TSUCHIYA: SURFACE GEOSTROPHIC FLOW 



On the other hand, set-and-drift observations are 

 affected by strong winds, sea, swell, and tidal cur- 

 rents. 



South Equatorial Current Region 



The South Equatorial Current is well defined 

 west of long. 90°W on the maps for February- 

 March 1967, August-September 1967, and 

 February- April 1968 (Figures 1,4, and 7). South of 

 lat. 10°S the direction of geostrophic flow is pre- 

 dominantly to the northwest, whereas the drift 

 charts (Meteorological Office, 1939; Wyrtki, 1965) 

 for the corresponding months indicate that the 

 surface current flows almost due west in this re- 

 gion. This disagreement may be due to the effect of 

 the Ekman drift. The trade winds in this area are 

 from the southeast to east, so that if the Ekman 

 drift is added to the geostrophic flow, the resultant 

 surface current would be nearly to the west. 



North of lat. 10°S the South Equatorial Current 

 becomes more zonal and flows almost due west. In 

 February-March 1967 (Figure 1) the westward 

 flow of the South Equatorial Current extends 

 across the equator to about lat. 5°N, and the cur- 

 rent near the equator west of the Galapagos Is- 

 lands is also westward. In April-May 1967 (Figure 

 2) the westward flow of the South Equatorial Cur- 

 rent is interrupted by an eastward current within 

 about 2° of the equator. Puis' (1895) charts of the 

 surface current for March and April clearly show 

 an eastward current at the equator between long. 

 110°W and the Galapagos Islands. Such an east- 

 ward current can be interpreted as the Equatorial 

 Undercurrent breaking the sea surface during 

 local weakening of the easterly trades (Cromwell, 

 Montgomery, and Stroup, 1954; Montgomery, 

 1962; Montgomery and Stroup, 1962:59-60). Jones 

 (1969) has presented evidence of a surfacing of the 

 Undercurrent on the basis of direct current mea- 

 surements made at long. 98°W in April 1968 

 (EASTROPAC cruise Thomas Washington 75). 

 The distribution of geopotential anomaly from 

 this cruise, however, does not suggest a surfacing 

 of the Undercurrent (Figure 7). 



On the other maps (Figures 3-7) flow is west- 

 ward from the equator to about lat. 5°N, and in 

 some longitudes an eastward current is revealed 

 just south of the equator, because geopotential 

 anomaly does not show a minimum at the equator 

 but a few degrees of latitude south of the equator. 

 The same distribution of geopotential anomaly 

 can be seen on Bennett's (1963, Figure 6) map 



based on EASTROPIC data. This distribution is 

 associated with a thermocline ridge that tends to 

 occur at lat. 1°-3°S (instead of the equator) in the 

 eastern Pacific. This southward displacement of 

 the ridge from the equator is clearly evident on 

 many of EASTROPAC vertical sections of tem- 

 perature or thermosteric anomaly and maps of the 

 topography of the 300-cl/t isanosteric surface, 

 which lies close to the center of the thermocline 

 (Love, 1971, 1972b, 1973, in press). According to 

 Cromwell's (1953) simple model of the wind- 

 driven meridional circulation, the direction of the 

 wind near the equator determines the position of 

 the maximum divergence of the Ekman transport 

 in the surface layer. He points out that the merid- 

 ional component of the southeast trades shifts the 

 maximum divergence, which would correspond to 

 a ridge of the thermocline, to the south of the 

 equator. 



The eastward geostrophic flow between the 

 equator and the ridge was first discussed by Aus- 

 tin ( 1960) and later commented upon by Stroup 

 (1969:35). It is not certain, however, that the ac- 

 tual surface current is eastward south of the 

 equator. Estimates of the magnitudes of terms in 

 the equation of motion suggest that the southward 

 pressure gradient is in approximate balance with 

 the northward component of wind stress. The M. 

 O. 435 drift chart for the May-July quarter shows 

 very weak easterly components just south of the 

 equator at long. 100°-110°W between strong 

 westward currents to the north and south. The 

 drift charts for the other quarters show no evi- 

 dence of an eastward current south of the equator. 



The southern-summer maps (Figures 1 and 7) 

 exhibit a weak eastward current along about lat. 

 10°S from long. 112°W to about 90°W. Examina- 

 tion of vertical sections of temperature and maps 

 of surface temperature in the EASTROPAC atlas 

 (Love, 1972a, in press) indicates that this current 

 is associated with a slight southward shoaling of a 

 shallow summer thermocline and with a merid- 

 ional temperature gradient developed during 

 summer between a pool of warm surface water 

 south of the equator and cold surface water farther 

 south that appears to be coming from the Chile 

 Current (Wooster, 1970). This suggests that the 

 eastward geostrophic current along lat. 10°S is 

 found only in southern summer. (It is interesting 

 to note that the North Equatorial Countercurrent, 

 which flows east at roughly the same latitude in 

 the northern hemisphere, is most strongly de- 

 veloped during the same season, i.e., northern 



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