Oceanographic observations in the Panama Bight, "Askoy" Expedition, 1941. Bulletin of the AMNH ; v. 118, article 3

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New York : [American Museum of Natural History]
"The Panama Bight may be defined as that part of the eastern tropical Pacific Ocean that lies between 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. Oceanographic observations discussed in this region were made by the 'Askoy' expedition during the period February 9 to May 26, 1941. Bathymetrically the region is part of the Panama Basin which is enclosed by Central and South America, Cocos Ridge, the Galapagos Islands, and Carnegie Ridge. Most of the Panama Bight is deeper than 3000 meters, the shelf being nowhere wider than 30 miles except in the Gulf of Panama which is less than 200 meters deep. Seasonal contrasts in weather within the Bight are related to movement of the Intertropical 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. In February surface temperatures of 26° to 28° C. and salinities of 34 to 35 parts per mille 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 33°‰). It is estimated that during the rainiest period the salinity of a 10-meter layer of water 300 miles long and 60 miles wide could be reduced by rainfall and runoff from 33 parts per mille to 28 parts per mille in a period of two to three months. During the 'Askoy' expedition, surface temperatures of 26° to 28° C. and surface salinities of somewhat less than 34 parts per mille were observed in most of the region. These high surface temperatures and relatively low surface salinities were characteristic of only a rather thin surface layer (reaching 25 meters or less) which was underlain by a shallow pycnocline. At 100 meters temperatures range from 15.8° to 19.5° C., salinities from 34.87 parts per mille to 35.05 parts per mille. In the Gulf of Panama high surface temperatures, low surface salinities, and the presence of a strong shallow pycnocline suggest 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-term measurements of sea level and surface temperature at Balboa by the Panama Canal Company. 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 Niño was observed off the coast of northern Peru. Schott's explanation of this phenomenon, based on its characteristics in 1891 and 1925, calls for a cold tongue extending from the Gulf of Panama nearly to the equator. Although this cold tongue was not detected by the 'Askoy,' unusually high temperature, low salinity, and a strong southward surface current measured west of Malpelo Island may be related to the influx of northern waters on the coast of Peru. Observation of a pronounced subsurface temperature discontinuity 200 miles west of Buenaventura, accompanied by other indications at the surface, suggests that a well-developed oceanic front was present on March 24, 1941. Examination of average surface current charts shows a northward coastal surface flow north of Cabo de San Francisco throughout the year, with a mean speed of about 25 cm. per second (0.5 knot) and a width of less than 100 miles. 'Askoy' measurements in the northern part of the Bight show a subsurface distribution of mass consistent with such a current which appears to be the eastern limb of the general counterclockwise circulation in the Panama Bight. It is proposed that it be called the 'Colombia Current'"--P. 149-150.
p. 117-151 : ill., maps ; 27 cm.
Includes bibliographical references (p. 150-151).