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Climate of the Past An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/cp-2020-75
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2020-75
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  12 Jun 2020

12 Jun 2020

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This preprint is currently under review for the journal CP.

Impact of mid-glacial ice sheets on deep ocean circulation and global climate: Role of surface cooling on the AMOC

Sam Sherriff-Tadano, Ayako Abe-Ouchi, and Akira Oka Sam Sherriff-Tadano et al.
  • Atmosphere and Ocean Research Institute, the University of Tokyo, Kashiwa, Japan

Abstract. This study explores the effect of southward expansion of mid-glacial ice sheets on the global climate and the Atlantic meridional overturning circulation (AMOC), as well as the processes by which the ice sheets modify the AMOC. For this purpose, simulations of Marine Isotope Stage (MIS) 3 and 5a are performed with an atmosphere-ocean general circulation model. In the MIS3 and MIS5a simulations, the global average temperature decreases by 5.0 °C and 2.2 °C, respectively, compared with the preindustrial climate simulation. The AMOC weakens by 3 % in MIS3, whereas it is enhanced by 16 % in MIS5a, both of which are consistent with a reconstruction. Sensitivity experiments extracting the effect of the expansion of glacial ice sheets from MIS5a to MIS3 show a global cooling of 1.1 °C, contributing to about 40 % of the total surface cooling from MIS5a to MIS3. These experiments also demonstrate that the ice sheet expansion leads to a surface cooling of 2 °C over the Southern Ocean as a result of colder North Atlantic deep water. We find that the southward expansion of the mid-glacial ice sheet exerts a small impact on the AMOC. Partially coupled experiments reveal that the global surface cooling by the glacial ice sheet tends to reduce the AMOC by increasing the sea ice at both poles, and hence compensates for the strengthening effect of the enhanced surface wind over the North Atlantic. Our results show that the total effect of glacial ice sheets on the AMOC is determined by the two competing effects, surface wind and surface cooling. The relative strength of surface wind and surface cooling depends on the ice sheet configuration, and the strength of the surface cooling can be comparable to that of surface wind when changes in the extent of ice sheet are prominent.

Sam Sherriff-Tadano et al.

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Sam Sherriff-Tadano et al.

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Short summary
We perform simulations of Marine Isotope Stage 3 and 5a with an atmosphere-ocean general circulation model to explore the effect of southward expansion of mid-glacial ice sheets on the Atlantic meridional overturning circulation (AMOC) and climate. We find that the southward expansion of the mid-glacial ice sheet causes a surface cooling over the North Atlantic and Southern Ocean, but exerts a small impact on the AMOC due to the competing effects of surface wind and surface cooling.
We perform simulations of Marine Isotope Stage 3 and 5a with an atmosphere-ocean general...
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