9 5° 20', a slight deepening of the top of the 

 layer with the extension of isopleths (except 

 for phosphate) from the top of the layer to 

 the sea surface, with indications of a local 

 shallow depression at 95° W. (station 84); 

 and further west still, a steep decline of the 

 top of the discontinuity layer to about 70 m. 

 at 970 W., with the disappearance from the 

 sea surface of isopleths associated with this 

 layer. 



Figure 7 shows the same properties along 

 the north-south section B-B, i.e., approxi- 

 mately along the axis of the ridge and a little 

 west of its crest. The deepening of the top of 

 the discontinuity layer at 14O40' N. (station 

 84) appears more clearly as a local depres- 

 sion; in this area and for about 40 miles 

 northwards the isopleths (temperature and 

 salinity) of the layer top extend to the sea 

 surface, which they do not at the southern 

 end of the section and do to a much less 

 extent at the northern (inshore) end. The 

 oxygen distribution is not very informative 

 in itself, but can be reconciled (with the aid 

 of figs. 4D and 6D) with other observations. 

 No phosphate isopleths reach the sea surface 

 in figure 6E or figure 7E, because of the 

 contour interval selected. There is no clear 

 evidence of a general change in slope of 

 isopleths at either end of section B-B. 



Interpretation 



At the time of the survey, the circulation 

 in the Gulf resembled the middle schematic 

 diagrams in figure 2B (October-January or 

 February-April, mostly the latter), as shown 

 by dynamic topography, isopleth slopes in 

 sections, and GEK measurements; the western 

 eddy was strong (1-2 knots), the eastern eddy 

 weak or nonexistent. The effect of the cir- 

 culation was to elevate the discontinuity layer 

 into a ridge running more or less north and 

 south from the head waters of the Gulf, with 

 its crest about 20 m. below the surface and 

 its western edge along meridian 95° W. (ap- 

 proximately). 



Waters of the upper part of the discontinuity 

 layer were mixed upwards to the surface 

 above the ridge, but more so along the western 

 flank of the ridge than elsewhere. This could 

 be interpreted as a result of greater wind 

 stress along 95° W. than further east, but if 

 that were the complete explanation the vertical 

 mixing should have been greater inshore than 

 in the middle of the Gulf, whereas the reverse 

 was the case. An area of divergence in the 



middle of the Gulf at about station 84 is there- 

 fore hypothesized: it could have occurred 

 between the western clockwise and the eastern 

 counterclockwise eddy mentioned in the pre- 

 ceding paragraph, although there is only slight 

 evidence that the latter eddy existed. 



Concentrations of phosphate and chlorophyll 

 a were not high in surface waters, even over 

 the ridge, nor was productivity. Zooplankton 

 concentration was about average for the eastern 

 tropical Pacific, and distributed in a way 

 that suggests formation on the ridge, dis- 

 persal downstream to the southwest, and 

 concentration in the eddy to the west of the 

 ridge, Micronekton concentration was above 

 average for the eastern tropical Pacific. 

 These results could mean that the Gulf was 

 declining, from a peak some weeks or months 

 earlier, in its capacity to produce biota at 

 the lower trophic levels, despite the incom- 

 plete stratification of its waters. 



OBSERVATIONS ON CRUISE 

 TO-58-2 (NOVEMBER 1958) 



There were three parts of this cruise in 

 the Gulf of Tehuantepec: part 1, November 

 8-9, consisted of a BT survey along 95° W. 

 north from about 14O30' N.; part 2, November 

 13-16, was a BT and GEK survey as shown 

 in figure 8A; and part 3, November 22-30, 

 was a station, BT, and GEK survey as shown 

 in figure 8B. The four section lines in figure 

 8A are comparable with those in figure 8B, 

 and two of them (A-A, B-B) are comparable 

 with the same section lines for the previous 

 cruise (fig. 3), 



The weather was calm on part 1 of the 

 cruise; on part 2 it was mild, winds being 

 mostly from north to west and not exceeding 

 Beaufort force 3; between parts 2 and 3, 

 on November 19 and 20, there was a north- 

 easter with force 5 to 7, which is identifiable 

 in the records of the Salina Cruz Meteorologi- 

 cal Observatory and appears to have been 

 the first strong norther for the season; on 

 part 3 of the cruise winds were predom- 

 inantly from the north and up to force 5. 



Horizontal distributions of properties 



Figures 9A and 9B are charts of surface 

 current (by GEK) and surface temperature 

 respectively, as observed on part 2 of the 

 cruise (before the Tehuantepecer). 



