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Freshwater forcing of the Atlantic Meridional Overturning Circulation revisited

Abstract

Freshwater (FW) forcing is widely identified as the dominant mechanism causing reductions of the Atlantic Meridional Overturning Circulation (AMOC), a climate tipping point that led to past abrupt millennial-scale climate changes. However, the AMOC response to FW forcing has not been rigorously assessed due to the lack of long-term AMOC observations and uncertainties of sea-level rise and ice-sheet melt needed to infer past FW forcing. Here we show a muted AMOC response to FW forcing (~50 m sea-level rise from the final deglaciation of Northern Hemisphere ice sheets) in the early-to-middle Holocene ~11,700–6,000 years ago. Including this muted AMOC response in a transient simulation of the Holocene with an ocean–atmosphere climate model improves the agreement between simulated and proxy temperatures of the past 21,000 years. This demonstrates that the AMOC may not be as sensitive to FW fluxes and Arctic freshening as is currently projected for the end of the twenty-first century.

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Fig. 1: Holocene meltwater–AMOC paradox.
Fig. 2: Comparison of data and models for regional temperature stacks of the past 21,000 years.
Fig. 3: Differences in modelled surface temperature and precipitation during the early Holocene (9 –6 ka) between simulations with and without FW forcing in the Holocene.

Data availability

The model datasets used for this study (Figs. 2 and 3) are available from the Open Science Framework (https://doi.org/10.17605/OSF.IO/NUQ2K). The TraCE-21K-I model data are available from the Earth System Grid (https://www.earthsystemgrid.org/project/trace.html), and the TraCE-21K-II model data are available from the Transient Climate Simulation Lab (https://trace-21k.nelson.wisc.edu).

Code availability

CCSM3 is freely available as open-source code from http://www.cesm.ucar.edu/models/ccsm3.0/

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Acknowledgements

We thank J. D. Shakun and C. Buizert for fruitful discussions and J. Lynch-Stieglitz for providing the AMOC data at the Florida Straits. This work was funded by the US National Science Foundation through grant numbers OPP-1936880, AGS-1502990 and AGS-1602771 (to F.H.), AGS-1503032 (to P.U.C.); the Climate, People, and the Environment Program (to F.H.); and the NOAA Climate and Global Change Postdoctoral Fellowship programme (to F.H.), administered by the University Corporation for Atmospheric Research. Support for this research was also provided by the University of Wisconsin–Madison Office of the Vice Chancellor for Research and Graduate Education with funding from the Wisconsin Alumni Research Foundation. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the US Department of Energy under contract number DE-AC05-00OR22725.

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F.H. designed and performed the research with input from P.U.C. and F.H. led the writing of the manuscript with input from P.U.C. All authors discussed the results and contributed towards improving the final manuscript.

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Correspondence to Feng He.

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Extended data

Extended Data Fig. 1 Site locations of the proxy data used in the regional temperature stacks.

Site locations for Greenland temperature stack and the Eastern Atlantic Ocean and Mediterranean Sea temperature stacks (top) and Antarctica temperature stack (bottom).

Extended Data Fig. 2 Comparison of data and models for regional temperature stacks of past 21,000 years over Greenland and Antarctica.

a-b, Surface air temperature (SAT) changes from the central Greenland at NGRIP and GISP2 sites. The composite of the central Greenland temperature was based on δ15N for 21-10 ka47 and δ18O for 10-0 ka at NGRIP74,75 and GISP2 sites70. c-f, SAT changes from the Antarctica at EDML76,77, Dome C76,77, Dome F78 and Vostok79 sites. g-h. Greenland and Antarctica stack. Proxy data in black; before and include the onset of the Bølling, TraCE-21K-I and TraCE-21K-II are identical (red); after the onset of the Bølling, simulation based on the protocol of prescribing the reconstructed AMOC in red (TraCE-21K-II) and prescribing the reconstructed FW forcing in cyan (TraCE-21K-I). See Methods for detailed information about the proxy data used in the regional temperature stacks. All modeled changes are referenced to the averages of corresponding proxy data during the Oldest Dryas (19-15 ka) to aid the comparison. LGM, Last Glacial Maximum. OD, Oldest Dryas. BA, Bølling-Allerød interstadial. YD, Younger Dryas. HMAP, Holocene Meltwater-AMOC Paradox. LH, Late Holocene. ka, thousand years before 1950.

Extended Data Fig. 3 Comparison of data and models for the regional temperature stack of past 21,000 years in the Eastern Atlantic Ocean/Mediterranean Sea.

a-h, sea surface temperature changes from a) core NA 87-22 in Northeast Atlantic Ocean80, b) Western Mediterranean (BS79-38)81, c) Iberian margin (SU81-18)82, d) Iberian margin (SU81-18)80, e) Gulf of Cadiz (M39-008)81, f) Alboran Sea (MD95-2043)83, g) Nile Delta (GeoB 7702-3)84 and h) Red Sea (GeoB 5844-2)85. i, Eastern Atlantic/Mediterranean sea surface temperature stack. Proxy data in black; before and include the onset of the Bølling, TraCE-21K-I and TraCE-21K-II are identical (red); after the onset of the Bølling, simulation based on the protocol of prescribing the reconstructed AMOC in red (TraCE-21K-II) and prescribing the reconstructed FW forcing in cyan (TraCE-21K-I). See Methods for detailed information about the proxy data used in the regional temperature stacks. All modeled changes are referenced to the proxy data during the Oldest Dryas (19-15 ka) to aid the comparison. LGM, Last Glacial Maximum. OD, Oldest Dryas. BA, Bølling-Allerød interstadial. YD, Younger Dryas. HMAP, Holocene Meltwater-AMOC Paradox. LH, Late Holocene. ka, thousand years before 1950.

Extended Data Fig. 4 Comparison of data and models for regional temperature stacks of past 21,000 years.

Same as Fig. 2, but with temperature anomalies of each regional stack (b-d) from the average value in the Oldest Dryas (19 ka-15 ka).

Extended Data Fig. 5 The temperature biases of the two Holocene simulations.

TraCE-21K-I in Cyan and TraCE-21K-II in red. Upper panel, the temperature differences and their uncertainties (one standard deviation) between the proxy data and the simulations during the early Holocene (9 ka - 6 ka) in the three regional temperature stacks in Fig. 2; Lower panel, the percentage of the biases with reference to the biases in TraCE-21K-I.

Extended Data Fig. 6 Location of FW forcing in TraCE-21K-I and TraCE-21K-II simulations.

The amount of FW forcing in TraCE-21K-I and TraCE-21K-II simulations was documented in Supplementary Tables 2 and 3. In the TraCE-21K-I simulation, locations of the FW forcing include the coastal region near the Mackenzie River (pink shaking), the Arctic Ocean in the Beaufort Sea and parts of the Canadian Arctic Archipelago (blue shading), the North Atlantic Ocean between 50°N and 70°N (hollow blue), the Barents Sea (orange shading), the North Sea (dark red shading), the Nordic Sea (hollow green), the coastal region near the St. Lawrence River (hollow black), Hudson Strait (hollow dark green), the Gulf of Mexico (red shading), the Weddell Sea (yellow shading) and Ross Sea (gray shading). In the TraCE-21K-II simulation, the FW forcing during the Younger Dryas was applied to the North Atlantic Ocean between 50°N and 70°N (hollow blue). In TraCE-21K-I, the Laurentide ice-sheet FW forcings are derived from Ref. 33 and Ref. 34 that are updated in timing by Ref. 35. The Scandinavian ice-sheet FW forcing comes from Ref. 36. Updates to these ice sheets’ deglacial chronologies by Ref. 30 and Ref. 31, respectively, have not changed the overall FW forcings. Ice divides were used to determine to which ocean basins the various fractions of the total ice-sheet FW were discharged following Ref. 34.

Extended Data Fig. 7 Total amount of freshwater forcing in the Northern Hemisphere (cyan) and Southern Hemisphere (blue) in TraCE-21K-I and TraCE-21K-II simulations.

m SLE 1000 yr-1, meter of sea level equivalent per thousand years. 1 m SLE 1000 yr−1 ~= 0.011 Sv. ka, thousand years before 1950. Refer Supplementary Tables 2 and 3 for the amount of meltwater discharge in specific locations in TraCE-21K-I and TraCE-21K-II simulations. Purple shading denotes the period of the HMAP.

Extended Data Fig. 8 Comparison of AMOC streamfunction and the changes of maximum mix layer depths between TraCE-21K-I and TraCE-21K-II during the early Holocene.

AMOC streamfunction (in sverdrup) during the early Holocene (9 ka – 6 ka) in TraCE-21K-II (top) and TraCE-21K-I simulations (middle) and the differences of maximum mix layer depths (m) in March between TraCE-21K-I and TraCE-21K-II during the early Holocene (9 ka – 6 ka) (bottom).

Extended Data Fig. 9 Comparison of global surface temperature evolution between TraCE-21K-I and TraCE-21K-II simulations.

Global surface temperature from TraCE-21K-I in cyan and TraCE-21K-II in red.

Extended Data Fig. 10 Comparison of regional surface temperature evolution between TraCE-21K-I and TraCE-21K-II simulations.

Northern Hemisphere mid-high latitude (top, 30N-90N), low-latitude (middle, 30S-30N) and Southern Hemisphere mid-high latitude (bottom, 30S-90S). Regional surface temperature from TraCE-21K-I in cyan and TraCE-21K-II in red.

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Supplementary Tables 1–3.

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He, F., Clark, P.U. Freshwater forcing of the Atlantic Meridional Overturning Circulation revisited. Nat. Clim. Chang. 12, 449–454 (2022). https://doi.org/10.1038/s41558-022-01328-2

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