Ocean Acidification trends and Carbonate System dynamics in the North Atlantic Subpolar Gyre during 2009–2019

Abstract. The CO₂-carbonate system dynamics in the North Atlantic Subpolar Gyre (NASPG) were evaluated between 2009 and 2019. Data was collected aboard eight summer cruises through the CLIVAR 59.5º N section. The Ocean Acidification (OA) patterns and the reduction in the saturation state of calcite (Ω_Ca) and aragonite (Ω_Arag) in response to the increasing anthropogenic CO₂ (C_ant) were assessed within the Irminger, Iceland and Rockall basins during a poorly-assessed decade in which the physical patterns reversed in comparison with previous well-known periods. The observed cooling, freshening and enhanced ventilation increased the interannual rate of accumulation of C_ant in the interior ocean by 50–86 % and the OA rates by close to 10 %. The OA trends were 0.0013–0.0032 units yr^-1 in the Irminger and Iceland basin and 0.0006–0.0024 units yr^-1 in the Rockall Trough, causing a decline in Ω_Ca and Ω_Arag of 0.004–0.021 and 0.003–0.0013 units yr^-1, respectively. The C_ant-driven rise in total inorganic carbon (C_T) was the main driver of the OA (contributed by 53–68 % in upper layers and >82 % toward the interior ocean) and the reduction in Ω_Ca and Ω_Arag (>64 %). The transient decrease in temperature, salinity and A_T collectively counteracts the C_T-driven acidification by 45–85 % in the upper layers and in the shallow Rockall Trough and by <10 % in the interior ocean. The present investigation reports the acceleration of the OA within the NASPG and expands knowledge about the future state of the ocean.