A modelling study of temporal and spatial <i>p</i>CO<sub>2</sub> variability on the biologically active and temperature-dominated Scotian Shelf

Abstract

Abstract. Continental 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 +1.7±0.2 mol C m−2 yr−1 for the Scotian Shelf, net uptake of CO2 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.

Elucidating Coastal Ocean Carbon Transport Processes: A Novel Approach Applied to the Northwest North Atlantic Shelf

“…Overall, this leads to oversaturation of carbon and drives the observed outgassing throughout much of the year on the SS (see Figure S1 in the Supporting Information S1). It is important to note that the model has been thoroughly validated and found to represent observed features reasonably well (Brennan et al, 2016;Laurent et al, 2021;Rutherford et al, 2021) and that the present study results are backed by observations (Shadwick et al, 2010).…”

“…The model ran for 16 years from 1999 to 2014, where the first year is considered model spin-up. Here, we focus on years 2006-2014, consistent with Rutherford et al (2021). Daily 3D snapshots of biogeochemical properties, dye tracers, salinity and temperature were saved.…”

“…The biogeochemical model is of medium complexity, based on the nitrogen cycle model with inorganic carbon chemistry of Fennel et al (2006); Fennel et al (2008), but was expanded to include two phytoplankton and two zooplankton groups by Laurent et al (2021). Detailed descriptions of the biological and inorganic carbon component in the current model version are given in Laurent et al (2021) and Rutherford et al (2021), respectively. Also used here are passive dye tracers (see Figure 1 for dye tracer source regions) for the calculation of water mass fractions in the three shelf regions of interest (SS, GB, and GoM) similar to Rutherford and Fennel (2018).…”

“…We posit that carbon-rich subpolar North Atlantic water, which enters the shelf north of Newfoundland and travels along the shelf across the Grand Banks (GB) to the SS, the GoM, and onward to the Middle Atlantic Bight as a distinctive water mass, becomes oversaturated with CO 2 due to local warming that occurs along its southwestward journey. We provide results from a high-resolution biogeochemical model of the northwest North Atlantic shelves that includes passive dye tracers for water mass tracking and has been shown to represent the seasonality reported in Shadwick et al (2010) well (see Rutherford et al, 2021). Our approach accounts for the inherently three-dimensional (3D) nature of coastal transport processes which are key to the proposed mechanism and is applicable to other shelf regions around the globe.…”

“…Shadwick et al (2010), and Burt et al (2013) suggested that outgassing of CO 2 on the SS occurs because of cross-shelf exchange and subsurface carbon-rich water being brought to the surface potentially via upwelling of slope water across the shelf break or upwelling of subsurface shelf water. Neither of these upwelling mechanisms can be responsible for the reported outgassing because (a) the shelfbreak current and its associated density fronts act as an effective barrier to cross-shelf exchange inhibiting the onwelling of slope water (Beardsley & Boicourt, 1981;Fratantoni & Pickart, 2007;Loder et al, 1998;Rutherford & Fennel, 2018), and (b) coastal upwelling events, while frequently occurring on the SS (Petrie et al, 1987;Shan et al, 2016), are temporarily lowering surface-ocean pCO 2 due to the colder temperatures of the upwelled water and are too short-lived to impact the annually integrated flux (see Rutherford et al, 2021 for a full Taylor decomposition of the effects of dissolved inorganic carbon (DIC) and temperature on surface pCO 2 during these upwelling events).…”