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

Abstract. The North Atlantic is one of the major sinks for anthropogenic CO2. In this study, we investigate the evolution of CO2 uptake and ocean acidification in the North Atlantic Subpolar Gyre (50° N–64° N) using repeated observations collected over the last three decades in the framework of the long-term monitoring program SURATLANT (SURveillance de l'ATLANTique). Data obtained between 1993 and 1997 suggest an important reduction in the capacity of the ocean to absorb CO2 from the atmosphere during summer, due to a rapid increase in the fugacity of CO2 (fCO2) in surface waters (5 times faster than the increase in the atmosphere). This was associated with a rapid decrease in surface pH (of the order of −0.014/yr) and was mainly driven by a significant warming and increase in DIC. Similar trends are observed between 2001 and 2007 during both summer and winter with a mean decrease of pH between −0.006/yr and −0.013/yr. These rapid trends are mainly explained by a significant warming of surface waters, a decrease in alkalinity during summer and an increase in DIC during winter. On the contrary, data obtained during the last decade (2008–2017) show a stagnation of surface fCO2 (increasing the ocean sink for CO2) and pH. These recent trends are explained by the cooling of surface waters, a small decrease of total alkalinity and the near-stagnation of dissolved inorganic carbon. Overall our results show that the uptake of CO2 and ocean acidification in the North Atlantic Subpolar Gyre is substantially impacted by multi-decadal variability, in addition to the accumulation of anthropogenic CO2. As a consequence, the future evolution of air-sea CO2 fluxes, pH and the saturation state of surface waters with regards to aragonite and calcite remain highly uncertain in this region.