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

• Published in: Biogeosciences
• Main Authors: C. Leseurre, C. Lo Monaco, G. Reverdin, N. Metzl, J. Fin, S. Olafsdottir, V. Racapé
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2020
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; North Atlantic; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-17-2553-2020; https://doaj.org/article/f6fdc61ab8b345438224f079a0304232

The North Atlantic is one of the major ocean sinks for natural and anthropogenic atmospheric CO 2 . Given the variability of the circulation, convective processes or warming–cooling recognized in the high latitudes in this region, a better understanding of the CO 2 sink temporal variability and associated acidification needs a close inspection of seasonal, interannual to multidecadal observations. In this study, we investigate the evolution of CO 2 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 CO 2 ( f CO 2 ) increases ( + 1.70 µ atm yr −1 ). The trend of f CO 2 in surface water is slightly less than the atmospheric rate ( + 1.96 µ atm 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 f CO 2 in summer (up to +14 µ atm yr −1 ) that was driven by a significant warming and an increase in DIC for a short period. Similar f CO 2 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 f CO 2 (between −4.4 and −2.3 µ atm yr −1 i.e., the ocean CO 2 sink increases). Surprisingly, during summer, pH increases up to +0.0052 yr −1 in the ...