Antarctic ice core records suggest that atmospheric CO2 increased by 15 to 20 ppm during Heinrich stadials (HS). These periods of abrupt CO2 increase are associated with a significant weakening of the Atlantic meridional overturning circulation (AMOC), and a warming at high southern latitudes. As such, modelling studies have explored the link between changes in AMOC, high southern latitude climate and atmospheric CO2. While proxy records suggest that the aeolian iron input to the Southern Ocean decreased significantly during HS, the potential impact on CO2 of reduced iron input combined with oceanic circulation changes has not been studied in detail. Here, we quantify the respective and combined impacts of reduced iron fertilisation and AMOC weakening on CO2 by performing numerical experiments with an Earth system model under boundary conditions representing 40,000 years before present (ka). Our study indicates that reduced iron input can contribute up to 6 ppm rise in CO2 during an idealized Heinrich stadial. This is caused by a 5% reduction in nutrient utilisation in the Southern Ocean, leading to reduced export production and increased carbon outgassing from the Southern Ocean. An AMOC weakening under 40ka conditions and without changes in surface winds leads to a ~0.5 ppm CO2 increase. The combined impact of AMOC shutdown and weakened iron fertilisation is almost linear, leading to a total CO2 increase of 7 ppm. Therefore, this study highlights the need of including changes in aeolian iron input when studying the processes leading to changes in atmospheric CO2 concentration during HS.

The Southern Ocean (SO) provides the largest oceanic sink of carbon. Observational datasets highlight decadal-scale changes in SO CO2 uptake, but the processes leading to this decadal-scale variability remain debated. Here, using an eddy-permitting ocean, sea-ice, carbon cycle model, we explore the impact of changes in Southern Hemisphere (SH) westerlies on contemporary (i.e. total), anthropogenic and natural CO2 fluxes using idealised sensitivity experiments as well as an interannually varying forced (IAF) experiment covering the years 1948 to 2007. We find that a strengthening of the SH westerlies reduces the contemporary CO2 uptake by leading to a high southern latitude natural CO2 outgassing. The enhanced SO upwelling and associated increase in Antarctic Bottom Water decrease the carbon content at depth in the SO, and increase the transport of carbon-rich waters to the surface. A poleward shift of the westerlies particularly enhances the CO2 outgassing south of 60S, while inducing an asymmetrical DIC response between high and mid southern latitudes. Changes in the SH westerlies in the 20th century in the IAF experiment lead to decadal-scale variability in both natural and contemporary CO2 fluxes. The ~10% strengthening of the SH westerlies since the 1980s led to a 0.016 GtC/yr^2 decrease in natural CO2 uptake, while the anthropogenic CO2 uptake increased at a similar rate, thus leading to a stagnation of the total SO CO2 uptake. The projected poleward strengthening of the SH westerlies over the coming century will thus reduce the capability of the SO to mitigate the increase in atmospheric CO2.

Model intercomparison studies of coupled carbon-climate simulations have the potential to improve our understanding of the processes explaining the pCO2 drawdown at the Last Glacial Maximum (LGM) and to identify related model biases. Models participating in the Paleoclimate Modelling Intercomparison Project (PMIP) now frequently include the carbon cycle. The ongoing PMIP-carbon project provides the first opportunity to conduct multimodel comparisons of simulated carbon content for the LGM time window. However, such a study remains challenging due to differing implementation of ocean boundary conditions (e.g. bathymetry and coastlines reflecting the low sea level) and to various associated adjustments of biogeochemical variables (i.e. alkalinity, nutrients, dissolved inorganic carbon). After assessing the ocean volume of PMIP models at the pre-industrial and LGM, we investigate the impact of these modelling choices on the simulated carbon at the global scale, using both PMIP-carbon model outputs and sensitivity tests with the iLOVECLIM model. We show that the carbon distribution in reservoirs is significantly affected by the choice of ocean boundary conditions in iLOVECLIM. In particular, our simulations demonstrate a ~250 GtC effect of an alkalinity adjustment on carbon sequestration in the ocean. Finally, we observe that PMIP-carbon models with a freely evolving CO2 and no additional glacial mechanisms do not simulate the pCO2 drawdown at the LGM (with concentrations as high as 313, 331 and 315 ppm), especially if they use a low ocean volume. Our findings suggest that great care should be taken on accounting for large bathymetry changes in models including the carbon cycle.