Variability of thermocline and intermediate waters in the South Atlantic

Abstract

This thesis investigates the variability of the intermediate and thermocline water mass properties in the South Atlantic, with a particular focus on salinity. These water masses form the upper branch of the Meridional Overturning Circulation in the Atlantic and have an important position in global ocean climate. Variability on various timescales is investigated: decadal changes are investigated using repeat hydrographic surveys, interannual variability is investigated using Argo float data and the two timescales are united by investigating a 40 year modern ocean model run. Salinity of thermocline water masses is shown to vary slowly on decadal timescales. Basinwide changes in salinity are evident at 24ºS over periods of 25 years. That these changes are representative of decadal changes and not merely aliasing shorter term variability is supported by analysis of the model. Increases in salinity at 24ºS from 1958 to 1983 and at 30?S from 1993 to 2003 are linked with influence from the Indian ocean while a freshening at 24ºS from 1983 to 2009 may be linked with an intensified hydrological cycle. Antarctic Intermediate Water (AAIW) is seen to increase in salinity at 24ºS from 1958 to 1983 and in the eastern basin at 30ºS from 1993 to 2003. This is different from the Indian and Pacific Oceans where AAIW has been observed to be freshening – a change linked with the intensification of the hydrological cycle. Using the highly correlated relationship between salinity and oxygen in AAIW, these changes are linked to the influence of the Indian Ocean. Further investigation of the changes in AAIW in the South Atlantic using Argo data and model data show that the variability of this water mass is dominated by westward propagating salinity anomalies. These anomalies originate in the Agulhas influenced Cape Basin of the South Atlantic and propagate westwards with speeds similar to those predicted for second mode baroclinic Rossby waves. The techniques developed for analysis of AAIW salinity using oxygen data are expanded to the other oceans of the southern hemisphere. The conclusions drawn from this analysis put the AAIW in the South Atlantic in a global context.

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