An assessment of the North Atlantic (25–75°N) air-sea CO2 flux in 12 CMIP6 models

Abstract

The global ocean stores about 1/4 of anthropogenic CO2, and the North Atlantic (NA) is the second largest region of the oceanic carbon sink. The Coupled Model Intercomparison Project Phase 6 (CMIP6) concentrates the latest Earth system models for studying the interaction between the carbon cycle and climate change. We find that the simulation of air-sea CO2 fluxes (FCO2) by CMIP6 models is consistent with the observation-based results in the NA. The spatial correlation coefficients of 12-model ensemble mean results and observations are 0.87 for the long-term annual mean results and above 0.62 for the seasonally averaged results. The main simulation biases of FCO2 are that most models overestimate the long-term annual mean carbon sink and seasonal variation of FCO2 in the subpolar NA because there the sink in winter is overestimated. In a large region of the subpolar NA, the bias of winter FCO2 is mainly caused by the bias of winter surface dissolved inorganic carbon (DIC) and total alkalinity. Differences in the simulated winter DIC among CMIP6 models in the subpolar North Atlantic are primarily caused by model differences in DIC's vertical structure and transport; this is mainly associated with differences in the meridional overturning at depth and surface CO2 solubility. We find the biological processes play a minor role in the surface and interior DIC differences among CMIP6 models in the subpolar NA. Compared with the CMIP5 models, the simulated FCO2 is closer to observation-based products in some models and this is mainly due to improved simulated wind fields.