Oceanic Circulation and Temperature 

 pret hydrographic observations we can obtain a 

 more thorough and extensive knowledge of the 

 hydrographic conditions of a region than is given 

 directly by a tabulation of the data. Because of 

 this fact it was thought advisable to include a 

 study of pur el V hydrographic problems with the 

 biological problems of marine ecology, in the pro- 

 gram of the Scripps Institution for Biological Re- 

 search. Accordingly in 1910 the writer commenced 

 a study of the problems suggested by the results of 

 hydrographic observations off the Pacific Coast. 



As the conclusions reached in this investigation 

 are based not only on hydrographic observations, 



z 





 



Z 



OeFLEcrmq Force 



Figure 15 

 Figure 14 Three forces that determine 



Constrained motion on the the velocity of the surface 

 surface of a spheroid. water. 



but also on recent theoretical results brought out 

 by Nansen and Ekman, I shall begin with a brief 

 sketch of Ekman's theory of oceanic circulation. 

 Suppose a wind is blowing horizontally over the 

 surface of a large body of water. Then the water 

 will move under the action of three forces, that 

 of the wind acting in the direction toward which 

 the wind blows, that of the deflecting force due to 

 the earth's rotation,* and that of the resistance 

 due to the friction of the water underneath. 



Hence, in order that the three forces (figure 15) 

 may be in equilibrium, the resultant surface velocity 

 must be directed to the right of the wind. This 

 upper layer of water will act upon the one under- 

 neath just as the wind acts upon its upper surface, 

 and so on downward. The mathematical theory 

 shows that if differences in density are neglected, 



• Whenever a body moves along the earth's surface, it 

 becomes acted upon by a force directed to the right of the 

 ntiotlon In the northern hemisphere, as shown by figure 14, 

 but to the left in the southern hemisphere. 



136 



