INDICES OF MEAN MONTHLY GEOSTROPHIC WIND 

 OVER THE NORTH PACIFIC OCEAN 



by 



Laurence E, Eber and Oscar E. Sette 

 U. S, Fish and Wildlife Service 

 Stanford, California 



ABSTRACT 



Geostrophic wind components computed from monthly mean atmospheric sea level pressure 

 charts are employed as indices of wind intensity over the principal current systems in the North 

 Pacific Ocean. Pressure differences across pairs of points at 36 locations and deviations from 

 long-term monthly means are tabulated for a 33-year period, 1926-58. 



The data for certain locations were combined to form regional indices representing wind 

 components tangential to the major North Pacific Ocean currents. The mean seasonal cycles of 

 tliese indices show marked differences in phase and amplitude for the various regions. Monthly 

 anomalies are found to have generally larger magnitudes in winter months than in other seasons. 

 Some evidence of long-term trends is indicated by graphical representation of time series and 

 decade means. In particular, weak wind components occurred over the Oyashio and Alaska cur- 

 rents during the winter months of the period 1946-55, while over the California current the wind 

 components were stronger, than during tlie previous two decades. 



Application of the wind index data is illustrated by a simple comparison with sea tempera- 

 ture. A correlation was found between paired values of wind indices and sea temperatures off 

 British Columbia for January, which is consistent with the concept of wind-induced advection. 

 Similarly paired values for later months were not significantly correlated. A tentative explana- 

 tion may lie in the larger magnitude of wind fluctuations in January, coupled with persistence of 

 the sea temperature anomalies. 



INTRODUCTION 



An understanding of the environmental 

 causes of fluctuations in the stocks of fish 

 comprising our commercial marine fishery 

 resources has been impeded by the complexity 

 of the environment and by the lack of ade- 

 quate information about it. This report 

 provides data on one element that plays a 

 major role in the complex train of events 

 affecting the physical environment of our 

 Pacific Ocean fish populations. This ele- 

 ment is atmospheric circulation. 



It is known that the surface circula- 

 tion of the Pacific Ocean is driven primarily 



by the winds. It follows that variations 

 in the winds will cause changes in the sur- 

 face ocean currents; they are also known 

 to alter the amount of vertical mixing cind 

 influence the intensity of upwelling. 

 These changes in turn affect the physical 

 and chemical properties of the sea water 

 and change the kinds and quantities of liv- 

 ing organisms supported by the several areas 

 of the Pacific. 



It would be desirable to measure 

 directly these various things throughout 

 all of the ocean and continuously through 

 time, but this is neither technically possi- 

 ble because we do not know what to measure, 



