especially as to the living organisms or 

 the trace chemicals, nor is it practically 

 feasible because of the high cost of making 

 multifold continuous observations at sea of 

 even a very few parameters. 



On the other hand, the far flung sys- 

 tem of recording atmospheric conditions at 

 sea and on neighboring land has amassed a 

 vast store of information with the required 

 continuity in time eind space. Our purpose 

 is to put at the disposal of marine biolo- 

 gists in general, suid fishery biologists in 

 particular, a numerical description of one 

 category of atmospheric events for their 

 use in studying the fluctuations of particu- 

 lar populations of living organisms. It is 

 hoped that the information also will assist 

 researchers interested in atmosphere-ocean 

 interrelationships. Since the events 

 described are several steps removed from 

 the events impinging on the organisms and 

 are related through intermediate processes, 

 we have preceded the tabular matter with 

 explanations of the nature of the data and 

 given one comparatively simple example 

 suggesting how the information may help to 

 elucidate population fluctuation problems. 



BASIC DATA 



Atmospheric pressure at sea level was 

 selected as the category of data from which 

 significant oscillations in the major wind 

 systems could be most readily inferred over 

 large axeas of the Pacific appropriate for 

 further inferences as to the movements and 

 properties of the several water masses of 

 the ocean. These data are readily available 

 for a reasonably long period of record in 

 the form of synoptic charts jind mean charts 

 for various time intervals. 



Sources and Definitions 



Specifically, we have used monthly 

 mean sea level pressure charts kindly sup- 

 plied by the U. S. Weather Bureau. The 

 charts utilized cover the period from 1926 

 to 1958 with a gap from July 1939 to Sep- 

 tember 1940. A few gaps also occur in the 

 data over the western Pacific for the peri- 

 od of World War II. Information from the 

 charts was reduced to a set of numerical 

 indices intended to describe simply, albeit 

 crudely, the variations which have occurred 

 over the past thirty years in those portions 

 of the wind system which appear most effec- 



tive in driving the Pacific Ocean surface 

 circulation. The index system Wtis designed 

 for study of both regional and ocean-wide 

 phenomena. 



The wind indices were derived from the 

 pressure charts using the geostrophic approx- 

 imation. This approximation, based on the 

 distribution of pressure, gives the motion 

 the air would assume under the force exerted 

 by the horizontal gradient of atmospheric 

 pressure in balance with the force imparted 

 by the earth's rotation. Because it assumes 

 a hypothetical condition in which friction, 

 acceleration, and boundary conditions are 

 neglected, the "geostrophic wind" differs 

 somewhat from the observed wind. For the 

 present application the differences are not 

 serious except possibly in areas south of 

 30° latitude. Owing to the space and time 

 scale used for constructing the indices, 

 small-scale, short-term perturbations are 

 not registered. Thus the indices pertain 

 to the general, broad-scale motion, which 

 is the part of the motion spectrum that we 

 desire to portray. There is also a system- 

 atic deviation of observed wind, at sea 

 level, to the left of the geostrophic wind 

 in the northern hemisphere. Since the wind 

 imparts to the water a motion that is to 

 the right of the wind stress direction, the 

 geostrophic wind more nearly approximates 

 the direction of induced water movement than 

 would the actual observed winds. Moreover, 

 of the data available we consider the pres- 

 sure observations to be more reliable than 

 the wind observations for this application. 

 Fortunately, the pressure gradients can be 

 computed quickly from the isobars appearing 

 on the conventional weather charts. 



In the data given here, pressure dif- 

 ferences were taken across 36 pairs of 

 geographically fixed points in selected 

 areas of the Pacific. The component of 

 geostrophic wind normal to the line joining 

 two points is proportional to the pressure 

 difference between them. This relation can 

 be expressed as follows: 



(/) 



v= 



I 



2Q.psinCt) As 



where l\p is the pressure difference be- 

 tween points separated by a distance /\q, 

 O is the angular velocity of the earth, O 

 is air density and OTis latitude. To facili- 

 tate computation of wind components, the 

 correction for latitude in the geostrophic 



