Jens Daniel Müller

and 13 more

Jens Terhaar

and 7 more

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 the (i) model set-up, such as the length of the spin-up, the starting date of the simulation, and carbon fluxes from rivers and into sediments, (ii) the ocean circulation, such as Atlantic Meridional Overturning Circulation and Southern Ocean mode and intermediate water formation, and (iii) the oceanic buffer capacity. Our analysis suggests that the 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 set-up prevents a robust assessment. For simulations of the ocean carbon sink, we recommend in the short-term to (1) start simulations in 1765, when atmospheric CO2 started to increase, (2) conduct a sufficiently long spin-up such that the GOBMs reach steady-state, and (3) 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.

Rémy Asselot

and 6 more

The ocean plays a major role in the moderation of anthropogenically-induced climate change by absorbing roughly a quarter of anthropogenic CO2 (Cant). This absorption of Cant by the ocean leads to ocean acidification, threatening marine’s life. The North Atlantic Ocean encompasses the highest ocean storage capacity of Cant per unit area. The subpolar North Atlantic gyre is subject to a large seasonal to decadal variability that might impact Cant storage. To investigate Cant evolution over the 2011-2021 period and its relationship with ocean dynamics in this region, we use the Argo-O2 array combined with neural networks and a back-calculation method (φCTO method). We compute monthly time-series of Cant in the Labrador and Irminger Seas. We show that Cant concentrations in the first 2000 dbar of the Labrador and Irminger Seas are strongly affected by winter deep convection, especially between winter 2015 and winter 2018. The Cant inventories in the top 2000 dbar of the Labrador and Irminger Seas increase through time, at rates of 1.63±0.32% yr-1 and 1.49±0.30% yr-1, respectively. Our monthly Argo-based Cant estimates complement high-quality ship-based measurements acquired at a biennial or lower frequency. Additionally, this study shows that Cant concentrations and Cant inventories in deep convection areas may depend on the method employed to calculate Cant. As a consequence, we take over the model ensemble idea and propose to use several methods to compute Cant, which would give its methodological uncertainty.