Continental shelves are thought to be affected disproportionately by climate change and are a large contributor to global air–sea carbon dioxide (CO 2) fluxes. It is often reported that low-latitude shelves tend to act as net sources of CO 2 , 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 CO 2 (p CO 2) on a seasonal scale. In contrast to the global trend, the Scotian Shelf acts as a net source. Surface p CO 2 undergoes a strong seasonal cycle with an amplitude of ∼ 200–250 µ atm. These changes are associated with both a strong biological drawdown of dissolved inorganic carbon (DIC) in spring (corresponding to a decrease in p CO 2 of 100–200 µ atm) 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 p CO 2 of ∼ 200–250 µ atm. Throughout the summer, events with low surface water p CO 2 occur associated with coastal upwelling. This effect of upwelling on p CO 2 is also in contrast to the general assumption that upwelling increases surface p CO 2 by delivering DIC-enriched water to the surface. Aside from these localized events, p CO 2 is relatively uniform across the shelf. Our model agrees with regional observations, reproduces seasonal patterns of p CO 2 , and simulates annual outgassing of CO 2 from the ocean of +1.7±0.2 mol C m -2 yr -1 for the Scotian Shelf, net uptake of CO 2 by the ocean of -0.5±0.2 mol C m -2 yr -1 for the Gulf of Maine, and uptake by the ocean of -1.3±0.3 mol C m -2 yr -1 for the Grand Banks. [ABSTRACT FROM AUTHOR]
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