We use an eddy-resolving, regional ocean biogeochemical model to investigate the main variables and processes responsible for the climatological spatio-temporal variability of pCO(2) and the air-sea CO2 fluxes in the southwestern Atlantic Ocean. Overall, the region acts as a sink of atmospheric CO2 south of 30 degrees 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 pCO(2) 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 pCO(2). Biological production and solubility are the main processes regulating pCO(2), with biological production being particularly important on the shelves. The role of mixing/stratification in modulating DIC, and therefore surface pCO(2), is shown in a vertical profile at the location of the Ocean Observatories Initiative (OOI) site in the Argentine Basin (42 degrees S, 42 degrees W).