An official website of the United States government
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.
more »
« less
This content will become publicly available on December 1, 2026
Impact of Atlantic Meridional Overturning Circulation Collapse on Carbon‐13 Components in the Ocean
Abstract Changes in the Atlantic Meridional Overturning Circulation (AMOC) are believed to have affected the cycling of carbon isotopes in both the ocean and the atmosphere. However, understanding how AMOC changes of Dissolved Inorganic Carbon (DIC) distributions in the ocean is limited, since models do not typically decompose the various processes that affect . Here, a new decomposition is applied to idealized simulations of an AMOC collapse, both for glacial and preindustrial conditions. The decomposition explicitly calculates the preformed and regenerated components of and separates between physical and biological effects. An AMOC collapse leads to a large and rapid decrease in in the North Atlantic, which is due to, in about equal parts, accumulation of remineralized organic matter and changes in preformed , both in glacial and preindustrial simulations. In the Pacific, Indian, and Southern Oceans increases by a smaller magnitude. This increase is dominated by changes in preformed in the glacial simulation and remineralized in the preindustrial simulation. An extensive evaluation of the decomposition shows that its errors are small in most cases, especially for large basin‐wide changes, whereas for small, local or global changes errors can be substantial. In contrast, approximations of the remineralized component based on Apparent Oxygen Utilization have large errors in most cases and are generally unreliable because they include contributions from oxygen disequilibrium.
more »
« less
- Award ID(s):
- 1924215
- PAR ID:
- 10650656
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 39
- Issue:
- 12
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Carré, Matthieu (Ed.)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 (δ13CDIC) 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 δ13CDIC, 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 δ13CDICby 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 δ13CDIC, and changes little between climate states, the biological disequilibrium is positive for DIC but negative for δ13CDIC, 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 δ13CDIC, 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 δ13CDICby an additional 0.52 ‰, and contribute 60 ppm to the lowering of atmospheric CO2. Spatial distributions of the δ13CDICcomponents are presented. Commonly used approximations based on apparent oxygen utilization and phosphate are evaluated and shown to have large errors.more » « less
-
Paleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean.more » « less
-
Abstract All else equal, if the ocean's “biological [carbon] pump” strengthens, the dissolved oxygen (O2) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO2) concentrations of the ice ages. Vollmer et al. (2022,https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were “regenerated” (transported as sinking organic matter from the ocean surface to the interior) rather than “preformed” (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed13C/12C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO2storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange.more » « less
-
Abstract We present idealized simulations to explore how the shape of eastern and western continental boundaries along the Atlantic Ocean influences the Atlantic meridional overturning circulation (AMOC). We use a state-of-the art ocean–sea ice model (MOM6 and SIS2) with idealized, zonally symmetric surface forcing and a range of idealized continental configurations with a large, Pacific-like basin and a small, Atlantic-like basin. We perform simulations with five coastline geometries along the Atlantic-like basin that range from coastlines that are straight to coastlines that are shaped like the coasts of the American and African continents. Changing the Atlantic basin coastline shape influences AMOC strength in a manner distinct from simply increasing basin width: widening the basin while maintaining straight coastlines leads to a 10-Sv (1 Sv ≡ 106m3s−1) increase in AMOC strength, whereas widening the basin with the geometry of the American and African continents leads to a 6-Sv increase in AMOC strength, despite both cases representing the same average basin-width increase relative to a control case. The structure of AMOC changes are different between these two cases as well: a more realistic basin geometry results in a shoaled AMOC while widening the basin with straight boundaries deepens AMOC. We test the influence of the shape of the both boundaries independently and find that AMOC is more sensitive to the American coastline while the African coastline impacts the abyssal circulation. We also find that AMOC strength and depth scales well with basin-scale meridional density difference, even with different Atlantic basin geometries, illuminating a robust physical link between AMOC and the North Atlantic western boundary density gradient.more » « less
