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1. OSU-UVic hosing run with preindistrial initial conditions (PI Full)

   https://doi.org/10.5281/zenodo.14774854  

   Schmittner, Andreas (January 2025, Zenodo)

   his directory contains model code, input, output, and scripts from a hosing (freshwater forcing in the North Atlantic) simulation with the OSU-UVic climate model (version 2.9.10) to investigate the effect of changes in the Atlantic Meridional Overturning Circulation (AMOC) on carbon and carbon-13 components in the ocean as described in Schmittner and Boling (2025) and Schmittner (2025). Model code is in the code/ subdirectory. Model input data is in the data/ subdirectory and in the control.in and mk.in files. Model output data is in the tavg*nc and tsi*nc files. Ferret scripts used to produce the figures are in the ferret/ subdirectory. Andreas Schmittner (andreas.schmittner@oregonstate.edu) References: Schmittner, A. and M. Boling (2025) Impact of Atlantic Meridional Overturning Circulation Collapse on Carbon Components in the Ocean, Global Biogeochemical Cycles, 39, e2025GB008526 doi: 10.1029/2025GB008526. Schmittner, A. (2025) Impact of Atlantic Meridional Overturning Circulation Collapse on Carbon-13 Components in the Ocean, Global Biogeochemical Cycles, 39, e2025GB008527 doi: 10.1029/2025GB008527. 

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2. Atlantic Meridional Overturning Circulation slowdown modulates wind-driven circulations in a warmer climate

   https://doi.org/10.1038/s43247-024-01907-5  

   Mimi, Mohima_Sultana; Liu, Wei (November 2024, Communications Earth & Environment)

   Abstract Wind-driven and thermohaline circulations, two major components of global large-scale ocean circulations, are intrinsically related. As part of the thermohaline circulation, the Atlantic Meridional Overturning Circulation has been observed and is expected to decline over the twenty-first century, potentially modulating global wind-driven circulation. Here we perform coupled climate model experiments with either a slow or steady Atlantic overturning under anthropogenic warming to segregate its effect on wind-driven circulation. We find that the weakened Atlantic overturning generates anticyclonic surface wind anomalies over the subpolar North Atlantic to decelerate the gyre circulation there. Fingerprints of overturning slowdown are evident on Atlantic western boundary currents, encompassing a weaker northward Gulf Stream and Guiana Current and a stronger southward Brazil Current. Beyond the Atlantic, the weakened Atlantic overturning causes a poleward displacement of Southern Hemisphere surface westerly winds by changing meridional gradients of atmospheric temperature, leading to poleward shifts of the Antarctic Circumpolar Current and Southern Ocean meridional overturning circulations. 

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3. Enhanced vertical mixing in the glacial ocean inferred from sedimentary carbon isotopes

   https://doi.org/10.1038/s43247-021-00239-y  

   Wilmes, Sophie-Berenice; Green, J. A. Mattias; Schmittner, Andreas (August 2021, Communications Earth & Environment)

   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. 

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4. Impact of Atlantic Meridional Overturning Circulation Collapse on Dissolved Inorganic Carbon Components in the Ocean

   https://doi.org/10.1029/2025GB008526  

   Schmittner, A; Boling, M (December 2025, Global Biogeochemical Cycles)

   The Atlantic Meridional Overturning Circulation (AMOC) impacts temperatures, ecosystems, and the carbon cycle. However, AMOC effects on Earth's carbon cycle remains poorly understood, in part because contributions of different physical and biological mechanisms that impact carbon storage in the ocean are not typically diagnosed in climate models. Here, we explore modeled effects of AMOC shutdowns on ocean Dissolved Inorganic Carbon (DIC) by applying a new decomposition that explicitly calculates preformed and regenerated DIC components and separates physical and biological contributions. An extensive evaluation in transient simulations finds that the method is accurate, especially for basin‐wide changes, whereas errors can be significant at global and local scales. In contrast, estimates of respired carbon based on Apparent Oxygen Utilization lead to large errors and are generally not reliable. In response to a shutdown of the AMOC under Last Glacial Maximum (LGM) background climate, ocean carbon increases and then decreases, leading to opposite changes in atmospheric carbon dioxide (CO2). DIC changes are dominated by opposing changes in biological carbon storage. Whereas regenerated components increase in the Atlantic and dominate the initial increase in global ocean DIC until model year 1000, preformed components decrease in the other ocean basins and dominate the long‐term DIC decrease until year 4000. Biological disequilibrium is an important contribution to preformed carbon changes. Biological saturation carbon decreases in the Pacific, Indian, and Southern Oceans due to a decrease in surface alkalinity. The spatial patterns of the DIC components and their changes in response to an AMOC collapse are presented. 

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5. Sensitivity of ocean circulation to warming during the Early Eocene greenhouse

   https://doi.org/10.1073/pnas.2311980121  

   Kirtland_Turner, Sandra; Ridgwell, Andy; Keller, Allison L; Vahlenkamp, Maximilian; Aleksinski, Adam K; Sexton, Philip F; Penman, Donald E; Hull, Pincelli M; Norris, Richard D (June 2024, Proceedings of the National Academy of Sciences)

   Multiple abrupt warming events (“hyperthermals”) punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ¹³C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation’s sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ¹³C and δ¹⁸O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ¹³C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ¹³C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ¹³C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ¹³C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future. 

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