FISHERY BULLETIN: VOL. 70, NO. 3 



The meridional component of geostrophic cur- 

 rent is given as a function of latitude, y, longi- 

 tude, X, and the zonal component of geostrophic 

 current, Vex: 



GY 



230— x 



Vgx, 



where x and y are the degrees of longitude and 

 latitude in the model ocean, with the coordinates 

 positive westward and northward. L{x) is given 

 in degrees latitude, and t is the time of the year 

 in months. The phase angle, a, equals 10°, and 

 in the dynamic height expression Aq equals 

 1.875 dyn m. The zonal component of geostroph- 

 ic flow in cm sec^ is given by 



„ 61.69 



sm y 



In the drift model it is most convenient to ex- 

 press the flow in nautical miles per month with 



r. 862.9 



Ai = — ■• • 



sm y 



The wind-driven current is based on the wind 

 stress values of the Scripps Institution of Ocean- 

 ography (University of Cahfornia, 1948). The 

 zonal and meridional components of stress were 

 averaged between long 120° and 160°W for each 

 month of the year and each latitude band 10°- 

 14°N, 15°-19°N, and 20°-24°N. The variation 

 with time, t, of the zonal components, tx, and the 

 meridional components, Xy, of the wind stress are 

 adequately defined by the harmonic functions in 

 Table 6, and shown in Figure 11. 



The meridional and zonal components of Ek- 

 man transport are calculated from the equations 

 Ey — — Tx/f, and Ex — r,y//, where /is the Cor- 

 iolis parameter. Most of the transport takes 

 place in the upper 100 m of ocean. Here, it is 

 assumed that all the transport takes place in 

 the upper 100 m. Vex = — K2 Ty and Vey = 



Table 6. — Harmonic functions of the zonal component, 

 Tx, and meridional component, Xy, of wind stress in dynes 

 cm~2. The time of year, t, is in months. 



a» 10''-14°N, T 



0.91 + 0.544 cos 30 ((-2.1) 



at I5°-19''N, T = 0.8 + 0. 188 cos 30 ((-1.2) 

 at 20°-24°N, 



T^ = 0.5<5 + 0.022 cos 30 ((+1.2) 



laj lO'-M-N, T^ = 0.56 - 0.330 cos 30 (t-2.5) 

 \af 15'>-19°N, T^ = 0.47 - 0.04 cos 30 ((-0.2) 

 lot 20°-24''N, T^ = 0.34 +. 0.045 cos 30 ((-1.4) 



K2TX then give the mean wind-driven current 

 for this depth in cm sec"' if Ko for latitude 

 bands 10°-14°N, 15°-19°N, and 20°-24°N is 

 3.17, 2.29, and 1.79, respectively. Again, for 

 application in the drift model it is most conven- 

 ient to express the drift current in nautical miles 

 per month, and K2 for latitude bands 10°-14°N, 

 15°-19°N, and 20°-24°N becomes 37.5, 27.0, and 

 21.3. 



Using the geostrophic and wind-driven cur- 

 rent speeds, the drift displacements can be cal- 

 culated. Once per month 11 objects are intro- 

 duced along the eastern boundary of the model 

 ocean, equally spaced from lat 10° to 20°N. Dis- 

 placements are calculated and the new position 

 at the end of the month is determined. Again, 

 displacements are calculated for the new time 

 and position, and the positions at the end of the 

 second month determined. These calculations 

 are repeated for 36 months or stopped before 

 that time if the western boundary (long 160°W) 

 is crossed or when the object drifts into easterly 

 (negative) flow at the northern boundary of the 

 westerly setting geostrophic current. It is as- 

 sumed that the drifting objects move up and 

 down in the upper 100 m much as a skipjack 

 school may be doing. 



The result of the numerical integration is 

 shown in Figure 12. The location of drifting 

 objects that were introduced at the beginning 

 of April, May, and June at lat 10° to 20° N along 

 long 120°W are traced across the model ocean 

 in steps of 3 months. After 12 months all drift- 

 ing objects are north of lat 15°N. The objects 

 that began north of lat 15 °N are the first to drift 

 into the northern, slow portion of the Equatorial 

 Current and are overtaken by the objects that 

 began at and to the south of lat 15°N. Thus a 

 meridional distribution of objects at the begin- 

 ning becomes oriented along the northern edge 

 of the model equatorial current after 24 months 

 of drifting. 



Another presentation of the results (Figure 

 13) shows the location, after 30 months, of all 

 objects that were introduced at the beginning of 

 each of the 30 months along long 120°W. The 

 most westerly position reached by objects during 

 each quarter is indicated by a dotted line. Again, 

 it is evident that objects initially located south 



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