SECT. 3] 



EASTERN BOUNDARY CURRENTS 



259 



even at 2000 m geostrophic flow is significant (Worthington, 1954; Stommel, 

 1958). 



The width of eastern boundary currents is difficult to determine since there 

 is no abrupt change of velocity or western "edge" to the flow. Both Gunther 

 (1936) and Hart and Currie (1960), in the Peru and Benguela Currents respec- 

 tively, have distinguished between a narrow coastal current and a broad 

 offshore oceanic current flowing in the same general direction. In both cases 

 the distinction is based on the contrast between the coastal regime, where 

 boundary processes predominate, and the offshore region, which is governed 

 by the trade winds. This point of view is in contrast with our picture of the 

 eastern boundary currents as the principal meridional circulation on the 

 eastern sides of oceans, the near-shore characteristics of which are modified 



500 



1000 



500 



DISTANCE OFFSHORE (km) 



1000 



Fig. 5. Vertical temperature structure across Gulf Stream, June, 1955. (After Fuglister, 

 1960). 



by the boundary processes. Both surface drift charts and the observed distribu- 

 tion of properties indicate that the bulk of equatorward transport occurs 

 within 1000 km of the coast with an appropriately directed pressure gradient 

 throughout this belt. The principal flow of the Gulf Stream or Kuroshio, on 

 the other hand, is confined to a band 100 km wide or less (Stommel, 1958; 

 NORPAC Committee, 1960). 



The difference in steric level across the Gulf Stream is of the order of one 

 meter. Across the eastern boundary currents, the difference is only about a 

 third as large (Table I), and since it is expressed across a distance ten times as 

 great, the resulting slope of the sea surface is very small. The average geo- 

 strophic surface velocity associated with this slope across the entire flow should 

 be about one-thirtieth that of the Gulf Stream, a figure comparable with that 

 observed. 



