SECKEL: SKIPJACK AND ENVIRONMENT 



longitude are equal, and there is a geostrophic 

 current and a wind-driven current. The incre- 

 ments of displacement of a fish school or a drift- 

 ing object by currents are expressed by 



-2.07 



A5 



w 



VwM = {Vg + Ve)M, 



w^here Vw, the velocity of the water, is the sum 

 of the geostrophic current, Vg, and the wind- 

 driven current, Ve. Vg and Ve are functions of 

 position and of time. Because the velocities are 

 vectors, numerical integration (summation of 

 increments) is facilitated by using zonal and 

 meridional components of the displacement 



LX = {Vgx + Vex) M, 



AY = (Vgy + Vey) M, respectively. 



The position of the fish school after n equal in- 

 crements of time, At, is 



jr„ = Xo + (Vgx + VEx)iAt + {Vgx + VEx)2At 



+, ... + (Vgx + VEx)nAt, 

 Yn = Yo + (Vgy + VEY)iAt + (Vgy + VEY)2At 



+ ... + (Vgy + VEY)nAt 



with an initial position Xo, Fo. 



In the model rectangle of ocean the meridional 

 distribution of dynamic height varies sinusoidal- 

 ly according to 



^0 + C(t) cos 



27r 



(y—a). 



The minimum dynamic height is at lat 10°N and 

 the maximum slopes northward from lat 20°30'N 

 at long 160°W to lat 26°30'N at long 120°W 

 (Figure 9) . Thus, the geostrophic flow is zonal 

 near lat 10 °N but acquires a meridional com- 

 ponent at higher latitudes. The amplitude of 

 dynamic height, C(t), in the sinusoidal distribu- 

 tion varies seasonally. The zonal component of 

 the geostrophic current is expressed analytically 

 by 



GX 



- -^^ (-7^ ^^^> ''- '" 



The seasonal variation of the amplitude, 

 C(t) = —[0.142 + 0.05 cos 30 (i + 1.13)], 



140 130 



WEST LONGITUDE 



Figure 9. — Dynamic topography of model ocean (dy- 

 namic meters) at the time of maximum velocity. 



0.09 



janIfeb ImarIaprImayIjunIjulIaugIsepI octInovIdecI 



MONTH 



Figure 10. — Seasonal variation in amplitude, C(t), for 

 the harmonic expression of dynamic height as a function 

 of latitude. 



is shown in Figure 10. 



The wave length, a function of longitude, x, is 

 twice the width of the current: L(x) =69 — 

 0.3a;. 



775 



