Basic Principles of the General Oceanic Circulation 



577 



in a direction exactly opposite to that of the sea level. This displacement of the internal 

 boundary surface will automatically reduce the pressure gradient imposed on the 

 lower layer from above. In the final equilibrium state of the lower layer there will be 

 no pressure gradient and therefore no motion. If io is the deviation of the internal 

 boundary surface from a level surface, the condition for this new state of equilibrium 

 is given by 



^' Ci. (XVIII.4) 



P2 



P\ 



This simple relationship will always be present if sufficient time is available. Con- 

 ditions at the outer boundaries of the current aX AC and BD will be considered later 

 (p. 622 et seq.). 



Fig. 263. Position of the physical sea surface and of the internal boundary surface of a 

 two-layered ocean for a forced movement of the upper layer in the interval AC-BD. 



The total effect of air currents on the ocean surface can be suitably illustrated by 

 the simple case of an ocean uniformly covering the entire earth (no continents). 

 This ocean can be assumed to have two layers, an upper troposphere and a lower 

 stratosphere, separated by a clearly defined density transition layer. To correspond to 

 actual conditions in the tropics and subtropics it can be assumed further on that the 

 troposphere in these regions is subdivided by a transition layer at about 100 m depth 

 separating the top layer from the subtropospheric water masses beneath. Only zonal 

 (east-west) currents will be present in this hydrosphere covering the total earth and it 

 can be regarded as a circular vortex as described by Bjerknes (1921), centred around 

 the axis of the earth. The movement of the water masses in this vortex will be east- 

 west, and the adjacent stream lines will not influence each others. The hydrosphere 

 will be affected only by the atmospheric currents at the sea surface, that is, by the trade 

 winds between the equator and the Ross latitudes (30° N. and S.), by the west winds in 

 middle latitudes between 30° and 60° N. and S., and by polar east winds polarwards 

 60° N. and S. The oceanic movements in the individual zones of the circular vortex 

 and the position of the boundary surface will then be a consequence of these effects. 

 Since conditions are symmetrical around the rotational axis it is only required to 

 consider a meridional section through such a wind-generated circulation. Fig. 264 



