Seasonality of biological and physical controls on surface ocean CO2 from hourly observations at the Southern Ocean Time Series site south of Australia.

The Subantarctic Zone (SAZ), which covers the northern half of the Southern Ocean between the Subtropical and Subantarctic Fronts is important for air-sea CO₂ exchange, ventilation of the lower thermocline, and nutrient supply for global ocean productivity. The first high-resolution autonomous observations of mixed layer CO₂ partial pressure (pCO₂) and hydrographic properties in the SAZ covering a full annual cycle will be presented. The annual cycle of pCO₂ is decomposed into physical and biological drivers: after the summer biological pCO₂ depletion (driven by an annual net community production of 2.45±1.47 mol C m^-2 yr^-1), the return to near atmospheric equilibrium proceeds slowly, driven by entrainment in early autumn when mixed layers deepen from <100 to ~200m, but only achieving full equilibration in late winter/early spring as respiration completes the annual cycle. The shutdown of winter convection and associated mixed layer shoaling proceeds intermittently, appearing to frustrate the initiation of production. Horizontal processes, identified from salinity anomalies, are associated with biological pCO₂ signatures, but with differing impacts in winter (when they reflect far-field variations in dissolved inorganic carbon and/or biomass) and summer (when they suggest promotion of local production by the relief of silicic acid or iron limitation). These results provide clarity on SAZ seasonal carbon cycling and demonstrate that the magnitude of the annual pCO₂ cycle is twice as large as that in the subarctic high-nutrient, low-chlorophyll waters, which can inform the selection of optimal global models in this region.