B 



TO-59-2 



SV« 



6,cL/t. 



C 



T069-2 



ST ST 



D 



T&-59-2 



Figure 23,--Vertical distributions of properties on 

 cruise TO-59-2, along section B-B (see fig. 20); depth 

 scale in m. 



SUMMARY 



It is concluded, from information obtained 

 on four cruises in 1958 and 1959 and data 

 available in published average charts, that the 

 prevailing transisthmian northerly wind af- 

 fects the upper waters of the Gulf of Tehuan- 

 tepec in the following ways in the period Sep- 

 tember through May (and possibly to a less 

 extent during the remaining 3 months, although 

 no observations have been made). 



1. The prevailing northerly winds cause a 

 current to flow from the head of the Gulf 

 to the south and southwest. The effect of this, 

 added to the effect of the normal flow north- 

 westwards along the Mexican coast, is to 

 produce in the Gulf a more or less permanent 

 flow pattern like that of a letter S rotated 

 90° clockwise, with its axis lying east and 

 west after rotation. As a result the discon- 

 tinuity layer (pycnocline, thermocline, etc.) 

 forms a meridional ridge in the eastern half 

 of the Gulf and a hollow in the western half. 



2. Stronger northerly winds (Tehuante- 

 pecers) increase the velocity of the southerly 

 current and the slope of the western side of 

 the ridge, and bring the top of the ridge closer 

 to the sea surface. This process starts in 

 October or November. 



3. As the discontinuity layer approaches the 

 windy sea surface, its top is stirred and mixed- 

 layer temperature, salinity, phosphate con- 

 centration, etc. become locally more like those 

 of the discontinuity layer. Production of biota, 

 first phytoplankton and later zooplankton and 

 micronekton, is stimulated in the near-surface 

 waters. Surface productivity is up to 161 

 mg.C/m.3/day. These phenomena are best 

 developed in the region where the ridge top 

 is most affected by the wind, i.e., along the 

 western shoulder of the ridge. Within that 

 meridional belt they tend to be best developed 

 at about latitude 150 N. instead of close to 

 the coast, for reasons that are not entirely 

 understood. 



4. The biota tend to be carried downstream 

 from the area of maximum production. If a 

 strong clockwise eddy forms to the west of 

 the ridge the effect is probably to concentrate 

 some of the biota in the eddy region, but 

 otherwise the effect is probably to transport 

 them through the southwest waters of the 

 Gulf. In any event the main concentration of 

 zooplankton standing crop (the most ubiquitous 

 biological measurement) is regularly in the 

 southern, southwestern, or western parts of the 

 Gulf. This concentration reaches about 600 

 ml./l,000 m.3. 



27 



