extends approximately from latitude 10° N, 

 to 20° N, along the coast in most months. 

 In the period October through February the 

 surface water in the Gulf is at least 1° C. 

 colder than that in immediately adjacent parts 

 of the ocean. The area of cold water extends 

 up to 400 miles in a south-southwesterly 

 direction from the head of the Gulf of Tehuan- 

 tepec, whereby there is more cool surface 

 water in the western half of the Gulf than in 

 the eastern half. Details are available in 

 published average monthly charts (U.S, Navy 

 Hydrographic Office, 1944; Meteorological Of- 

 fice, London, 1956). 



Mean surface current 



Cromwell and Bennett (1959) presented 

 charts of the main features of surface cir- 

 culation (average surface current set reported 

 by ships) in the eastern tropical Pacific in 

 each calendar month. An inspection of the 

 information for the Gulf of Tehuantepec showed 

 four main patterns of circulation, characteris- 

 tic of the following periods: June through 

 August, October through January, February 

 through April, and May and September. These 

 patterns are shown schematically in figure 2B: 

 unbroken arrows indicate stronger current 

 (at least 10 miles per day) and the broken 

 arrows indicate weaker current. 



The simplest pattern is the summer one 

 which shows a simple drift west-northwestward 

 through the Gulf. With the beginning of per- 

 sistent northerly winds in October this current 

 receives a strong contribution from the north 

 and the main line of flow becomes sinuous; 

 there is a weaker contribution from the north- 

 west which indicates possible continuity of 

 drift (a clockwise eddy) in the western part of 

 the region, and this is shown in the January 

 chart of Cromwell and Bennett (1959). In the 

 period February through April the northwest 

 contribution becomes more important than the 

 northern, and there is some development not 

 only of the above-mentioned clockwise eddy 

 in the western half of the region but also of 

 an anticlockwise eddy in the eastern half. May 

 and September are months of transition to 

 and from the summer circulation pattern; the 

 data are consistent with a sinuous line of 

 weak flow from east to west. 



Transitory eddies, clockwise in the west of 

 the Gulf and anticlockwise in the east, seem 

 to occur rather regularly in association with 

 northerly winds, to judge from the following 

 remarks on inshore currents (U.S. Navy Hy- 

 drographic Office, 1951): 



During northers the current sets strongly to the 

 northward and eastward along the shore on the western 

 side of the Gulf, and to the northward and westward 

 inshore on the east side of the Gulf; at other times the 

 current sets in the opposite direction. This confusion 

 of currents may be accounted for in this way: the fury 

 of the norther blows the water out of the Gulf to the 

 southward and, as the waters lower at the head, there 

 is a rush along each shore to the northward to supply 

 or fill the vacancy. When the norther moderates or 

 ceases to blow, the water that was banked up, as it 

 were, flows back into the Gulf, and the extra amount 

 rushes out along each shore to the southward. 



Mean depth of mixed layer 



According to the quarterly charts of ther- 

 mocline topography of the eastern tropical 

 Pacific by Cromwell (1958), the mixed layer 

 in the Gulf of Tehuantepec has an average 

 depth of between 10 and 30 m. except in the 

 quarter October-December, when it is less than 

 10 m. There were, however, very few observa- 

 tions available from the Gulf region. The 

 charts were based on all available bathy- 

 thermograms filed at the Scripps Institution 

 of Oceanography. 



Mean standing crop of zooplankton 



Brandhorst (1958) presented a contour chart 

 of average zooplankton standing crop for the 

 quarter October-December, based on data from 

 expeditions made in 1955 and 1956. It indicates 

 that the Gulf of Tehuantepec is a region of 

 fairly high standing crop of zooplankton at 

 this season (200-400 ml. /1, 000 m.3, twice as 

 much as in adjacent coastal waters), but is 

 based on very few observations in that area. 



Data from Expedition EASTROPIC 



Sections and charts by Brandhorst (1958) 

 summarize the physical and chemical infor- 

 mation gathered at the seven stations 

 (Spencer F. Baird Stations 80-86) occupied in 

 this region in early December 1955, A Te- 

 huantepecer occurred during the survey. Sec- 

 tions of temperature, salinity, thermosteric 

 anomaly, and dissolved oxygen concentration 

 are all similar: they show the discontinuity 

 layer rising away from the coast to a depth 

 of about 10 m, below the surface at 15° 10' N., 

 95°29' W. (station 83) and then falling to about 

 50 m, below the surface in the southwestern 

 part of the Gulf region. Brandhorst called this 

 topography a "dome," a term introduced into 

 oceanography by Cromwell (1958) to describe 

 a similar feature off the Costa Rican coast, 

 and noted its resemblance to the Costa Rican 



