Model constraints on the anthropogenic carbon budget of the Arctic Ocean

Abstract. The ArcticOcean is projected to experience not only amplified climate change but also amplified ocean acidification. Modeling future acidification depends on our ability to simulate baseline conditions and changes over the industrial era. Such centennial-scale changes require a global model to account for exchange between the Arcticand surrounding regions. Yet the coarse resolution of typical global models may poorly resolve that exchange as well as critical features of ArcticOcean circulation. Here we assess how simulations of ArcticOcean storage of anthropogenic carbon (C ant ), the main driver of open- ocean acidification, differ when moving from coarse to eddy admitting resolution in a global ocean circulation- biogeochemistrymodel (NEMO-PISCES). The Arctic's regional storage of C ant is enhanced as model resolution increases. While the coarse- resolution model configuration ORCA2 (2°) stores 2.0 Pg C in the ArcticOcean between 1765 and 2005, the eddy-admitting versions ORCA05 and ORCA025 (1/2° and 1/4°) store 2.4 and 2.6 Pg C. That result from ORCA025 falls within the uncertainty range from a previous data-based C ant storage estimate (2.5 to 3.3 Pg C). Yet those limits may each need to be reduced by about 10 % because data-based C ant concentrations in deep waters remain at ∼ 6 μmol kg −1 , while they should be almost negligible by analogy to the near-zero observed CFC-12 concentrations from which they are calculated. Across the three resolutions, there was roughly three times as much anthropogenic carbon that entered the ArcticOcean through lateral transport than via the flux of CO 2 across the air-sea interface. Wider comparison to nine earth systemmodels that participated in the Coupled Model Intercomparison ProjectPhase 5 (CMIP5) reveals much larger diversity of stored anthropogenic carbon and lateral transport. Only the CMIP5 models with higher lateral transport obtain C ant inventories that are close to the data-based estimates. Increasing resolution also enhances acidification, e.g., with greater shoaling of the Arctic's average depth of the aragonitesaturation horizon during 1960–2012, from 50 m in ORCA2 to 210 m in ORCA025. To assess the potential to further refine modeled estimates of the ArcticOcean's C ant storage and acidification, sensitivity tests that adjust model parameters are needed given that century-scale global ocean biogeochemical simulations still cannot be run routinely at high resolution.