Fi>;ure 8,--(;iirrenl, temperalurc, •iiiU ztioplaiikion charts for tlie 

 Gulf of Tehuaiiicpec on S(;(1T Expedition. 



November 1958 a few days before a northerly- 

 gale, and (below) similar information along 

 the same sections a few days after the gale. 

 The upper east-west section shows a gentle 

 north-south ridge in the thernnocline with its 

 crest at about 94° W. where the thernnocline top 

 is about 20 m. below the surface, and the 



corresponding north-south section (along 95° 

 W., the meridian of Chivela Pass) shows the 

 thermocline top undulating at about the same 

 level; a few isotherms from the thermocline 

 top extend to the surface along the western 

 shoulder of the ridge, especially near the coast. 

 The lower sections show marked changes in 

 the thermocline topography including the ele- 

 vation of the top of the ridge to 12 m. below 

 the sea surface; and additional isotherms 

 reaching the surface along the western shoulder 

 of the ridge, especially in midgulf at 15° N., 

 where the thermocline almost breaks down. 

 Isopleths of salinity, thermosteric anomaly, 

 oxygen, and phosphate are distributed in almost 

 the same way in the same sections. 



Figure 8 presents horizontal distributions, 

 from another cruise, of dynamic height anomaly 

 and surface current by GEK, surface tem- 

 perature, and zooplankton standing crop in the 

 upper 300 m. (approximately), with some values 

 for micronekton in the upper 90 m. (approxi- 

 mately). The western flank of the meridional 

 ridge is seen to run south- southwest in the 

 direction of the wind, and there is some 

 clockwise circulation along this ridge and 

 around the hollow to the west of it; surface 

 current agrees well with geostrophic current. 

 The coolest area is over the ridge but es- 

 pecially along its western flank at about 15 N. 

 Zooplankton is most abundant in the cool area 

 and south and west of it. 



The following conclusions were drawn from 

 the data of all cruises and from information in 

 published average charts of wind, current, 

 temperature, etc. They refer to the period 

 September through May; the same processes 

 may operate to a less extent from June through 

 August, but no cruise 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- 

 westward 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 discontinuity 

 layer (pycnocline, thermocline, etc.) forms a 

 ridge in the eastern half of the Gulf and a 

 hollow in the western half. 



2. Stronger northerly winds (Tehuantepec- 

 ers) 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 



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