mixed layer temperature, salinity, phosphate 

 concentration, 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 ^ re 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 15° 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 to concentrate some of the 

 biota in the eddy region, but otherwise the ef- 

 fect is to transport them through the southwest 

 waters of the Gulf. Average current charts sug- 

 gest that the former event occurs more fre- 

 quently in spring {February to April-May) than 

 in winter. In any event the main concentration 

 of zooplankton standing crop (the most ubiqui- 

 tous biological measurement) is regularly in 

 the southern, southwestern, or western parts 

 of the Gulf. This concentration reaches about 

 600 ml. /I, 000 m.3. 



5. Toward the end of the Tehuantepecer 

 season (May) the ridge and hollow flatten out, 

 upward transfer of water properties dimin- 

 ishes, and productivity and standing crops of 

 biota (except possibly micronekton, for which 

 available measurements of standing crops are 

 few) all decline. 



6. There is no clear evidence of upwelling 

 at any season. Enrichment is attributed to 

 ridging, followed by vertical mixing and stir- 

 ring of the ridge top by wind, as postulated 

 by Cromwell (1958) for the offing of the Pacific 

 coast of Costa Rica. 



A paper entitled "An oceanographic study of 

 the Gulf of Tehuantepec" has been submitted 

 for publication as a Fishery Bulletin toSpecial 

 Scientific Report- -Fisheries. 



Oceanography of Panama Bight 



Observations on Askoy Expedition, 1941 

 (W . S.V/ooster) .--Ihe Panama Bight liesbetween 

 the Isthmus of Panama (about latitude 9 N.) 

 and Punta Santa Elena (about latitude 2 S.) 

 and extends westward from the coasts of 

 Panama, Colombia, and Ecuador to about 

 longitude 81 W. (fig. 1). Oceanographic ob- 

 servations were made in this region by the 

 Askoy Expedition during the period February 

 9 to May 26, 1941. 



Seasonal contrasts in weather within the 

 Bight are related to movement of the Inter- 



tropical Convergence. In January-March the 

 Intertropical Convergence is farthest south, 

 offshore northerlies cause upwelling in the Gulf 

 of Panama, and the Gulf experiences its dry 

 season. In July-September the Intertropical 

 Convergence is farthest north, and the region 

 south of Cabo de San Francisco receives its 

 least rain. The central part of the Bight 

 receives heavy rainfall throughout the year. 

 From June to November most of the Panama 

 Bight is dominated by southwest winds. 



These seasonal changes are reflected in the 

 average surface distribution of temperature 

 and salinity. InFebruary surface temperatures 

 of 26° to 28° C. and salinities of 34 to 35 /60 are 

 found in most parts of the Bight, with lower 

 temperatures in the Gulf of Panama. By August 

 low surface temperatures have disappeared 

 in the Gulf, and the whole region is covered 

 with surface waters of low salinity (less than 

 33A)o. It is estimated that during the rainiest 

 period the salinity of a 10-m. layer of water 

 300 miles long and 60 miles \vide could be 

 reduced by rainfall and runoff from 33 to 28 /{jo 

 in a period of 2 to 3 months. 



During the Askoy Expedition, surface tenn- 

 peratures of 26° to 28° C. and surface salini- 



o» 



ties of somewhat less than 34 Ado were observed 

 in most of the region. These high surface 

 temperatures and relatively low surface sa- 

 linities were characteristic of only a rather 

 thin surface layer (reaching 25 m. or less) 

 which was underlain by a shallow pycnocline. 

 At 100 m. temperatures ranged from 15.8° 

 to 19.5° C, salinities from 34.87 to 35.05?oo. 



In the Gulf of Panama high surface tem- 

 peratures, low surface salinities, and the 

 presence of a strong shallow pycnocline sug- 

 gested that upwelling in early 1941 was less 

 intense than usual. This was confirmed by 

 comparison with Hannibal observations in 

 March 1933, and by examination of long-ternn 

 measurements of sea level and surface tem- 

 perature at Balboa by the Panama Canal Com- 

 pany, 



Correlation of a northerly wind-stress index 

 for February-March with the average sea level 

 for the same months over a 42-year period 

 gave the significant correlation coefficient 

 of -0.54. However, although the 1941 northerly 

 wind-stress index was somewhat lower than 

 average, it was not low enough to account for 

 the unusually high sea level observed in 1941. 

 Thus, it seems likely that some other large- 

 scale process affecting sea temperatures over 

 a large area was operating. 



During the first half of 1941 El Nino was 

 observed off the coast of northern Peru. 

 Schott's explanation of this phenomenon, based 

 on its characteristics in 1891 and 1925, calls 



21 



