We analyze the variability of air-sea fluxes of carbon dioxide and oxygen in the Southern Ocean during the period 1993-2004, in a biogeochemical and physical simulation of the global ocean. Our results suggest that ENSO and the Southern Annular Mode are of comparable significance in driving interannual variability; both climatic indices are associated with surface heat fluxes, which in turn control the mixed-layer depth variability in the model. Because carbon-rich, oxygen-poor waters are entrained into the mixed layer during winter convection episodes, soluble gas fluxes are correlated with changes in entrainment. We adopt a Lagrangian view of tracers propagating along the Antarctic Circumpolar Current to highlight the importance of convective mixing in inducing anomalous air-sea fluxes of carbon dioxide and oxygen. The idealized Lagrangian model captures the principal features of the variability simulated by the more complex model. Distinct spatial and temporal patterns arise from the different equilibration timescales of the two gases.