Ocean carbonate system variability in the North Atlantic Subpolar surface water (1993–2017)

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.

Decadal Dynamics of the CO2 System and Associated Ocean Acidification in Coastal Ecosystems of the North East Atlantic Ocean

“…In this study, the DIC/TA dataset revealed a significant increase in DIC concomitant with a somewhat unexpected increase of TA at the two nearshore stations and relatively stable TA at SOMLIToffshore during the study period (Figure 5). Recent observations based on similar DIC/TA datasets also revealed an increase in TA for the 2007-2015 period, albeit in very different ecosystems at the coastal Point B site in the Mediterranean Sea (Kapsenberg et al, 2017) and in the North Atlantic Subpolar Gyre (Leseurre et al, 2020). In our study, similarly, to the observations made at point B and contrary to the North Atlantic Subpolar Gyre, the increase in TA was concomitant with a significant increase in pCO 2 of 3.52 ± 0.47, 2.95 ± 1.04, and 3.11 ± 0.49 µatm year −1 and decrease in pH of −0.0029 ± 0.0005, −0.0028 ± 0.0010, and −0.0027 ± 0.0004 year −1 at SOMLIT-Brest, SOMLIT-pier, and SOMLIT-offshore, respectively (Figure 5).…”

“…These trends were also close to values previously reported at several coastal observatories, such as in the Irminger Sea (−0.0026 ± 0.0006 year −1 ), in the Cariaco Basin (−0.0025 ± 0.0004 year −1 ) (see review of Bates et al, 2014), or in the Mediterranean Sea at Point B (−0.0028 ± 0.0003 year −1 ) (Kapsenberg et al, 2017), although care must be taken when comparing such diverse ecosystems. The North Atlantic trends ranged from −0.0013 ± 0.0009 year −1 (period 1995-2013, Kitidis et al, 2017) to −0.0017 year −1 (period 1993-2017, Leseurre et al, 2020). In open ocean regions of the northern hemisphere, pH trends have been observed concomitantly with sea surface pCO 2 increases close to the atmospheric CO 2 forcing (Kitidis et al, 2017;Wang et al, 2017;Leseurre et al, 2020).…”

“…In open ocean regions of the northern hemisphere, pH trends have been observed concomitantly with sea surface pCO 2 increases close to the atmospheric CO 2 forcing (Kitidis et al, 2017;Wang et al, 2017;Leseurre et al, 2020). Deviations from these atmospheric trends have been reported for specific periods or regions and may reflect local changes in oceanic buffering capacity as discussed in Section "Deconvolution of Carbonate System Trends During the 2008-2017 Period" and elsewhere (Lauvset and Gruber, 2014;Lauvset et al, 2015;Leseurre et al, 2020). These deviations can be even more significant in coastal margins, as reported in the Mediterranean Sea (Kapsenberg et al, 2017) or in the SAB, with decrease in pH reaching 0.004 year −1 based on multidecadal observations (Reimer et al, 2017b).…”

“…The North Atlantic trends ranged from −0.0013 ± 0.0009 year −1 (period 1995-2013, Kitidis et al, 2017) to −0.0017 year −1 (period 1993-2017, Leseurre et al, 2020). In open ocean regions of the northern hemisphere, pH trends have been observed concomitantly with sea surface pCO 2 increases close to the atmospheric CO 2 forcing (Kitidis et al, 2017;Wang et al, 2017;Leseurre et al, 2020). Deviations from these atmospheric trends have been reported for specific periods or regions and may reflect local changes in oceanic buffering capacity as discussed in Section "Deconvolution of Carbonate System Trends During the 2008-2017 Period" and elsewhere (Lauvset and Gruber, 2014;Lauvset et al, 2015;Leseurre et al, 2020).…”

“…Global estimates of OA in the open ocean were estimated at −0.0018 ± 0.0004 year −1 for the period 1991-2011 (Lauvset et al, 2015) and relate to pCO 2 increase of 1.2 ± 0.5 µatm year −1 to 2.1 ± 0.5 µatm year −1 in different ocean basins (Takahashi et al, 2009(Takahashi et al, , 2014. These trends are highly linked to the studied periods and can vary regionally and seasonally (Leseurre et al, 2020). Longterm monitoring in nearshore ecosystems is particularly relevant since these ecosystems host large stocks of shellfish species sensitive to ongoing OA (Doney et al, 2020), yet remain scarce.…”