RRS Charles Darwin Cruise 139, 01 Mar-15 Apr 2002. Trans-Indian Hydrographic Section across 32°S

A modern hydrographic section was made across the Indian Ocean at a latitude of about 32°S during a 46-day voyage from Durban to Fremantle aboard RRS Charles Darwin in March-April 2002. The principal goal of this work was to measure the flows of mass, heat, freshwater, inorganic and organic nutrients, and carbon dioxide across the southern boundary of the Indian Ocean in order to determine the meridional overturning circulation for the Indian Ocean, to define the heat, freshwater, nutrient and carbon transports across 32°S, and to produce overall physical and biogeochemical budgets for the Indian Ocean. A second goal was to examine the climate variability in ocean circulation from comparisons of these new measurements with previous surveys in 1936, 1965, 1987 and 1995. A total of 146 hydrographic stations were made along this transoceanic section. At each station an instrument package consisting principally of a CTD, 3 Lowered ADCP's and 24 10-litre sampling bottles was lowered from the surface down to the ocean bottom to measure temperature, salinity, oxygen and eastward and northward current profiles throughout the water column. On the way back to the surface, 24 water samples were collected at various depths and these samples were analysed on board ship for salinity and oxygen (to calibrate the continuous electronic profiles), for inorganic nutrients, constituents of the carbon system, and chlorofluorocarbons. Samples were also collected and stored for later, shore-based analyses of helium, tritium, and organic nutrients. Throughout the cruise velocity data in the upper few hundred meters of the water column were provided by an ADCP mounted in the ship's hull, meteorological variables were monitored and samples of air and rainfall were periodically collected. In addition, 25 Argo floats were launched along the section to provide continuing profiles over the next 5 years. This report describes the methods used to acquire and process the measurements on board ship during the cruise.