Stronger Southern Ocean carbon uptake in high-resolution ocean biogeochemistry simulations

It is increasingly recognized that the way Southern Ocean mesoscale eddies are represented in ocean models influences air-sea CO2 fluxes and their response to climate change. In this study, we assess the Southern Ocean carbon uptake since the 1960s in a hierarchy of global ocean biogeochemistry models (GOBMs) based on the NEMO-MOPS and FESOM-REcoM models. The horizontal resolutions of the GOBMs range from 1 and 0.5 resolutions (eddy-parameterized) to 0.25 and 0.1 resolutions (eddy-rich, where eddies are explicitly represented). We find that the eddy-rich models have steeper density surfaces across the ACC with respect to eddy-parameterized models, in better agreement with observations. A larger amount of deep waters low in anthropogenic carbon (Cant) is thereby transported to the surface, leading to a 10% higher Cant uptake and storage. Natural CO2 (Cnat), which integrated over the whole Southern Ocean is directed into the ocean, shows a somewhat higher ingassing in the eddy-rich models. As a result, the net CO2 uptake is about 14% higher in the eddy-rich with respect to the eddy-parameterized models. Trends over the 1958-2018 period reveal a gradual wind-driven reduction of Cnat uptake in all configurations, but this trend is about 40% weaker in the 0.1 model with respect to the lower resolution models. At the same time, the upward trend in the residual meridional overturning circulation (MOC) is weaker in the 0.1 model, supporting the hypothesis of a more pronounced eddy-compensation of the wind-driven Cnat trends. Our study suggests that GOBMs using standard eddy parameterizations may underestimate the net and anthropogenic CO2 uptake by about 10%, and emphasizes the importance of adequately simulating mesoscale eddies for better constraining the Southern Ocean carbon uptake in changing climate conditions.