AbstractContinental shelves are thought to be affected disproportionately by climate change and are a large contributor to global air–sea carbon dioxide (CO2) fluxes. It is often reported that low-latitude shelves tend to act as net sources of CO2, whereas mid- and high-latitude shelves act as net sinks. Here, we combine a high-resolution regional model with surface water time series and repeat transect observations from the Scotian Shelf, a mid-latitude region in the northwest North Atlantic, to determine what processes are driving the temporal and spatial variability of partial pressure of CO2 (pCO2) on a seasonal scale. In contrast to the global trend, the Scotian Shelf acts as a net source. Surface pCO2 undergoes a strong seasonal cycle with an amplitude of ∼ 200–250 µatm. These changes are associated with both a strong biological drawdown of dissolved inorganic carbon (DIC) in spring (corresponding to a decrease in pCO2 of 100–200 µatm) and pronounced effects of temperature, which ranges from 0 ∘C in the winter to near 20 ∘C in the summer, resulting in an increase in pCO2 of ∼ 200–250 µatm. Throughout the summer, events with low surface water pCO2 occur associated with coastal upwelling. This effect of upwelling on pCO2 is also in contrast to the general assumption that upwelling increases surface pCO2 by delivering DIC-enriched water to the surface. Aside from these localized events, pCO2 is relatively uniform across the shelf. Our model agrees with regional observations, reproduces seasonal patterns of pCO2, and simulates annual outgassing of CO2 from the ocean of mol C m−2 yr−1 for the Scotian Shelf, net uptake of CO2 by the ocean of mol C m−2 yr−1 for the Gulf of Maine, and uptake by the ocean of mol C m−2 yr−1 for the Grand Banks.