698 Main Features of General Oceanic Circulation and their Physical Exploration 



Fig. 333. The three regions of an ocean model according to Morgan, 1956, Ij, interior 

 region; lb, frictionless stream region; II, northern region, non-steady, and lateral friction 

 eflfects possibly in an important way. 



an investigation was required to show whether a lateral eddy viscosity is needed to 

 explain the intense current in the region close to the western shore. 



A general discussion based on the momentum balance alone shows that if the 

 frictional torque is essential to the torque balance it is certainly not the only contribu- 

 tion to it. On the contrary, a boundary layer analysis, together with an accurate esti- 

 mate of the order of magnitude of the separate terms, shows the predominance in this 

 region of the pressure terms, the non-linear inertia terms and the terms arising from 

 the variation of the Coriolis parameter with latitude. This is in complete agreement 

 with the theoretical results of Charney (1955) for the Gulf Stream (p. 627) but not 

 with the result of Munk which presumes here a large lateral friction. In region II, 

 the non-linear terms, the lateral eddy viscosity and non-stationary effects become of 

 the greatest importance. Transitions from one region to the other must, of course, 

 be considered more closely, but it appears that this leads to no further difficulties, so 

 that it seems possible to obtain a comprehensive picture of the entire ocean circulation. 



Very recently Stommel (1957), in an extremely interesting and instructive survey 

 article, has compared the different theories of ocean currents and discussed their 

 basic physical ideas. Avoiding mathematical ballast he tried to represent the three- 

 dimensional oceanic movements by means of schematic block-diagrams, which are, 

 however, based on strict theoretical principles. It seems not possible to describe all 

 the details here but only the most essential points in connection with the upper wind- 

 driven circulation and the deep-sea circulation shall be dealt with. Figure 334 shows a 

 rudimentary simplified model of the Atlantic Ocean with meridional boundaries 60° 

 apart in which a certain zonal wind-stress distribution (indicated on the left) generates 

 a wind-driven circulation. Westerlies prevail between 30° lat. and the poles and the 

 trade winds extend across the equator from 30° S. to 30° N. The lines shown are 

 isobars parallelling the geostrophic flow. A certain contribution of the Ekman wind- 

 driven transport in the surface layers has been omitted in order to retain clarity in the 

 picture. Obviously, a system of gyres and western currents is obtained as in previous 

 theoretical investigations. The boundaries between the gyres correspond to the latitudes 



