582 



Basic Principles of the General Oceanic Circulation 



If in addition bottom friction of the type described by Ekman is taken into account, 

 the current field will be slightly altered; now the bottom current must aiso contribute 

 in order to satisfy the convergences and divergences appearing in the current field of 

 the Ekman top layer. 



Fig. 266. Sverdrup-type solution in a homogeneous ocean of uniform depth, bounded by a 

 meridional coastal wall on its eastern side. The wind system with sinusodial pattern is 

 indicated by shaded arrows hovering above the surface. The curved lines with arrows 

 are isobars and give the direction of the geostrophic horizontal flow (independent of the 

 depth). At a number of subsurface depths the velocity components are shown by solid 

 arrows (according to Stommel 1957). 



Considerably more complicated models of this type can, of course, be developed, 

 but they will all show that the boundary conditions at any coast to the west cannot be 

 satisfied except by taking into account processes involving the dissipation of energy. 



(b) Conditions East of a Meridionally Oriented Coast 



In the western part of the oceans, and particularly along the western boundary, the 

 vorticity related to lateral friction must also be taken into account with an additional 

 term in order to satisfy mass conservation and space continuity conditions in the 

 vorticity equation (XVII. 5). With this equation Stommel (1949, 1951) was the first to 

 give an explanation of the westward intensification of ocean currents. He took the case 

 of a symmetrical anticyclonic wind circulation over a closed rectangular oceanic area 

 in the Northern Hemisphere. The wind stress vorticity is thus negative over the entire 

 ocean. The effect of the wind stress can be expected to cause an anticyclonic circula- 

 tion in the sea. The horizontal eddy viscosity will tend to counteract the effect of wind 

 stress. In the western parts of this ocean the anticyclonic flow will transport water 

 northward, in the eastern parts southward; in equation (XVII. 5) the planetary vorticity 

 effect is therefore negative at the western side of the ocean and positive at the eastern 

 side. This is a consequence of the conservation of angular momentum or, what amounts 

 to the same, of the variation of Coriolis parameter with latitude. 



