The cause of the Davidson Current is not 

 clearly understood. Off the coast of Oregon it 

 appears to result from local wind stress, but 

 direct measurements during October 1958 and 

 January 1959 (Reid and Schwartzlose, 1963) 

 indicate that the driving force of this current is 

 not local winds ; it may be a surface manifesta- 

 tion of a deeper northward-flowing countercur- 

 rent that develops when winds weaken season- 

 ally (Sverdrup, Johnson, and Fleming, 1942). 

 This northward countercurrent which opposes 

 the offshore California Current has been re- 

 ported off central California below 200 m. 

 throughout the year (Reid, Roden, and Wyllie, 

 1958). Northward flow also was reported off 

 Washington and Oregon below 200 m. during 

 the summers of 1955-57 and 1959 ; this report 

 was based upon limited observations (Dodi- 



mead, Favorite, and Hirano, 1963). Our closely 

 spaced observations during 1963 permit a 

 more detailed evaluation of the size and con- 

 tinuity of the surface Davidson Current, the 

 subsurface California Undercurrent, and other 

 major features of the circulation off the coasts 

 of British Columbia and Washington. 



GEOSTROPHIC CURRENTS 



Geostrophic currents reflect the general cir- 

 culation associated with the distribution of 

 mass. They are calculated from an arbitrarily 

 selected reference depth and are, therefore, 

 relative currents. The 1,000-db. (decibar) sur- 

 face has been used as a reference surface in the 

 North Pacific Ocean by Reid (1961) , Dodimead 

 et al. (1963), Budinger, Coachman, and Barnes 

 (1964), and Favorite (1966) because they had 



Figure 2.— Geopotential topography, 0/1,500 m., spring 1963. (The 183- and 1,829-m. depth con- 

 tours are shown.) 



GEOSTROPHIC CIRCULATION 



225 



