A modeling 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). In contrast to the global trend, the Scotian Shelf acts as a net source. Surface pCO2 undergoes a strong seasonal cycle associated with both a strong biological drawdown of Dissolved Inorganic Carbon (DIC) in spring, and pronounced effects of temperature, which ranges from 0 °C in the winter to near 20 °C in the summer. Throughout the summer, events with low surface-water pCO2 occur nearshore 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 +1.9 ± 0.2 mol C m−2 yr−1 for the Scotian Shelf, net neutral CO2 flux of −0.09 ± 0.16 mol C m−2 yr−1 for the Gulf of Maine and uptake by the ocean of −0.88 ± 0.4 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 ). 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). We believe these model results can therefore be extended to make inferences about the real system and that the evidence presented confirms our hypothesis, which we henceforth refer to as a mechanism.…”

“…(2016) simulated significant uptake (−30 mg C m −2 yr −1 ) for the SS and GoM combined (see Figure 2 in Fennel et al., 2019). In contrast, a high‐resolution regional model simulated that the SS acts as a large source of CO 2 (+1.7 ± 0.2 mol C m −2 yr −1 ), very similar to the observation‐based estimates of Shadwick and Thomas (2014), and the SS and GoM combined as a weak net source at +0.4 ± 0.2 mol C m −2 yr −1 (Rutherford et al., 2021).…”

“…(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).…”

“…(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 shelf‐break 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 p CO 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 p CO 2 during these upwelling events).…”

Comment on bg-2021-48

“…For calculating air-sea CO2 flux, according to the carbonate chemistry model of Zeebe & Wolf-Gladrow (2001), we use dissociation constants (K1 and K2) from Millero et al (1995) using Mehrbach et al (1973) data on the seawater scale.…”