FISHERY BULLETIN: VOL. 70, NO. 3 



of travel, that must be considered — namely, 

 randomly swimming skipjack drift westward in 

 the North Equatorial Current. This mode of 

 travel means that skipjack entering the North 

 Equatorial Current in the eastern Pacific do not 

 migrate in the sense that they are actively swim- 

 ming towards a destination. These skipjack are 

 concentrated, or converge with the trade wind- 

 driven water near lat 15°N, and near lat 20°N 

 they are concentrated by the southward com- 

 ponent of the geostrophic current and the north- 

 ward component of wind-driven current. As the 

 fish schools approach Hawaii, they further ac- 

 cumulate because of decreasing geostrophic flow. 



COMPARISON OF THE MODEL WITH 

 THE REAL OCEAN 



In the model the effects of characteristic 

 features in the geostrophic and wind-driven cur- 

 rents on the distribution of drifting objects were 

 demonstrated. The simplest analytic expres- 

 sions for the North Equatorial Current and the 

 wind distribution were used in order to facili- 

 tate numerical integration. How well do these 

 simple expressions reflect the average conditions 

 as we know them to exist in the ocean? 



First, consider the field of geostrophic flow. 

 In the model area, data from historic oceano- 

 graphic cruises are sparse, but a chart of the 

 Pacific Ocean dynamic topography prepared by 

 Reid (1962) shows that maximum and minimum 

 dynamic heights diff"er by about 0.4 dyn m as they 

 do in Figure 9. Average geostrophic speeds in 

 the model and the ocean are therefore of the 

 same magnitude. 



There is some uncertainty in the width of the 

 North Equatorial Current at long 120°W. At 

 this meridian Reid's chart shows the northern 

 edge to be at about lat 15° N. A qualitative geo- 

 strophic interpretation of Barkley's (1968) 

 depth of the sigma-t 25.4 surface, in the main 

 pycnocline, places the northern edge near lat 

 25°N at long 120° W and near lat 21 °N at long 

 160°W. 



Many more bathythermograph data than 

 oceanographic station data are available. A geo- 

 strophic interpretation of Wyrtki's (1964) 

 depths of the center of the permanent thermo- 



cline places the northern edge of the Equatorial 

 Current to the north of lat 20 °N in the eastern 

 part of the model area. More recently, charts 

 of the temperature distribution at 400 ft pre- 

 pared from bathythermograph data by Robinson 

 and Bauer' places the northern edge of the Equa- 

 torial Current near lat 18° or 19°N at long 160°W 

 and from about lat 20° to 25°N at long 120°W. 

 In the vicinity of the Hawaiian Islands, the 

 northern edge of the Equatorial Current inferred 

 from Robinson's charts compares well with the 

 results of the TWZO investigation (Charnell, 

 Au, and Seckel, 1967a,b,c,d,e,f), 



The seasonal variation of the geostrophic flow 

 in the North Equatorial Current observed during 

 the TWZO investigation was reported by Seckel 

 (1970c). The meridional slope of dynamic 

 height between lat 10° and 20°N computed for 

 each cruise of this investigation, reflecting the 

 mean, zonal component of geostrophic current 



• Robinson, M. K., and R. A. Bauer, Atlas of monthly 

 mean sea surface and subsurface temperature and depth 

 of the top of the thermocline, North Pacific Ocean. Un- 

 published manuscript reproduced by Fleet Numerical 

 Weather Central, 1971. 



o 



I- 



lij 

 Q. 



Z 



o 



1964 



1965 



Figure 15. — Meridional slopes of dynamic height, lat 

 10° to 20°N for every month from February 1964 to 

 June 1965 of the Trade Wind Zone Oceanography cruises, 

 connected by straight lines. Numerals indicate the mer- 

 idional slopes of dynamic height for the cruises of the 

 Carnegie, 1929, and Hugh M. Smith (HMS), 1950-1957. 



780 



