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Abstract Anthropogenic perturbations from fossil fuel burning, nuclear bomb testing, and chlorofluorocarbon (CFC) use have created useful transient tracers of ocean circulation. The atmospheric¹⁴C/C ratio (∆¹⁴C) peaked in the early 1960s and has decreased now to pre‐industrial levels, while atmospheric CFC‐11 and CFC‐12 concentrations peaked in the early 1990s and early 2000s, respectively, and have now decreased by 10%–20%. We present the first analysis of a decade of new observations (2007 to 2018–2019) and give a comprehensive overview of the changes in ocean ∆¹⁴C and CFC concentration since the WOCE surveys in the 1990s. Surface ocean ∆¹⁴C decreased at a nearly constant rate from the 1990–2010s (20‰/decade). In most of the surface ocean ∆¹⁴C is higher than in atmospheric CO₂while in the interior ocean, only a few places are found to have increases in ∆¹⁴C, indicating that globally, oceanic bomb¹⁴C uptake has stopped and reversed. Decreases in surface ocean CFC‐11 started between the 1990 and 2000s, and CFC‐12 between the 2000–2010s. Strong coherence in model biases of decadal changes in all tracers in the Southern Ocean suggest ventilation of Antarctic Intermediate Water was enhanced from the 1990 to the 2000s, whereas ventilation of Subantarctic Mode Water was enhanced from the 2000 to the 2010s. The decrease in surface tracers globally between the 2000 and 2010s is consistently stronger in observations than in models, indicating a reduction in vertical transport and mixing due to stratification. 

Abstract We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group database, of oxygen and carbon stable isotope ratios from benthic foraminifera in deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-19 ky) to the Holocene (<10 ky) with a particular focus on the early last deglaciation (19-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models, and sites with at least millennial resolution were preferred. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data, even though its coverage is still sparse in many regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation. 

Abstract Ocean geochemical tracers such as radiocarbon, protactinium and thorium isotopes, and noble gases are widely used to constrain a range of physical and biogeochemical processes in the ocean. However, their routine simulation in global ocean circulation and climate models is hindered by the computational expense of integrating them to a steady state. Here, a new approach to this long‐standing “spin‐up” problem is introduced to efficiently compute equilibrium distributions of such tracers in seasonally‐forced models. Based on “Anderson Acceleration,” a sequence acceleration technique developed in the 1960s to solve nonlinear integral equations, the new method is entirely “black box” and offers significant speed‐up over conventional direct time integration. Moreover, it requires no preconditioning, ensures tracer conservation and is fully consistent with the numerical time‐stepping scheme of the underlying model. It thus circumvents some of the drawbacks of other schemes such as matrix‐free Newton Krylov that have been proposed to address this problem. An implementation specifically tailored for the batch HPC systems on which ocean and climate models are typically run is described, and the method illustrated by applying it to a variety of geochemical tracer problems. The new method, which provides speed‐ups by over an order of magnitude, should make simulations of such tracers more feasible and enable their inclusion in climate change assessments such as IPCC. 

Abstract Reconstructing the circulation, mixing and carbon content of the Last Glacial Maximum ocean remains challenging. Recent hypotheses suggest that a shoaled Atlantic meridional overturning circulation or increased stratification would have reduced vertical mixing, isolated the abyssal ocean and increased carbon storage, thus contributing to lower atmospheric CO₂concentrations. Here, using an ensemble of ocean simulations, we evaluate impacts of changes in tidal energy dissipation due to lower sea levels on ocean mixing, circulation, and carbon isotope distributions. We find that increased tidal mixing strengthens deep ocean flow rates and decreases vertical gradients of radiocarbon andδ¹³C in the deep Atlantic. Simulations with a shallower overturning circulation and more vigorous mixing fit sediment isotope data best. Our results, which are conservative, provide observational support that vertical mixing in the glacial Atlantic may have been enhanced due to more vigorous tidal dissipation, despite shoaling of the overturning circulation and increases in stratification. 

The performance of global ocean biogeochemical models can be quantified as the misfit between modeled tracer distributions and observations, which is sought to be minimized during parameter optimization. These models are computationally expensive due to the long spin‐up time required to reach equilibrium, and therefore optimization is often laborious. To reduce the required computational time, we investigate whether optimization of a biogeochemical model with shorter spin‐ups provides the same optimized parameters as one with a full‐length, equilibrated spin‐up over several millennia. We use the global ocean biogeochemical model MOPS with a range of lengths of model spin‐up and calibrate the model against synthetic observations derived from previous model runs using a derivative‐free optimization algorithm (DFO‐LS). When initiating the biogeochemical model with tracer distributions that differ from the synthetic observations used for calibration, a minimum spin‐up length of 2,000 years was required for successful optimization due to certain parameters which influence the transport of matter from the surface to the deeper ocean, where timescales are longer. However, preliminary results indicate that successful optimization may occur with an even shorter spin‐up by a judicious choice of initial condition, here the synthetic observations used for calibration, suggesting a fruitful avenue for future research. 

Despite their importance for Earth’s climate and paleoceanography, the cycles of carbon (C) and its isotope¹³C in the ocean are not well understood. Models typically do not decompose C and¹³C storage caused by different physical, biological, and chemical processes, which makes interpreting results difficult. Consequently, basic observed features, such as the decreased carbon isotopic signature (δ¹³C_DIC) of the glacial ocean remain unexplained. Here, we review recent progress in decomposing Dissolved Inorganic Carbon (DIC) into preformed and regenerated components, extend a precise and complete decomposition to δ¹³C_DIC, and apply it to data-constrained model simulations of the Preindustrial (PI) and Last Glacial Maximum (LGM) oceans. Regenerated components, from respired soft-tissue organic matter and dissolved biogenic calcium carbonate, are reduced in the LGM, indicating a decrease in the active part of the biological pump. Preformed components increase carbon storage and decrease δ¹³C_DICby 0.55 ‰ in the LGM. We separate preformed into saturation and disequilibrium components, each of which have biological and physical contributions. Whereas the physical disequilibrium in the PI is negative for both DIC and δ¹³C_DIC, and changes little between climate states, the biological disequilibrium is positive for DIC but negative for δ¹³C_DIC, a pattern that is magnified in the LGM. The biological disequilibrium is the dominant driver of the increase in glacial ocean C and the decrease in δ¹³C_DIC, indicating a reduced sink of biological carbon. Overall, in the LGM, biological processes increase the ocean’s DIC inventory by 355 Pg more than in the PI, reduce its mean δ¹³C_DICby an additional 0.52 ‰, and contribute 60 ppm to the lowering of atmospheric CO₂. Spatial distributions of the δ¹³C_DICcomponents are presented. Commonly used approximations based on apparent oxygen utilization and phosphate are evaluated and shown to have large errors. 

Marine and terrestrial biogeochemical models are key components of the Earth System Models (ESMs) used to project future environmental changes. However, their slow adjustment time also hinders effective use of ESMs because of the enormous computational resources required to integrate them to a pre-industrial equilibrium. Here, a solution to this spin-up problem based on sequence acceleration, is shown to accelerate equilibration of state-of-the-art marine biogeochemical models by over an order of magnitude. The technique can be applied in a black box fashion to existing models. Even under the challenging spin-up protocols used for Intergovernmental Panel on Climate Change (IPCC) simulations, this algorithm is 5 times faster. Preliminary results suggest that terrestrial models can be similarly accelerated, enabling a quantification of major parametric uncertainties in ESMs, improved estimates of metrics such as climate sensitivity, and higher model resolution than currently feasible. 

OSU-UVic Climate Model of Intermediate Complexity with Model of Ocean Biogeochemistry and Isotopes (MOBI2.2). New features in this release include Nathaniel Fillman's carbon and C13 decomposition code and Samar Khatiwala's Pa/Th code. 

Input data required for a simulation of the preindustrial control (PIC) simulation with the OSU version of the University of Victoria climate model (version 2.9) with the Model of Ocean Biogeochemistry and Isotopes (MOBI2.2). 

Input data required for a simulation of the Last Glacial Maximum (LGM) simulation with the OSU version of the University of Victoria climate model (version 2.9.10) with the Model of Ocean Biogeochemistry and Isotopes (MOBI2.2).