SECKEL: SKIPJACK AND ENVIRONMENT 



30°- 



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 o 



<20* 



on 

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10° 



160° 



150° 140° 130° 



WEST LONGITUDE 



120° 



Figure 16. — Schematic presentation of geostrophic flow 

 in the North Equatorial Current with meridional com- 

 ponents of geostrophic flow and wind-driven current 

 (insert). 



is shown in Figure 15. Also shown are the me- 

 ridional slopes computed from historic cruises. 

 These values fall into the seasonal pattern. 

 Qualitative interpretations of Wyrtki's and Rob- 

 inson's charts referred to above confirm the TW- 

 ZO results and show that the seasonal variation 

 extends to long 120°W. 



The model east-west component of geostrophic 

 flow, therefore, compares well with available 

 oceanographic station and bathythermograph 

 data. Although a small north-south component 

 of geostrophic flow is computed from the ana- 

 lytical expression of dynamic height used in the 

 model, this component is not adequately repre- 

 sented. A schematic presentation of geostrophic 

 flow in Figure 16 illustrates that south of lat 

 15°N the Equatorial Current is essentially zonal 

 but north of this latitude, the meridional com- 

 ponent increases to north of lat 20 °N where the 

 geostrophic current is mostly meridional. 



The underestimate of the meridional compo- 

 nent of geostrophic flow in the northern portion 

 of the model has the effect of shifting the main 

 concentration of drifting objects north. The 

 underestimate, however, does not affect the dur- 

 ation of east to west drift. 



In the model only drift within the North Equa- 

 torial Current is considered and objects that pass 

 across its northern boundary are not plotted in 

 Figures 12 and 13. In the ocean, drifting objects 

 that pass across the northern boundary are car- 

 ried back into the convergence by easterly and 

 southerly components of geostrophic flow. Thus, 



the concentration of drifting objects in the con- 

 vergence near Hawaii is expected to be larger 

 than shown in the model. 



The wind stress values of the Scripps Insti- 

 tution of Oceanography (University of Califor- 

 nia, 1948) that were used represent average con- 

 ditions in the trade wind region. Important in 

 the drift model is a northward component of 

 wind drift that is proportional to the zonal com- 

 ponent of wind stress. This characteristic fea- 

 ture is not lost by averaging wind stress values 

 in 5-degree latitude bands between long 120° 

 and 160°W. 



The simple procedure of dividing the Ekman 

 transport by 100 m to obtain the wind-driven 

 current may be questioned. This procedure, 

 however, affects the magnitude of wind drift 

 and not the characteristic feature essential in the 

 drift model; a meridional component of wind- 

 driven current. 



The results of the drift model are therefore 

 the same as those to be expected in the portion 

 of ocean under consideration if average condi- 

 tions prevail. Drifting objects or skipjack 

 schools uniformly distributed along long 120°W 

 between lat 10° and 20°N drift westward and 

 southward with the geostrophic North Equator- 

 ial Current. A component of surface wind-driv- 

 en current in the direction to the right of the 

 wind stress vector displaces the skipjack schools 

 northward. In consequence of the converging 

 flow fields, the concentration of drifting schools 

 increases westward much as in the model. 



The drift model is based on a smooth North 

 Equatorial Current representing average con- 

 ditions. From Figure 17, showing the time var- 

 iation of the east-west component of geostrophic 

 flow at long 148°W during the TWZO investiga- 

 tion, it is evident that the North Equatorial Cur- 

 rent is not a smooth flowing stream. Cells of 

 high westerly speeds alternate with low speeds 

 or even easterly flow directions. A fish school 

 caught in high westerly flow of more than 25 cm 

 sec~\ such as occurred near lat 15°N during 

 August to October 1964, would in 2 months be 

 carried about 1,300 km (700 nautical miles) 

 westward. Thus, much shorter drift durations 

 can occur than those given in the model. An 



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