Assessment of Global Ocean Biogeochemistry Models for Ocean Carbon Sink Estimates in RECCAP2 and Recommendations for Future Studies

The ocean is a major carbon sink and takes up 25%-30% of the anthropogenically emitted CO2. A state-of-the-art method to quantify this sink are global ocean biogeochemistry models (GOBMs), but their simulated CO2 uptake differs between models and is systematically lower than estimates based on statistical methods using surface ocean pCO2 and interior ocean measurements. Here, we provide an in-depth evaluation of ocean carbon sink estimates from 1980 to 2018 from a GOBM ensemble. As sources of inter-model differences and ensemble-mean biases our study identifies (a) the model setup, such as the length of the spin-up, the starting date of the simulation, and carbon fluxes from rivers and into sediments, (b) the simulated ocean circulation, such as Atlantic Meridional Overturning Circulation and Southern Ocean mode and intermediate water formation, and (c) the simulated oceanic buffer capacity. Our analysis suggests that a late starting date and biases in the ocean circulation cause a too low anthropogenic CO2 uptake across the GOBM ensemble. Surface ocean biogeochemistry biases might also cause simulated anthropogenic fluxes to be too low, but the current setup prevents a robust assessment. For simulations of the ocean carbon sink, we recommend in the short-term to (a) start simulations at a common date before the industrialization and the associated atmospheric CO2 increase, (b) conduct a sufficiently long spin-up such that the GOBMs reach steady-state, and (c) provide key metrics for circulation, biogeochemistry, and the land-ocean interface. In the long-term, we recommend improving the representation of these metrics in the GOBMs. In this study, we evaluate the performance of state-of-art global ocean biogeochemistry models (GOBMs) in simulating CO2 fluxes across the air-sea interface from 1980 to 2018 for the Global Carbon Budget. Across these GOBMs, the simulated CO2 uptake is systematically lower than that of observation-based estimates and the estimates differ also substantially between GOBMs. As reasons for the too low carbon sink of the GOBMs, we find that the simulations of several GOBMs were initialized after the start of the industrial revolution and that the majority of the considered GOBMs underestimate the large-scale ocean circulation in the Atlantic. The different initialization times of the simulations as well as different strengths of the simulated ocean circulation across the global ocean also partly explain the inter-model differences for the ocean carbon sink. Our analysis of the influence of GOBM dynamics on their simulated carbon sink was impeded by the fact that not all GOBMs had the same initial stability and that the riverine component of the ocean carbon sink is highly uncertain in both observations and GOBMs. Based on our evaluation, we give recommendations for follow up studies. The simulated CO2-uptake by global ocean biogeochemistry models (GOBMs) in the second phase of the REgional Carbon Cycle Assessment and Processes project is systematically lower than observation-based estimates This underestimation can, to first order, be explained by the simulation setup as well as biases in surface chemistry and ocean circulation Concrete steps forward are proposed to improve simulations of the ocean carbon sink by GOBMs