sufficient data only to 1,000 m. Bennett (1959), 

 using a deeper reference level, obtained greater 

 surface velocities for the Gulf of Alaska. In the 

 absence of a known depth of no motion, the 

 deepest level compatible with all the data 

 (1,500 db.) was selected for the reference sur- 

 face in this study. Other assumptions that limit 

 the accuracy of geostrophic currents are: (1) 

 synoptic data, (2) unaccelerated flow, (3) lack 

 of internal wave or tidal influence, and (4) 

 absence of friction. 



Caution must be used in the estimation of the 

 surface velocity from geostrophic currents 

 alone. The Ekman Current (Sverdrup et al., 

 1942) caused by local, variable wind stress 

 must be added to the geostrophic current, for 

 it is reasonable to assume in the absence of di- 

 rect measurements of current that the Ekman 



velocities at the surface may exceed the surface 

 geostrophic velocities. Examination of trans- 

 port values computed from mean monthly pres- 

 sure charts (FofonoiT and Ross, 1961) suggests 

 that Ekman velocities of 3 to 10 cm. /sec. gen- 

 erally toward the southeast may be expected in 

 this coastal area during spring and fall. Al- 

 though the short-term Ekman Currents, aver- 

 aged on a daily basis, may be even greater, they 

 are negligible below 200 m. They were neglect- 

 ed in this discussion which is concerned pri- 

 marily with the main portion of the water 

 column below 200 m. 



The data from the cruises of the Kelez per- 

 mit construction of the first geostrophic cur- 

 rent charts off the Washington coast from a 

 reference level of 1,500 db. Relative currents 

 flow along contour lines of equal geopotential 



Figure .3.— Geopotential topography, 0/1,500 db., fall lOfiS. (The 183- and 1,829-m. depth contours 

 are shown.) 



226 



U.S. FISH AND WILDLIFE SERVICE 



