596 



The Tropospheric Circulation 



and convection and they have an extremely low concentration of oxygen which is 

 largely due to the almost total stagnation and also due to biological causes. 



The internal forces, providing the motive force for the entire current system of the 

 tropics and the subtropics, are produced, on the one hand, by the wind system present 

 in these zones and on the other hand, by the internal pressure field set up by the thermo- 

 dynamic conditions. Figure 271 shows the absolute topography of the physical sea level 

 in the Atlantic Ocean pictured by isobaths drawn at intervals of 5 dyn/cm between 

 35° N. and 35° S. and at intervals of 10 dyn/cm outside this area (Defant, 1941^). 

 The direction of this stationary gradient current, which corresponds to this pressure 

 field is indicated by arrow-heads on the dynamic isobaths. Comparison of this topo- 

 graphy in the tropical and subtropical area with that of the tropospheric density transi- 

 tion layer (Fig. 269) shows that they are almost mirror images; in deeper layers the 



Fig. 271. Absolute topography of the physical sea surface (dynamic isobaths drawn from 

 5 to 5 dyn cm, 10 to 10 respectively). 



