Upper ocean temperature and salinity sections in the South Atlantic in 1911

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Journal of Geophysical Research-Oceans
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Mar 15
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Hydrographic cruises conducted in July-October 1911 by the ''Fram,'' the ''Deutschland,'' and the ''Mowe'' and in January-February 1911 by the ''Stephan'' provide a survey of the upper South Atlantic subtropical gyre during the cold season and of the equatorial Atlantic. The data accuracy is not as high as in more recent surveys, in particular, the depth of the measurement is not known precisely: the Fram temperature is only good to within 0.10 degrees C accuracy and salinity to within 0.02. The South Atlantic cold season stations often present a 100-m or thicker surface mixed layer overlaying a thermocline with maximum vertical gradient increasing from the southwest to the northeast. A layer of minimum vertical temperature gradient of 0.01 degrees C m(-1) at a temperature close to 13 degrees C is embedded in the upper thermocline between 40 degrees W and 20 degrees W on the southern section of the Fram (30 degrees S to 35 degrees S). This defines a mode water at a density (26.60) close to the surface density of deep mixed layers offshore of the Brazil current found between 34 degrees S and 39 degrees 21'S but which is warmer and saltier (14.9 degrees C, 35.70). The 1911 T-S characteristics in the subtropical South Atlantic thermocline present little zonal variations, with the warmest, saltiest water on isopycnal surfaces found in the southwestern portion of the survey and the coldest, freshest waters near Namibia. The 1911 South Atlantic surface waters are much colder than the average situation in recent decades (average of -0.82 degrees C) and are a little fresher (-0.04) than the hydrographic climatology for that season. In the subtropical gyre, the isopycnals were probably deeper in 1911 than in 1926, except in the southwestern corner, and the slope of the T-S relationship was larger in 1911 than in 1925-1926, with warmer and saltier waters in the upper thermocline (above gamma=26.6-26.4) and fresher and cooler waters in the lower thermocline. On the other hand, T-S characteristics in the equatorial thermocline are comparable in 1911 and 1926. Both surveys present water masses which are colder and fresher on isopycnal surfaces than the surveys in the 1960s.


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