section (Defant, 

 1941Z>), and one obtains thereby all the characteristics of the disturbances which occur. 

 The bottom topography in this part of the South Atlantic was earlier assumed (p. 435) 

 to be the cause of the wave-form current pattern appearing in the region of the sub- 

 tropical convergence (Fig. 187). It should be emphasized, on the other hand, however, 

 that the vortical disturbances originate on the shelf of the South American continent 

 between 45° and 35° S. far in the west, and from here extend as a continuous chain of 

 regular vortices throughout the entire area as far as the southern tip of Africa. This 

 source region or birth place, is the region where the denser water of the Falkland 

 Current meets the lighter water of the Brazil Current and where the tendency 

 for a vortex formation is extremely large. Here a strong solenoidal field is continuously 

 regenerated, which can be considered as the necessary condition out of which vortices 

 are formed and the disturbance field then stretches far out into the Atlantic. 



A probable explanation of these wave-form disturbances can be derived by means of 

 the arguments put forward by Rossby and co-workers (1939) in a discussion of the 

 sinusoidal disturbances in zonal atmospheric air currents. In a wave-like disturbance, 

 which is superimposed on a pressure field that decreases to the south (Fig. 309, 



P+2 



W ^+ 



P+2 



P + l 



Fig. 309. Wave flow for a uniform towards south decreasing pressure field. 



Southern Hemisphere) the water transport through the cross-sections A and C where 

 there is an anticyclonic curvature of the isobars will be greater because of the occurring 

 centrifugal force than that through section B where there is a cyclonic curvature. There 

 will therefore be a horizontal divergence and pressure fall between sections B and C 

 and a horizontal convergence and pressure rise between A and B. The wave disturbance 

 will thus move eastwards and since the centrifugal force is larger when the curvature is 

 greater the shorter waves will travel eastwards faster than the longer ones. In addition 

 to this effect, there will be a pure latitude effect which originates from the relation of 

 the geostrophic flow to the pressure gradient. Due to the Coriolis force the mass 

 transport across the section 5 in a lower latitude will be greater than that across sections 

 A and C in higher latitudes. This gives rise to convergence and pressure rise between 

 A and B. This latitude effect which is independent from the wavelength causes a west- 

 ward movement of the wave. Both effects are of the same order of magnitude and it is 

 easily understood that for a particular wavelength the wave disturbance will be 



