Basic Principles of the Geiieral Oceanic Circulation 



581 



Fig. 265. Stream lines of the flow representing the field of mass transport; differences of the 

 values of the stream function between two stream lines represent the net mass transport 

 in 10® metric tons per second flowing between these stream lines (from the surface down to a 



depth of no motion). 



Figure 265 shows stream lines of flow representing the field of mass transport. The 

 principal troughs and ridges are accounted for by the wind stress function. Off the 

 coast in the east the currents are weak and the meridional component is directed south- 

 wards in middle latitudes. 



The integrated equations give no information on the distribution of vertical motions 

 in the deep oceanic layers. A better comprehension of these currents can be gained by 

 accurate calculations for the very simple model of Sverdrup. Stommel (1957) has 

 recently given a very instructive description of such a case, in which zonal wind stress 

 was assumed to act on a homogeneous ocean surface with an eastern coast line. 

 Figure 266 shows the solution. At the surface there is a zonal wind stress with a similar 

 distribution as that shown in Fig. 265. The stream lines will therefore also be similar to 

 those in the diagram. The transport in the thin Ekman layer, indicated by the upper 

 arrows, will produce a vertical downward velocity in the central part of the diagram. 

 Outside the zonal belt of westerly winds the vertical velocity will be directed upwards. 

 These vertical components from the bottom of the Ekman layer to the bottom of the 

 ocean decrease linearly to zero. The divergence and convergence system of the meri- 

 dional components of geostrophic velocity are coupled with this vertical velocity field. 

 At the latitude of maximum westerly wind, where there is no impressed vertical 

 velocity, the geostrophic flow will be entirely zonal and will decrease linearly towards 

 the eastern coast. The topography of the physical sea surface, which determines the 

 pressure field associated with the geostrophic flow, is also shown in Fig. 265. 



