Abstract. We use an eddy-resolving, regional ocean biogeochemical model to investigate the main variables and processes responsible for the climatological spatio-temporal variability of pCO₂ and the air-sea CO₂ fluxes in the southwestern Atlantic Ocean. Overall, the region acts as a sink of atmospheric CO₂ south of 30° S, and is close to equilibrium with the atmospheric CO₂ to the north. On the shelves, the ocean acts as a weak source of CO₂, except for the mid/outer shelves of Patagonia, which act as sinks. In contrast, the inner shelves and the low latitude open ocean of the southwestern Atlantic represent source regions. Observed nearshore-to-offshore and meridional pCO₂ gradients are well represented by our simulation. A sensitivity analysis shows the importance of the counteracting effects of temperature and dissolved inorganic carbon (DIC) in controlling the seasonal variability of pCO₂. Biological production and solubility are the main processes regulating pCO₂, with biological production being particularly important on the shelves. The role of mixing/stratification in modulating DIC, and therefore surface pCO₂, is shown in a vertical profile at the location of the Ocean Observatories Initiative (OOI) site in the Argentine Basin (42° S, 42° W).