Advective Controls on the North Atlantic Anthropogenic Carbon Sink

Abstract

Though it is clear that the North Atlantic is the site of the highest storage of anthropogenic carbon (C ant) per area, it is uncertain whether the air-sea C ant fluxes contributing to North Atlantic C ant storage occur in the subpolar gyre or upstream in the subtropical gyre. Using data and models, we show that air-sea C ant uptake capacity is advected into the subpolar gyre along the same subsurface pathway as nutrients. This pathway is known as the nutrient stream. On the A22 section between Woods Hole and Bermuda, nutrient stream waters are the oldest in the upper 2,000 m and contain low C ant. These northward moving waters are sufficiently depleted in C ant such that they could sustain a subpolar air-sea C ant flux of −0.19 Pg C ant year −1. The ocean hindcast model used here indicates that despite some subtropical re-circulation, uptake capacity is transported into the subpolar gyre where it sustains subpolar air-sea C ant uptake. With this model, we show that high-and low-end estimates of subpolar air-sea C ant flux are reconciled by accounting for a factor of two difference in their respective study areas. If half of the observed air-sea C ant uptake capacity transport at A22 is ventilated in the subpolar region, it can fully support high-end estimates of the subpolar air-sea C ant sink (−0.09 ± 0.01 Pg C ant year −1). Plain Language Summary The ocean has absorbed approximately 40% of the CO 2 from fossil fuel burning and cement production, lowering atmospheric CO 2 and limiting climate change. Ocean storage of this additional carbon from human activities (anthropogenic carbon, C ant) is most intense in the North Atlantic. In the subpolar North Atlantic, C ant is injected into the deep ocean and thus sequestered from the atmosphere for thousands of years. It is currently uncertain whether any of this sequestered C ant was absorbed from the atmosphere in the subpolar North Atlantic. Here we present evidence that the upper limb of the ocean's overturning circulation supplies the subpolar North Atlantic with the capacity to absorb C ant from the atmosphere. Subpolar uptake capacity travels along the same subsurface pathways as the nutrients that support subpolar phytoplankton growth. Waters following this pathway likely started their northward journey decades ago, in the Southern Ocean and thus have not been in contact with the high atmospheric carbon concentrations that exist today. When these waters are exposed to the atmosphere in the subpolar North Atlantic, they can absorb a large amount of C ant. We show that this is the mechanism by which the subpolar North Atlantic absorbs a substantial amount of C ant .

Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

“…The sign of f ant,CO2 is everywhere negative, indicating CO 2 uptake by the ocean, with the highest uptake rates in the North Atlantic, Southern Ocean, western boundary currents, and the equatorward flanks of the subtropical gyres (see also Mikaloff‐Fletcher et al., 2006). These regions experience the re‐emergence of waters whose anthropogenic carbon content is not in equilibrium with the atmosphere, and have strong surface winds that promote fast gas exchange rates (Caldeira & Duffy, 2000; Ishii et al., 2020; Ridge & McKinley, 2020; Sarmiento et al., 1992; Toyama et al., 2017).…”

How Well Do We Understand the Land‐Ocean‐Atmosphere Carbon Cycle?

“…Current uncertainties in ocean models suggest that, despite the fact that the current ensemble of models largely agrees as to the recent evolution of the sink (Figure 8), there may be substantial divergence in feedback strength and ocean sink response to emission mitigation. Since the majority of the anthropogenic carbon is held in the ocean's thermocline (Gruber, Clement, et al., 2019), the circulation here is critical to the ocean sink's near‐term response to mitigation (Iudicone et al., 2016; Ridge & McKinley, 2020; Rodgers et al., 2020). There is substantial spread in the regional distribution of ocean carbon uptake in current models (Fay & McKinley, 2021; Hauck et al., 2020; McKinley et al., 2016), and major differences in representations of seasonality (Mongwe et al., 2018), which illustrates knowledge gaps with respect to physical and biological processes and their representations in models.…”

“…On centennial timescales under high emissions scenarios, slowing of the overturning circulation and reduced buffer capacity will significantly reduce the rate of ocean carbon uptake (Randerson et al., 2015; Ridge & McKinley, 2020, 2021). But how will the ocean sink evolve under the increasingly more likely scenario of substantial emissions mitigation (Hausfather & Peters, 2020)?…”

How Well Do We Understand the Land-Ocean-Atmosphere Carbon Cycle?

“…On centennial timescales under high emissions scenarios, slowing of the overturning circulation and reduced buffer capacity will significantly reduce the rate of ocean carbon uptake (Randerson et al, 2015;Ridge & McKinley, 2020. But how will the ocean sink evolve under the increasingly more likely scenario of substantial emissions mitigation (Hausfather & Peters, 2020)?…”

“…Current uncertainties in ocean models suggest that, despite the fact that the current ensemble of models largely agrees as to the recent evolution of the sink (Figure 8), there may be substantial divergence in feedback strength and ocean sink response to emission mitigation. Since the majority of the anthropogenic carbon is held in the ocean's thermocline (Gruber, Clement, et al, 2019), the circulation here is critical to the ocean sink's near-term response to mitigation (Iudicone et al, 2016;Ridge & McKinley, 2020;Rodgers et al, 2020). There is substantial spread in the regional distribution of ocean carbon uptake in current models Hauck et al, 2020;McKinley et al, 2016), and major differences in representations of seasonality (Mongwe et al, 2018), which illustrates knowledge gaps with respect to physical and biological processes and their representations in models.…”