Coastal Upwelling Indices, West Coast of North America, 



1946-71 



ANDREW BAKUN' 



ABSTRACT 



A series of monthly indices of intensity of large-scale, wind-induced coastal upwelling at 

 selected locations along the west coast of North America is presented for the period 1946 

 through 1971. The indices are based on calculations of offshore Ekman surface wind transport 

 from monthly mean surface atmospheric pressure data. Summaries by quarter and by year are 

 included. 



The effect of using monthly mean pressure data is evaluated by comparison to a similar 

 series of monthly means of transports computed 6-hourly over a 43^-yr period. The correla- 

 tion between the two series at an\ particular location was found to be high. However, the slope 

 of the regression line varies at tlifferent locations. Also values off southern California during 

 summer may be amplified relati\ e to other locations as a result of the discontinuity in the atmos- 

 pheric pressure gradient caused by the coastal mountain range between the thermal low in 

 the interior of southern California and the higher pressure offshore. The conclusion is that these 

 series may be satisfactory indicators of temporal variations of upwelling at each location, but 

 less satisfactory indicators of spatial distributions. 



INTRODUCTION 



Coastal upwelling can profoundly affect the 

 physical environment of marine organisms. An 

 extreme example is the "El Niiio" of the Peru 

 Current where high surface temperatures re- 

 lated to cessation of upwelling have apparently 

 resulted in such dislocation of marine fauna as 

 to have caused starvation of great numbers of 

 guano-producing seabirds (Wooster, 1960). Per- 

 haps even more important from a fishery stand- 

 point is the role of upwelling in the cycle of organ- 

 ic production in the sea, both in transporting 

 nutrient-rich deeper waters into the illuminated 

 surface layers where they are available for pro- 

 duction, and in providing virgin water which is 

 sufficiently free from predators to allow accumu- 

 lation of large phytoplankton blooms (Gushing, 

 1969). Estimates have run as high as Ryther's 

 (1969) suggestion that upwelling areas com- 



' Pacific Environmental Group. National Marine Fish- 

 eries Service. NOAA. Monterey. CA 93940. 



prising about one-tenth of 1% of the ocean surface 

 may produce one-half of the world's harvestable 

 fish supply. Evidently, some indication of fluctua- 

 tions in the upwelling regime is extremely impor- 

 tant to understanding fluctuations in marine pop- 

 ulations. This report is an attempt to provide 

 indices of intensity of coastal upwelling off west- 

 ern North America on time and space scales use- 

 ful to fishery research in general, and to the 

 MARMAP (Marine Resources Monitoring, As- 

 sessment and Prediction) program of the Na- 

 tional Marine Fisheries Service in particular. 



METHOD 



It is generally recognized that coastal upwell- 

 ing in broad, diffuse eastern boundary currents 

 such as the California Current and the eastern 

 portion of the Gulf of Alaska Gyre is largely due 

 to replacement from below of surface water 

 transported offshore by the stress of the wind on 

 the sea surface (Sverdrup et al., 1942, p. 501). 

 Other possible mechanisms producing coastal up- 



