t 



1 



NORTH 

 ATLANTIC 



SOUTH 

 ATLANTIC 



SUMMER 

 WINTER 

 AUTUMN 



35 30 25 20 15 10 S .5 -10 



30 25 20 15 10 5 0-5 -10 



( Kg /cm /sec. ) 



Figure 6.--Offshore Ekman transport of water, Mp, computed for each season from mean wind stress values and averaj^e coastline orientations 

 for 5-degree rectaPKles along tlie eastern sides of oceans. Summer refers to the summer months in the northern or southern hemisphere as the 

 case requires; similarly for winter, autumn, and spring. 



bounciary coasts showed that, with the ex- 

 ception of the Indian Ocean, behavior of the 

 index is in qualitative agreement with what 

 is known about seasonal and geographical 

 variations of coastal upwelling. 



These tests suggest that a simple model of 

 dependence of vertical motion on the com- 

 ponent of wind stress parallel to the coast 

 and the resulting offshore Ekman transport 

 is applicable. With better wind data it might 

 be possible to make a more sophisticated 

 analysis of the relationship, which might then 

 have some predictive value for particular 

 seasons and years. The reason for failure of 

 the index in the Indian Ocean is not understood. 



In regions of the better known eastern 

 boundary currents there are occasional indi- 

 cations of surface poleward countercurrents 

 close inshore. There are also indications that 

 coastal undercurrents are a common feature 

 of the circulation on the eastern sides of 

 oceans. Indirect evidence for these is found in 

 the frequent observations of near-shore weak- 

 ening of the vertical density gradient with a 

 deepening of the isopycnals below a few 

 hundred meters. Such counterflows provide 



a mechanism for the retention of plankton 

 populations in an area through which there 

 is a predominant flow. 



The eastern boundary current regions are 

 well known for their high productivity. Stand- 

 ing crops of phytoplankton and zooplankton, 

 rates of carbon fixation, and populations of 

 fishes of commercial value are known to be 

 large. In part, the upward transfer by up- 

 welling of nutrients from the subsurface reser- 

 voir is considered responsible. High produc- 

 tivity is also favored by the shallow ther- 

 mocline near such coasts, since the mixed 

 layer is usually shallower than the "critical 

 depth" (which depends on the amount of in- 

 coming radiation, transparency of the water, 

 and the energy level at the compensationdepth), 

 a condition essential for maximal development 

 of phytoplankton populations. 



A paper entitled "Eastern boundary cur- 

 rents" is in press in The Sea: Ideas and 

 Observations (Interscience Publishers). 



The work was supported in part by the 

 Office of Naval Research (about 25 percent) 

 and California Marine Research Committee 

 (about 50 percent). 



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