Oceanography of the Giilf of Tehuantepec 



(H. Blackburn) 



The Gulf of Tehuantepec is a bight which 

 forms the southern boundary of the Isthmus 

 of Tehuantepec in southeastern Mexico, near 

 Guatemala (fig. 1). The isthmus runs east 

 and west, is only 120 n-iiles in breadth between 

 the Pacific and the Gulf of Mexico, and is only 

 735 ft. above sea level at its lowest point 

 (the narrow Chivela Pass, oriented north and 

 south along 95° W. through the Sierra Madre). 



Because of this topography the Gulf of Te- 

 huantepec is exposed to northerly winds from 

 the Gulf of Mexico and the Plains States. The 

 northerly is the predominant wind in frequency 

 and strength throughout the year, especially 

 from October through April when gales, called 

 Tehuantepecer s, are fairly common in the Gulf. 

 Sea surface temperatures are lower in the 

 affected area of the Gulf than elsewhere over 

 most of this period, as shown for instance 

 in figure 5. These facts have long been known: 

 in conjunction with observations of average 

 monthly surface current in the Gulf (Cromwell 

 and Bennett, 1959), and observations made at 

 a few oceanographic stations on Expedition 



EASTROPIC (Brandhorst, 1958), they sug- 

 gested a pattern of wind-connected ocean 

 phenomena leading to seasonal enrichment of 

 near- surface waters and parallel changes in 

 abundance of yellowfin tuna. 



The area was selected for intensive study 

 because it was thought to offer good prospects 

 for explaining changes in tuna availability in 

 terms of well-marked changes in sea and 

 wind conditions, and so helping fishermen to 

 minimize their scouting time. The intermittent 

 windiness of the area limits the time that tuna 

 boats can spend in it and makes it especially 

 desirable that they locate fish without delay. 



Six surveys of the Gulf were made on four 

 cruises by STOR investigators. The cruises 

 were TO- 58-1 (Expedition SCOT) with one 

 survey in May- June 1958; TO-58-2 with two 

 surveys in November 1958; TO-59-1 with two 

 surveys in January-February 1959; and TO- 

 59-2 with one survey in September 1959. 



Figures 7 and 8 exemplify the information 

 obtained on these cruises. Figure 7 shows 

 (above) vertical distributions of temperature 

 along east-west and north-south sections in 



70 58-2 

 RftflT 3 



Figure 7.--( Above) Temperature profiles along east-west and north-south sections in the C.ulf of Tehuantepec in November 1958, before a 

 northerly gale. (Below) Profiles along the same sections after the gale. Depth scales in meters. 



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