SECT. 3] EASTERN BOUNDARY CURRENTS 273 



4. Poleward Eastern Boundary Currents 



A. Subtropical Coastal Counter currents and Undercurrents 



In regions of the better-known eastern boundary currents, there are occa- 

 sional indications of surface poleward countercurrents close inshore. There are 

 reports of such flow in the Benguela region (Hart and Currie, 1960), and the 

 El Nino condition off northern Peru may represent another such situation 

 (Wooster. 1960). The best known of these countercurrents is present along the 

 North American coast during winter from near the tip of Baja California to 

 45°N and is known north of 35°N as the Davidson Current. There is also north- 

 westward flow from about 32°N to 35°N inside the Channel Islands throughout 

 the year. This has been explained by Munk (1950) as due to the fact that the 

 northerly winds reach their maximum speed some distance from shore. There- 

 fore, on approaching the coast, the wind-stress curl changes sign as does the 

 meridional Sverdrup transport. An analysis of the nearshore wind stress in 

 other eastern boundary regions may show the same feature. 1 



There is also evidence that coastal undercurrents are a common feature of 

 the circulation on the eastern sides of oceans. As has been noted before, cross - 

 stream density profiles frequently show a nearshore weakening of the vertical 

 density gradient with a deepening of the isopycnals below 100-200 m. A similar 

 nearshore deepening of isopleths of other properties occurs. The picture is 

 reminiscent of the distribution of properties near the equator (Wooster and 

 Cromwell. 1958) which has been related to the Equatorial Undercurrent 

 (Wooster and Jennings, 1955; Knauss, 1960). 



Such a distribution of mass indicates a poleward geostrophic flow which is 

 apparent on dynamic charts of the 200-db surface (Fig. 15). Off the California 

 coast, such an undercurrent has been discussed by Reid, Roden and Wyllie 

 (1958). This current, which is present throughout the year, flows to the north- 

 west along the coast from Baja California to at least 40°N, bringing warmer, 

 more saline water great distances along the coast. Gunther (1936) describes an 

 undercurrent off the Chilean coast, identifying it by the presence of waters of 

 high salinity and temperature, and of low oxygen content. Recently Brandhorst 

 (1959) has attributed important effects on the Chilean hake fishery to this 

 "Gunther Current". The feature is also evident on the 200-db dynamic chart 

 and, presumably, persists throughout the year. 



Hart and Currie (1960) observed poleward flow along the southwest African 

 coast on dynamic charts of the 200-db surface (their fig. 34) which they consider 

 a compensatory movement connected with the process of upwelling. (This is 

 not evident on our 200-db chart, which for the most part is based on Meteor 

 data.) They consider the presence of waters of low oxygen content close to 



1 The Sverdrup (1947) equation refers to vertically integrated transport rather than to 

 surface velocity and thus includes the possibility of sub-surface countercurrents. The 

 width of such currents, where present, seems to be of the order of one or two hundred 

 kilometers, so that Hidaka's (1958) wind-stress values for five-degree squares are spaced 

 on too coarse a grid to be used in detailed analysis. 



10— s. ii 



