Main Features of General Oceanic Circulation and their Physical Exploration 699 



of no Ekman layer convergence (see p. 581, Fig. 265); the regions of maximum geo- 

 strophic meridional flow in each gyre correspond to the latitudes of maximum con- 

 vergence (meridional flow towards the equator) or of maximum divergence (meridional 

 flow towards the poles) of the Ekman wind-driven layer. Western boundary currents 

 corresponding to continuity requirements have been introduced along the western 

 boundary region. 



Fig. 334. The steady circulation produced in an ocean of uniform depth bounded by 

 meridional coasts 60^ apart, acted upon by a distribution of zonal winds, which are indicated 

 on the left. The western boundary current is shown schematically by the double line at the 

 western coast and its transport is indicated by heavy arrows (according to Stommel, 1957). 



This simple circulation, derived from the application of previously described 

 mathematical principles to a homogeneous or a vertically integrated ocean, can now 

 be interrelated with the internal oceanic circulation of the deep layers which corres- 

 ponds to a thermo-hahne circulation. For this purpose Stommel subdivided the total 

 ocean into two layers by means of a level surface half way to the bottom, for instance, 

 at 1500-2000 m. Across this level surface there is a vertical mass transport which is 

 specified geographically. The geostrophic flow of the wind-driven circulation super- 

 imposes on the water transports of the internal circulation and the continuity condi- 

 tions require that the vertically integrated transport over both layers together should 

 vanish. Figure 336 shows this model given by Stommel, a similar kind of presentation as 

 used for the previous model. A level surface L divides the ocean into an upper and a 

 lower layer. The thermo-haline convection processes allow a sinking of the water 

 masses across the level surface in sub-Arctic latitudes (p. 684) and a corresponding 

 rise across the level surface in sub-Antarctic latitudes (p. 675). 



These convection processes are indicated by vertical transport lines drawn through 

 the level surface in Fig. 335. The remainder of the thermo-haline circulation is com- 

 pletely determined by continuity and dynamic reasoning; the transport of water 

 between the two hemispheres takes place in a narrow western boundary current 

 according to the dynamic principles frequently mentioned above that are effective 

 on the rotating earth. The field of motion in this circulation is entirely internal; its 



