TSUCHIYA: SURFACE GEOSTROPHIC FLOW 



these maps is an artifact of short-period 

 fluctuations of the mass field.) In these months 

 the Countercurrent shows a drastic change in in- 

 tensity; its rapid disappearance and reappear- 

 ance on this sequence of maps (Figures 1-3, 6, and 

 7) are remarkable. The maps demonstrate that 

 the Countercurrent can either disintegrate or 

 reestablish itself on a time scale less than 2 mo. 



A comparison of the present maps (Figures 1-7) 

 with meteorological charts in the EASTROPAC 

 atlas (Love, 1971, 1972a, 1972b, 1973, in press. 

 See also the position of the ITCZ indicated at the 

 eastern and western ends of each map.) reveals a 

 rather good correlation between the positions of 

 the North Equatorial Countercurrent and the 

 ITCZ. The northern boundary of the Countercur- 

 rent (minimum of geopotential anomaly) coin- 

 cides approximately with the ITCZ except in 

 February-March and August-September. In 

 February-March the southern boundary of the 

 Countercurrent, if it is present, more nearly coin- 

 cides with the ITCZ; in August-September the 

 ITCZ is located far to the north of the northern 

 boundary. This finding is in agreement with what 

 can be seen on Wyrtki's ( 1965) long-term average 

 charts. 



There is a high degree of similarity in 

 geostrophic-flow patterns between February- 

 March 1967 (Figure 1) and February- April 1968 

 (Figure 7). The major features are much the same 

 for the two maps; even the development of the 

 cyclonic and anticyclonic eddies off Costa Rica, 

 the discontinuity of the North Equatorial Coun- 

 tercurrent near long. 98°W, and the eastward 

 flow along about lat. 10°S are similar. In view of 

 the large variations observed between the two 

 cruise periods, this similarity is perhaps surpris- 

 ing. The only notable difference is the latitude of 

 the minimum of geopotential anomaly near the 

 equator (discussed in the preceding section) west 

 of the Galapagos Islands; the minimum is located 

 at the equator in 1967 (Figure 1), while it is lo- 

 cated a few degrees south of the equator in 1968 

 (Figure 7). The southward shift of the minimum 

 in Figure 7 is probably due to the more southerly 

 trade winds near the equator in 1968 than in 

 1967 (Cromwell, 1953). 



The eastward current indicated along about 

 lat. 10°S on the southern-summer maps (Figures 

 1 and 7) is of particular interest, because no pre- 

 vious data from the eastern Pacific have sug- 

 gested it. This countercurrent is very weak (the 

 average geostrophic speed from seven meridional 



sections is 7 cm/s) and was not noticed in earlier 

 examination of vertical sections based on EAS- 

 TROPAC data (Tsuchiya, 1972). It is a narrow and 

 thin current (about 50 m thick) and is completely 

 separate from a subsurface eastward current 

 which has its maximum speed at a depth of 70-200 

 m and which flows along about lat. 6°S (Stroup, 

 1969; Tsuchiya, 1972).=^ More data are needed to 

 determine the relation of this surface eastward 

 countercurrent with the South Equatorial Count- 

 ercurrent, which is well developed near lat. 10°S 

 at the sea surface of the central and western 

 Pacific (Reid, 1959, 1961; Merle, Rotschi, and Voi- 

 turiez, 1969; Rotschi, 1970; Tsuchiya, 1970; Don- 

 guy and Rotschi, 1970). As was mentioned in the 

 preceding section, the former countercurrent is 

 defined only between long. 112°W and long. 90°W 

 and is not found in the west of the EASTROPAC 

 area. 



ACKNOWLEDGMENTS 



This work was part of the Scripps Tuna Ocean- 

 ography Research Program, Institute of Marine 

 Resources, Scripps Institution of Oceanography, 

 and was supported by National Science Founda- 

 tion Grant GA-29748 and by Contract 03-3-208-36 

 between the National Marine Fisheries Service 

 and the Institute of Marine Resources. Support 

 was also provided by the Marine Life Research 

 Program of the Scripps Institution of Oceanog- 

 raphy. 



I especially wish to thank Daniel R. Cayan for 

 his assistance throughout the course of this study. 

 I also thank Forrest R. Miller for supplying some 

 of the EASTROPAC meteorological data prior to 

 publication. 



LITERATURE CITED 



Austin, T. S. 



1960. Oceanography of the east central equatorial Pacific 

 as observed during expedition Eastropic. U.S. Fish Wildl. 

 Serv., Fish. Bull. 60:257-282. 

 Bennett, E. B. 



1963. An oceanographic atlas of the eastern tropical Pacific 

 Ocean, based on data from EASTROPIC Expedition, 

 October-December 1955. [In Engl, and Span.] Bull. 

 Inter-Am. Trop. Tuna Comm. 8:31-165. 

 Brandhorst, W. 



1958. Thermocline topography, zooplankton standing 

 crop, and mechanisms of fertilization in the eastern trop- 

 ical Pacific. J. Cons. 24:16-31. 



^A detailed report on this subsurface eastward countercurrent 

 is being prepared for publication elsewhere. 



1085 



