Gray, Alison R.Riser, Stephen C.Sauvé, Jade2025-10-022025-10-022025-10-022025SauvxE9_washington_0250E_28714.pdfhttps://hdl.handle.net/1773/54078Thesis (Ph.D.)--University of Washington, 2025The Southern Ocean plays a central role in the global carbon cycle, yet the processes that govern its air-sea carbon exchange remain incompletely understood. This dissertation advances our process-based understanding of dissolved inorganic carbon (DIC) variability, a key determinant of surface ocean pCO2 and air-sea carbon fluxes. A circumpolar DIC budget framework is developed and applied to both biogeochemical observations and global ocean models to quantify the drivers of mixed layer DIC variability. The approach is further refined through a closed budget analysis in a high-resolution, data-assimilating model, enabling more detailed process-level insight into the Southern Ocean carbon cycle. Chapter 2 constructs the first observation-based, monthly resolved mixed layer DIC budget for the circumpolar Southern Ocean, using six years of autonomous biogeochemical float data. Analysis of two regions, the Sea Ice Zone (SIZ) and the Antarctic Southern Zone (ASZ), shows that biological processes dominate seasonal DIC variability, with northward Ekman transport playing a secondary but important role. On annual scales, vertical mixing supplies DIC to the mixed layer in both regions, but the fate of this DIC diverges: in the ASZ it is either consumed by biological production or outgassed, while in the SIZ it is exported northward. This wind-driven DIC redistribution emerges as a key mechanism supporting carbon outgassing, with implications for future changes under reduced sea ice cover. The results also provide a robust observational benchmark for evaluating climate models. Chapter 3 extends the DIC budget framework to evaluate seasonal carbon fluxes across multiple models, including OMIP simulations, the B-SOSE data-assimilating model, and observations. All datasets identify vertical mixing as the primary DIC source and biological uptake as the main sink, but models consistently underestimate and delay the biological drawdown, leading to a negative entrainment flux, opposite in sign to observational estimates. Advective terms show the largest model spread, reflecting differences in simulated circulation. These results highlight that even models performing well in global metrics can misrepresent key seasonal processes, emphasizing the value of process-based diagnostics to improve model fidelity. Chapter 4 shifts focus to zonal variability, revealing that zonal asymmetries in the physical drivers of carbon flux are central to explaining spatial variability in Southern Ocean air-sea carbon exchange. Using a closed budget in the B-SOSE model, the analysis identifies carbon outgassing hotspots in the Indo-Pacific ASZ linked to intense vertical mixing downstream of topographic features. Zonal advection spreads this DIC eastward, sustaining outgassing further along the path of the ACC. Regional variation in mixing, entrainment, and advection explains much of the zonal variability in air-sea carbon exchange, underscoring the limitations of zonally averaged approaches and the necessity of resolving regional processes in both modelsand diagnostics. Across all chapters, this dissertation demonstrates that carbon fluxes in the Southern Ocean result from tightly coupled physical and biological processes whose variability plays out across both space and time. It highlights the importance of moving beyond global and annual averages to resolve seasonal and regional dynamics, which are critical for understanding and predicting carbon cycle behavior. Looking forward, extending the budget framework to additional float data, higher-resolution models, and climate projections will help clarify how DIC fluxes and carbon outgassing may evolve in a warming world. These advances are essential for improving Earth system models and supporting more reliable climate policy.application/pdfen-USCC BY-NC-SAautonomous observationscarbon cycleglobal ocean biogeochemistry modelsmixed layer budgetseasonal variabilitySouthern OceanPhysical oceanographyChemical oceanographyBiogeochemistryOceanographyThesis