AN OCEANOGRAPHIC STUDY OF THE OF TEHUANTEPEC' 



By Maurice Blackburn 



ABSTRACT 



The results of four cruises to the Gulf of Tehuantepec show that chemical 

 enrichment and biological production in near-surface waters result from the 

 effect of transisthmian northerly winds in (a) setting up a circulation of water 

 which results in a domed or ridged discontinuity layer and (b) stirring the upper 

 part of that layer in the region where it is closest to the surface. The wind-induced 

 circulation also acts on the zooplankton produced, sometimes dispersing it down- 

 stream from the enriched area and sometimes concentrating it in a clockwise 

 eddy to the west of the ridge. 



INTRODUCTION 



The Gulf of Tehuantepec is a bight which 

 forms the southern boundary of the Isthmus 

 of Tehuantepec in southeastern Mexico. It is 

 the southernmost major geographic feature 

 of the Mexican Pacific coast, the last one to 

 be passed by coastal ships before they reach 

 the coast of Guatemala. 



The first attempt to make an organized 

 oceanographic survey in the Gulf of Tehuan- 

 tepec was in December 1955, during the 

 Expedition EASTROPIC. The survey was brief, 

 partly because of bad weather. However, 

 the results assembled and interpreted by 

 Brandhorst (1958), together with information 

 from average charts, suggested a pattern of 

 wind-connected seasonal changes in the waters 

 of the Gulf that could be checked by further 

 oceanographic surveys.* 



1 Contribution from the Scripps Institution of Ocea- 

 nography. 



' A different attack upon this problem, by statistical 

 analysis of surface temperature and wind speed data 

 averaged for 2-degree rectangles and 10-day periods 

 for the years 1949 through 1957, has been made by 

 Roden (1961). This work formed part of the same 

 research program as the present paper. 



Six surveys of the Gulf were then made on 

 four cruises by investigators of the Tuna 

 Oceanography Research program of the Scripps 

 Institution of Oceanography. The cruises were: 

 TO-58-1 (Expedition SCOT) in April, May, and 

 June 1958; TO-58-2 in November and Decem- 

 ber 1958; TO-59-1 in January and February 

 1959; and TO-59-2 In August and September 

 1959. There was one survey on the first cruise, 

 two on the second, two on the third (incom- 

 plete, because of bad weather), and one on the 

 fourth. 



The purpose of this paper is to present and 

 discuss the results of these surveys, together 

 with information about the Gulf of Tehuantepec 

 from other sources, as a contribution to the 

 oceanography of the eastern tropical Pacific. 



The work was made possible by financial 

 support from the U.S. Fish and Wildlife 

 Service, Bureau of Commercial Fisheries. 

 The Inter-American Tropical Tuna Commis- 

 sion helped to staff cruises, and the late 

 Townsend Cromwell, of the Commission, gave 

 much valuable advice about the planning of 

 the investigation. The research vessels were 

 the Spencer F. Baird (TO-58-1, TO-58-2), 

 Stranger (TO-59-1), and Hugh M. Smith (TO- 

 59-2). The paper was read by or discussed 

 with the following persons who made 



