Abstract
Abstract. The North Atlantic is one of the major ocean sinks for natural and anthropogenic atmospheric CO2. Given the variability of the circulation, convective processes or warming–cooling recognized in the high latitudes in this region, a better understanding of the CO2 sink temporal variability and associated acidification needs a close inspection of seasonal, interannual to multidecadal observations. In this study, we investigate the evolution of CO2 uptake and ocean acidification in the North Atlantic Subpolar Gyre (50–64∘ N) using repeated observations collected over the last 3 decades in the framework of the long-term monitoring program SURATLANT (SURveillance de l'ATLANTique). Over the full period (1993–2017) pH decreases (−0.0017 yr−1) and fugacity of CO2 (fCO2) increases (+1.70 µatm yr−1). The trend of fCO2 in surface water is slightly less than the atmospheric rate (+1.96 µatm yr−1). This is mainly due to dissolved inorganic carbon (DIC) increase associated with the anthropogenic signal. However, over shorter periods (4–10 years) and depending on the season, we detect significant variability investigated in more detail in this study. Data obtained between 1993 and 1997 suggest a rapid increase in fCO2 in summer (up to +14 µatm yr−1) that was driven by a significant warming and an increase in DIC for a short period. Similar fCO2 trends are observed between 2001 and 2007 during both summer and winter, but, without significant warming detected, these trends are mainly explained by an increase in DIC and a decrease in alkalinity. This also leads to a pH decrease but with contrasting trends depending on the region and season (between −0.006 and −0.013 yr−1). Conversely, data obtained during the last decade (2008–2017) in summer show a cooling of surface waters and an increase in alkalinity, leading to a strong decrease in surface fCO2 (between −4.4 and −2.3 µatm yr−1; i.e., the ocean CO2 sink increases). Surprisingly, during summer, pH increases up to +0.0052 yr−1 in the southern subpolar gyre. Overall, our results show that, in addition to the accumulation of anthropogenic CO2, the temporal changes in the uptake of CO2 and ocean acidification in the North Atlantic Subpolar Gyre present significant multiannual variability, not clearly directly associated with the North Atlantic Oscillation (NAO). With such variability it is uncertain to predict the near-future evolution of air–sea CO2 fluxes and pH in this region. Thus, it is highly recommended to maintain long-term observations to monitor these properties in the next decade.
Highlights
The ocean plays an important role in climate regulation by absorbing between one-quarter and one-third of anthropogenic carbon dioxide (CO2) emitted to the atmosphere (Le Quéré et al, 2018; Gruber et al, 2019)
The uptake of CO2 in the NA is mainly due to extensive biological activity during spring–summer and considerable heat loss during winter, both processes being subject to significant spatiotemporal variability related to climate mode such as the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Variability (AMV), called the Atlantic Multidecadal Oscillation (AMO)
Whatever the NAO variability, the long-term fugacity of CO2 (f CO2) and pH trends we evaluate over 1993–2017 in the NASPG for summer or winter are mainly explained by the increase in dissolved inorganic carbon (DIC) associated with the uptake of anthropogenic CO2 (Fig. 6); that is, the DIC or sDIC trends (Table 2) are not significantly different from the anthropogenic DIC trend estimated between 1994 and 2007 in this region (Gruber et al, 2019a)
Summary
The ocean plays an important role in climate regulation by absorbing between one-quarter and one-third of anthropogenic carbon dioxide (CO2) emitted to the atmosphere (Le Quéré et al, 2018; Gruber et al, 2019). In the North Atlantic Subpolar Gyre (NASPG, around 50–60◦ N) where the oceanic CO2 sink is strong (e.g., Watson et al, 2009), recent years in the mid-2010s have witnessed large negative surface temperature anomalies (Josey et al, 2018) and an extreme freshening (Holliday et al, 2020). Associated changes in the ocean circulation and water masses in the NASPG are for example presented in Chafik et al (2014, 2019), Desbruyères et al (2015) and Nigam et al (2018) These physical processes linked to the NAO and/or AMV directly impact the sea surface f CO2 and air–sea CO2 fluxes through warming–cooling or deeper convection as this has been observed in the NASPG for specific periods (Corbière et al, 2007; Fröb et al, 2019). A direct link of the CO2 uptake variability with the NAO depends on the period investigated, and this is not always clearly revealed from observations (Metzl et al, 2010; Schuster et al, 2013)
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