Data

Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling

University of Tasmania, Australia

Sauermilch, Isabel ; Whittaker, Joanne ; Klocker, Andreas

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ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adc&rfr_id=info%3Asid%2FANDS&rft_id=info:doi10.25959/5eb21fc078c99&rft.title=Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling&rft.identifier=10.25959/5eb21fc078c99&rft.description=Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4°C, up to 5°C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.Maintenance and Update Frequency: notPlannedStatement: The ocean model used is the MIT general circulation model (MITgcm, version MITgcmUV checkpoint62x) in an ocean-only configuration with no sea ice. The model domain is circumpolar and extends between 84°S and 25°S (and 0°, see model run in SI) in latitude. The horizontal grid spacing is 0.25°, equivalent to a zonal grid spacing of ~27.8 km and a meridional grid spacing of ~3 km to ~25 km at the southern and northern boundary, respectively. The model configuration uses 50 vertical levels, ranging from 10 m at the sea surface to 368 m at the bottom. Our model uses a linear drag coefficient of 0.0011, a nonlinear equation of state, a seventh-order advection scheme for temperature and salinity and the K-profile parameterization. No parameterization is used for the advection and diffusion due to mesoscale eddies. Partial cells are used in the vertical for a more accurate representation of bathymetry. Restoring boundary conditions for the surface forcing are taken from a coupled atmosphere-ocean model (GFDL CM2.1) simulating Late Eocene conditions with atmospheric CO2 concentrations of 800 ppm. The surface forcing for sea surface temperature, sea surface salinity, and zonal and meridional wind stresses are temporally and zonally averaged values from the coupled model (Figure S1). The surface forcing is zonal averaged, in order to be able to modify the bathymetry configurations and continent-ocean distributions, without causing artificial disturbances of the ocean circulations. A sponge layer of ~300 km is used at the northern boundary of the model to relax to a temporal and zonal mean of salinity and temperature output from the coupled model with a restoring time scale of 10 days.&rft.creator=Sauermilch, Isabel &rft.creator=Whittaker, Joanne &rft.creator=Klocker, Andreas &rft.date=2021&rft.coverage=westlimit=-180.00; southlimit=-85.00; eastlimit=180.00; northlimit=0.00&rft.coverage=westlimit=-180.00; southlimit=-85.00; eastlimit=180.00; northlimit=0.00&rft_rights=The data described in this record are the intellectual property of the University of Tasmania through the Institute for Marine and Antarctic Studies.&rft_rights= http://creativecommons.org/licenses/by/4.0/&rft_rights=http://i.creativecommons.org/l/by/4.0/88x31.png&rft_rights=WWW:LINK-1.0-http--related&rft_rights=License Graphic&rft_rights=Creative Commons Attribution 4.0 International License&rft_rights=http://creativecommons.org/international/&rft_rights=WWW:LINK-1.0-http--related&rft_rights=WWW:LINK-1.0-http--related&rft_rights=License Text&rft_rights=Cite data as: Sauermilch, I., Klocker, A & Whittaker, J. (2020). Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling. Institute for Marine and Antarctic Studies (IMAS), University of Tasmania (UTAS). doi:10.25959/5eb21fc078c99&rft_rights=Creative Commons Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0&rft_subject=oceans&rft_subject=Southern Ocean, ACC, Cenozoic Greenhouse-Icehouse Transition, Sea Surface Temperatures, Ocean Gyres, Antarctic cooling, Eocene-Oligocene transition, Tectonic gateways&rft_subject=EARTH SCIENCE | CLIMATE INDICATORS | PALEOCLIMATE INDICATORS | PALEOCLIMATE RECONSTRUCTIONS | SEDIMENTS&rft_subject=EARTH SCIENCE | CLIMATE INDICATORS | PALEOCLIMATE INDICATORS&rft_subject=EARTH SCIENCE | CLIMATE INDICATORS | PALEOCLIMATE INDICATORS | PALEOCLIMATE RECONSTRUCTIONS&rft_subject=EARTH SCIENCE | CLIMATE INDICATORS | PALEOCLIMATE INDICATORS | PALEOCLIMATE RECONSTRUCTIONS | SEA SURFACE TEMPERATURE RECONSTRUCTION&rft_subject=EARTH SCIENCE | CLIMATE INDICATORS | PALEOCLIMATE INDICATORS | PALEOCLIMATE RECONSTRUCTIONS | STREAMFLOW RECONSTRUCTION&rft_subject=EARTH SCIENCE | CLIMATE INDICATORS | PALEOCLIMATE INDICATORS | PLATE TECTONICS&rft_subject=EARTH SCIENCE | PALEOCLIMATE | OCEAN/LAKE RECORDS | MICROFOSSILS&rft_subject=EARTH SCIENCE | PALEOCLIMATE | OCEAN/LAKE RECORDS | ISOTOPES&rft_subject=EARTH SCIENCE | OCEANS | MARINE GEOPHYSICS&rft_subject=Physical Oceanography&rft_subject=EARTH SCIENCES&rft_subject=OCEANOGRAPHY&rft.type=dataset&rft.language=English Access the data

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Cite data as: Sauermilch, I., Klocker, A & Whittaker, J. (2020). Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling. Institute for Marine and Antarctic Studies (IMAS), University of Tasmania (UTAS). doi:10.25959/5eb21fc078c99

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Brief description

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4°C, up to 5°C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.

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Maintenance and Update Frequency: notPlanned

Statement: The ocean model used is the MIT general circulation model (MITgcm, version MITgcmUV checkpoint62x) in an ocean-only configuration with no sea ice. The model domain is circumpolar and extends between 84°S and 25°S (and 0°, see model run in SI) in latitude. The horizontal grid spacing is 0.25°, equivalent to a zonal grid spacing of ~27.8 km and a meridional grid spacing of ~3 km to ~25 km at the southern and northern boundary, respectively. The model configuration uses 50 vertical levels, ranging from 10 m at the sea surface to 368 m at the bottom. Our model uses a linear drag coefficient of 0.0011, a nonlinear equation of state, a seventh-order advection scheme for temperature and salinity and the K-profile parameterization. No parameterization is used for the advection and diffusion due to mesoscale eddies. Partial cells are used in the vertical for a more accurate representation of bathymetry. Restoring boundary conditions for the surface forcing are taken from a coupled atmosphere-ocean model (GFDL CM2.1) simulating Late Eocene conditions with atmospheric CO2 concentrations of 800 ppm. The surface forcing for sea surface temperature, sea surface salinity, and zonal and meridional wind stresses are temporally and zonally averaged values from the coupled model (Figure S1). The surface forcing is zonal averaged, in order to be able to modify the bathymetry configurations and continent-ocean distributions, without causing artificial disturbances of the ocean circulations. A sponge layer of ~300 km is used at the northern boundary of the model to relax to a temporal and zonal mean of salinity and temperature output from the coupled model with a restoring time scale of 10 days.

Notes

Credit
This research was undertaken with support from Australian Research Council Special Research Initiative for Antarctic Gateway Partnership (Project ID SR140300001), Discovery Project 180102280, and computational resources from the Australian National Computational Infrastructure. We thank Dave Hutchinson for providing the forcing input data for our model. DRM is supported by the ORCHESTRA project (NE/N018095/1). KH's research has been funded by the Deutsche Forschungsgemeinschaft (DFG) under the project GO724/15-1 and institutional resources from the Research Program PACES-II, Workpackage 3.2, of the Alfred Wegener Institute (AWI). The European Research Council under the European Community’s Seventh Framework Program provided funding by ERC Starting Grant #802835 to PKB. DRM, KH and JHL received a “Visiting Scholarship” by the University of Tasmania.

Data time period: 2020-04-21 to 2030-04-21

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text: westlimit=-180.00; southlimit=-85.00; eastlimit=180.00; northlimit=0.00

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(DATA ACCESS - ocean gyre model [index of files for download])

uri : https://data.imas.utas.edu.au/attachments/44e47376-8627-4daa-a49a-7c2d31294537 External Link

Identifiers

DOI : 10.25959/5EB21FC078C99
global : 44e47376-8627-4daa-a49a-7c2d31294537

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