A Modeling and Data Study of Dissolved Oxygen and pCFC Age Variability in the Thermocline of the Southern Ocean

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0525874Intellectual MeritRecent observations of dissolved oxygen during repeat occupations of hydrographic sections have found increased apparent oxygen utilization within the thermocline of the South Indian, South Pacific, and south of Australia. The goal of this proposed research is to identify the role of variability in the physical processes or biological processes which result in changes in dissolved oxygen within the Southern Ocean thermocline, including Subantarctic Mode and Antarctic Intermediate Waters. Data analysis of the World Ocean Circulation Experiment (WOCE) and CLImate VARiability and predictability (CLIVAR) Repeat Hydrography Programs will be combined with model simulations with variable physical forcing using the Hallberg Isopycnal Model. Chlorofluorocarbons (CFCs) will be included in these studies. Where CFC measurements are available during repeat measurements, increases in the pCFC ages are co-located with the decreases in oxygen within the thermocline. Since CFCs are not affected by variability in biological processes, their distributions imply that variability in the physical forcing has produced the increased apparent oxygen utilization. The modeling study will confirm whether this is true as well in the southern ocean and will be used to determine the relative importance of the variability in physical forcing due to variability in the winds, surface buoyancy forcing, and gas exchange on the ventilation and circulation of the thermocline and mode waters of the Southern Ocean. Similar studies of the North Pacific have shown that the same model reproduces the observed decreases of oxygen in the deep thermocline from the 1980s to the 1990s. Because of the fidelity of the model, the basin wide results can be used to provide a context for the observed changes. In that work, changes in ventilation have been shown to be important near the formation region of the water mass in question, while circulation changes were more important further downstream. Broader Impacts of Proposed WorkThe results from this project will contribute to the interpretation of the carbon dioxide system measurements during the CLIVAR Repeat Hydrography. The fractional carbon method for determination of the anthropogenic carbon dioxide concentration relies on the assumption of steady state circulation, and then corrects for remineralization using Apparent Oxygen Utilization (AOU) and for air-sea disequilibrium using pCFC ages. It is clear that variability in physical forcing needs to be considered in the calculation of carbon uptake. The close collaboration of a CFC chemist and a physical oceanographer should provide new insight into this important problem. In addition, collaborations with the modeling group at GFDL will be continued to ground their modeling effort in interpretation of observations. The project will also contribute to the development of the biogeochemical component of the Hallberg Isopycncal Model and will make the offline tracer advection code available to the community in collaboration with GFDL. Two post-doctoral fellows and a graduate student will receive training in this interdisciplinary research project. The co-PI will continue to be involved in supporting women in physical oceanography through an effort supported by NSF and ONR to developing a mentoring network for junior women in the field.