WILLIAMS: MODELS OF MIGRATION OF YOUNG SKIPJACK 



Figure 3. — Schematic chart of surface circulation in the 

 tropical central-east Pacific Ocean. NEC = North Equa- 

 torial Current; NECC = North Equatorial Countercur- 

 rent; SEC = South Equatorial Current; SECC = 

 South Equatorial Countercurrent. 



April absent, east of long: 120°W. Although 

 Tsuchiya (1968) showed that below a depth of 

 50-100 m the NECC eastward transport is con- 

 tinuous throughout the year, more recent calcu- 

 lations of geostrophic flow from EASTROPAC 

 data (M. Tsuchiya, unpublished data and per- 

 sonal communication) show that the subsurface 

 transport is separate (deeper and slightly far- 

 ther south) and that the surface NECC does 

 cease or become minute and variable from Jan- 

 uary to May. 



A striking relationship between development 

 of the surface NECC and the latitudinal position 

 of the Intertropical Convergence Zone (ITCZ) 

 (meteorological equator) at the meridian of 

 120° W was presented in Wyrtki (1965). The 

 NECC ceases east of long 120°W with the an- 

 nual movement of the ITCZ south of lat 8°N 

 about January, and when it returns north of lat 

 8°N about May, the NECC is reestablished. 

 Hence, variations in the ITCZ position at long 

 120°W may be reflected in the time of cessation 

 and resumption of the surface NECC, i.e., dur- 

 ation of the interruption. Abrupt interruption 

 of the current is apparently not the case, and at 

 least in January and May, it is weak and inter- 

 mittent, and eddies and meanders may occur. 

 [Superimposed on the overall monthly trend is 



also variability due to short-term (daily) fluctu- 

 ations in ITCZ position.] Monthly ITCZ posi- 

 tions at long 120°W and NECC position based 

 on Wyrtki (1965, Figure 18) are reproduced 

 here as Figure 4, together with ITCZ monthly 

 positions based on a 77-year mean of observa- 

 tions from the National Weather Records Center 

 and prepared by the Goddard Space Flight 

 Center, NASA (Allison et al., 1969). The po- 

 sition of the ITCZ is controlled by the variations 

 in the location and strength of the wind fields 

 in the central-east Pacific, the northeast and 

 southeast trades, and the pressure fields associ- 

 ated with them. 



Wyrtki (1966) stated the California Current 

 is strong and penetrates farthest south and con- 

 tributes most of the water to the NEC in the 

 period February to April. The California Cur- 

 rent is weak and located north of lat 20 °N from 

 August to December while in January and May- 

 July it shows intermediate positions and 

 strengths. 



Tsuchiya (1968, 1970) has reviewed the pre- 

 sent scanty information on the zonally narrow 



Sept. 



FiGURR 4. — Sea.sonal variations in the position of the 

 Intertropical Convergence Zone (ITCZ) and of the 

 boundaries of the North Equatorial Countercurrent 

 (NECC) at long 120°W. 



745 



