Variability in water mass formation and ventilation rates in the southwestern Pacific Ocean based on Repeat Hydrography CFC measurements

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In this project, the US investigator will collaborate with Australian oceanographers to reoccupy two hydrographic sections that have been identified as essential by the U.S. CLIVAR/Carbon (CLImate VARiability and predictability) Repeat Hydrography Program. The dissolved chlorofluorocarbons (CFCs) on sections between Tasmania and Antarctica along 140'E in 2007-8 and along 170'W between 50'S and the equator in 2008-9 will be measured. This will be the fourth occupation of both these sections with chlorofluorocarbon tracers (CFC) and carbon system measurements since 1990. All three Chlorofluorocarbons: CFC-11, CFC-12, and CFC-113 will be measured on both sections, and SF6 will be measured on at least the pacific line. These tracer measurements complement the hydrographic observations that will include dissolved nutrients and carbon system parameters. The CFCs emphasize the contrast between newly-ventilated cold waters and the older, circumpolar waters of northern origin, and provide a "clock" to help understand observed changes in water mass properties in a circumpolar band located on the north side of the Antarctic Circumpolar Current, associated with the thick cold mixed layers where Subantarctic Mode Water and Antarctic Intermediate Water. These measurements will be interpreted in collaboration with the Australian colleagues. A variety of approaches to data analysis, including comparison with general circulation model output, will be utilized to interpret changes in the CFC concentrations and derived ages. Both numerical models and transit time distributions will be utilized to interpret the pCFC ages. Broader Impacts of Proposed WorkThis proposed research continues a long-term collaboration with Australian scientists, continues the training of a graduate student in model-data comparisons who will participate one of the expeditions, and provides data on changes in intermediate water formation rates and bottom waters that will help us better understand the role of the Southern Ocean in climate change and anthropogenic CO2 uptake. There is no seawater CFC measurement laboratory in the Australian oceanographic community. These CFC measurements make an important contribution to the interpretation of both the hydrographic and carbon system measurements of the Australian researchers. The Australian oceanographers have agreed to the data policy of the CLIVAR/Carbon Repeat Hydrography Program. These data from one of the regions of the ocean that is potentially important to studies of the ocean's response to changes in surface forcing will be publicly available soon after the voyages.