VARIATION OF THE SURFACE GEOSTROPHIC FLOW IN 

 THE EASTERN INTERTROPICAL PACIFIC OCEAN 



MiZUKI TSUCHIYA^ 



ABSTRACT 



A sequence of seven maps is presented to show the distribution of geopotential anomaly at the sea 

 surface in the eastern intertropical Pacific Ocean. Each map represents a 2-mo period during the 

 EASTROPAC expedition from February 1967 to April 1968. 



The most striking feature revealed by these maps is the variation of the North Equatorial Counter- 

 current in response to the annual variation of the atmospheric circulation. In February-April (both 

 1967 and 1968), when the atmospheric intertropical convergence zone (ITCZ) lay near its southernmost 

 position at lat. 2°-6°N, the Countercurrent was discontinuous and was rapidly changing in intensity. In 

 August-September, when the ITCZ lay near its northernmost position at lat. 11°-15°N, the Countercur- 

 rent was strong, broad, and extended east all the way to the coast of Costa Rica. 



In southern summer a weak and narrow eastward current was indicated along about lat. 10°S 

 between long. 112° and 90°W within the westward flow of the South Equatorial Current. This current 

 is so weak that it is probably buried in the westward Ekman drift due to the southeast trades and can be 

 observed only when the trade winds are unusually weak. 



An eastward current, which can be interpreted as the Equatorial Undercurrent breaking the sea 

 surface, was indicated within about 2° of the equator in April-May, when the southeast trades were 

 relatively weak near the equator. 



The distributions of relative geostrophic flow in February-March 1967 and February-April 1968 

 were remarkably similar over the entire study area. 



The purpose of this paper is to present a sequence 

 of seven maps (Figures 1-7) showing the distribu- 

 tion of geopotential anomaly at the sea surface of 

 the eastern intertropical Pacific Ocean from Feb- 

 ruary 1967 to April 1968 and to discuss the varia- 

 tions of the circulation revealed by these maps. 

 Each map represents a 2-mo period during the 

 EASTROPAC expedition, which was an interna- 

 tional cooperative oceanographic investigation 

 coordinated by the Bureau of Commercial 

 Fisheries (now National Marine Fisheries Ser- 

 vice). 



Prior to EASTROPAC a considerable number of 

 expeditions took place in the eastern intertropical 

 Pacific Ocean; consequently, its principal ocean- 

 ographic features were reasonably well known 

 (e.g., Wooster and Cromwell, 1958; Bennett, 1963; 

 Wyrtki, 1966, 1967; Tsuchiya, 1968; Stroup, 

 1969). However, the accumulated data were too 

 sparse in time and space to give insight into 

 monthly or seasonal variations in the distribution 

 of oceanographic properties. The EASTROPAC 

 expedition was designed to acquire data to bring to 

 light these time variations. 



'Institute of Marine Resources, Scripps Institution of 

 Oceanography, University of California at San Diego, La JoUa, 

 CA 92037. 



Manuscript accepted January 1974. 

 FISHERY BULLETIN: VOL. 72, NO. 4, 1974. 



The expedition was divided into seven 2-mo 

 cruise periods. During each period, a single-ship 

 or multiship cruise was carried out. Multiship 

 cruises took place in February-March, August- 

 September 1967, and February- April 1968. These 

 cruises covered the area between lat. 20°N and 

 20°S (15°S in August-September 1967) and from 

 the coast of the American continents westward to 

 long. 119°W (126°W in February-March 1967). 

 Single-ship cruises took place in April-May, 

 June-July, October-November 1967, and De- 

 cember 1967-January 1968 in time intervals be- 

 tween the multiship cruise periods. Each of these 

 cruises covered the area between lat. 20°N and 3°S 

 and between long. 98° and 119°W. 



Details of the observational program, the list of 

 participating vessels, and track charts have been 

 published in the EASTROPAC atlas (Love, 

 1972a). 



DATA 



Almost all EASTROPAC stations with observa- 

 tions to 500 m or deeper were used in this study. 

 These stations are listed in Table 1 and can be 

 identified on the track charts included in the 

 EASTROPAC atlas (Love, 1972a). 



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