156 Thermohaline Features 



Let us now form the horizontal divergence of the Ekman wind drift, 



div^Mg, from equations (7): 



div,^ J-^^";-'^> . (9) 



This is a quantity which has often been contoured for various oceanic 

 regions in the beHef that it has something to do with upwelling. 



Because of the variation of the Coriohs parameter with latitude, aU 

 northward or southward geostrophic motions exhibit a horizontal diver- 

 gence, div^Mg, which can be obtained formally from equations (8): 



f 



divgjf,= -'^. (10) 



In a stationary process the total horizontal divergence, divgil/, must 

 vanish, hence the sum of equations (9) and (10) vanishes, and we are led 

 back to equation (5). The meaning is clearer, however, because we now see 

 that what equation (5) expresses is the condition under which the divergence 

 of the Ekman wind drift, produced by the wind, is compensated for by the 

 divergence of the geostrophic flow. In Ekman's original picture of the 

 stationary wind-driven currents set up by the wind on a plane ocean with 

 constant Coriohs parameter, the geostrophic flow is nondivergent, and hence 

 he was forced to consider a divergent frictional layer at the bottom to com- 

 pensate for the surface Ekman wind-drift convergence. Intense lateral 

 friction could produce a divergence, too, but we rule it out by hypothesis. 

 It is worth emphasizing that the role assigned to friction in the present 

 hypothesis is comparatively insignificant: we suppose that it is dynamically 

 important only in the Ekman wind-drift layer. This is a very different 

 point of view from that espoused by Neumann, by Hidaka and the Japanese 

 school, and by Stockmann and the Russian school. As we have seen, it is 

 consistent with the original studies of Sverdrup, Reid, and Munk concerning 

 the interior regions of the ocean; and it is possible that friction (in a chmato- 

 logical-mean sense) is important in certain regions of decay of the intense 

 western currents (for example, in the detaching eddies in the Gulf Stream). 

 The impossibility of stating definitely whether or not friction plays an 

 important dynamical role in the interior of the ocean is one of the reasons 

 why the present discussion is not, strictly speaking, a theory, but merely 

 a hypothesis. 



