ZOOPLANKTON OF CENTRAL PACIFIC 



373 



tercurrent sandwiched in between. Although 

 the boundaries of the Countercurrent may vary 

 meridionally witli longitude and season, its south- 

 ern and northern boundaries ordinarily occur 

 near 5° X. and 10° X. latitude in the mid-Pacific. 

 The South Equatorial Current is therefore on 

 both sides of the Equator while the Xorth Equa- 

 torial Current is confined entirely to the Xorthern 

 Hemisphere. As previously stated, the Equator 

 is the site of upwelling resulting from divergence 

 of the surface waters. It is also the location of 

 the newly discovered subsurface Equatorial Un- 

 dercurrent flowing to the eastward (Cromwell 

 and others, 1954). The region between the 

 Equator and the southern boundary of the Coun- 

 tercurrent is a zone of convergence. Under 

 certain conditions, as described by Cromwell 

 (1953) and Cromwell and Reid (1956), a sharply 

 defined convergence or "front" * may be formed 

 in the South Equatorial Current between the 

 Equator and the southern boundary of the 

 Countercurrent. 



The motion of these currents is either directly 

 or indirectly the result of wind stress on the 

 surface of the ocean, and it is logical that varia- 

 tions in these currents are a reflection of variations 

 in the prevailing winds or "trades." 



The Climatic Charts of the Oceans (U. S. 

 Weather Bureau, 1938), based on averages of 50 

 years of observations, provide a general picture 

 of the velocity and direction of prevailing winds 

 in the equatorial Pacific. Average wind condi- 

 tions for the months of March and August, 

 which represent the extremes of the seasonal 

 variation, are shown in figure 6. 



In the region of our zooplankton studies (110° 

 W. to 180° long.), the charts show longitudinal 

 and latitudinal as well as seasonal variations 

 in tlie tradewinds. In an east-west direction 

 along the Ecjuator there is a general decrease in 

 intensity from Beaufort force 3 and 4 east of 160° 

 W. longitude to force 1 and 2 west of that merid- 

 ian. Between 100° W. and 140° W. the south- 

 east trades are dominant O60 percent constant) 

 along the Equator in all months of the year. 

 Between 140° W. and 160° W. they are dominant 

 from May to January; between 160° W. and 

 180° they are only of importance from Julj' to 



s Defined hy Cromwell (\9^) as "a pronounced oceanic convergence," 

 and hy Cromwell and Reid (IQ.Vi^ as "a narrow band along the sea surface 

 across which the density change,s abruptly" and "the surface temperature 

 gradient Is often of the order of degrees per l/10() mile." 



October. At other months of the year the re- 

 sultant wind at the Equator is from the east 

 between 140° W. and 160 W., and from the north- 

 east between 160° W. and 180°. 



Xorth of the Equator in the region of the 

 Countercurrent, the period of strongest winds is 

 from December to May when the northeast 

 trades prevail. At other months of the year the 

 winds are light and variable; in the eastern part 

 of the region, from 120° W. to 140° W., the 

 southeast trades exert a slight influence. Longi- 

 tudinally the northeast trades reach their highest 

 velocity between 140° W. and 170° W. longitude. 



According to the wind drift model of Cromwell 

 (1953), convergence and sinking of the surface 

 waters will occur to the north of the Equator 

 in the South Equatorial Current during a south 

 or southeast wind, and conversely to the south of 

 the Equator under the influence of a north or 

 northeast wind. A pronounced convergence or 

 front has been encountered south of the Equator 

 on only one of the many POFI hydrographic 

 and fishing surveys. This is not surprising in 

 view of the slight influence of north or northeast 

 winds at the Equator in the eastern and central 

 Pacific. Evidence of convergence north of the 

 Equator has been observed, though, on several 

 occasions. 



When the generally westward current near the 

 Equator has a northward component, as during 

 southeast trade winds, we anticipate that the 

 zone of greatest zooplankton abundance will be 

 north of the Equator, due both to the physical 

 displacement of the organisms and the time lag 

 in their development, with the peak of abundance 

 occurring somewhere in the zone of convergence 

 between the region of upwelling and the southern 

 boundary of the Countercurrent. With a pre- 

 vailing northeast wind the zooplankton maximum 

 should theoretically occur to the south of the 

 Equator and, with an east wind, more nearly 

 on the Equator or with a double peak.* 



In summary, then, as a result of the direction 

 and relative high velocity of the trade winds, we 

 expect to find larger concentrations of zooplankton 



« Murphy and Shomura (IQ.Wb) have shown that the latitudinal variation 

 in the zone of best-yellowfln-catch also coincides with differences in the pre- 

 vailing winds. Fishing sections along 120° W. and 13n^ W. longitude, asso- 

 ciated with southeast winds. Indicated the [>eak abundance to be north of 

 the Equator; the catch along l.'iS^ W. and lfi9* W., associated with variable 

 winds, showed the peak abundance to be nearly centered on the Equator, 

 while a section along 180°, associated with northeast winds. Indicated the peak 

 of yellowfin abundance to be displaced to the south. 



