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. This study explores the effect of southward expansion of Northern Hemisphere (American) mid-glacial ice sheets on the global climate and the Atlantic Meridional Overturning Circulation (AMOC) as well as the processesby which the ice sheets modify the AMOC. For this purpose, simulations ofMarine Isotope Stage (MIS) 3 (36 ka) and 5a (80 ka) are performed with anatmosphere–ocean general circulation model. In the MIS3 and MIS5asimulations, the global average temperature decreases by 5.0and 2.2 ∘C, respectively, compared with the preindustrial climatesimulation. The AMOC weakens by 3 % in MIS3, whereas it strengthens by16 % in MIS5a, both of which are consistent with an estimate based on231Pa ∕ 230Th. Sensitivity experiments extracting the effect of thesouthward expansion of glacial ice sheets from MIS5a to MIS3 show a globalcooling of 1.1 ∘C, contributing to about 40 % of the totalsurface cooling from MIS5a to MIS3. These experiments also demonstrate thatthe ice sheet expansion leads to a surface cooling of 2 ∘C overthe Southern Ocean as a result of colder North Atlantic Deep Water. We findthat the southward expansion of the mid-glacial ice sheet exerts a smallimpact on the AMOC. Partially coupled experiments reveal that the globalsurface cooling by the glacial ice sheet tends to reduce the AMOC byincreasing the sea ice at both poles and, hence, compensates for thestrengthening effect of the enhanced surface wind over the North Atlantic.Our results show that the total effect of glacial ice sheets on the AMOC isdetermined by two competing effects: surface wind and surface cooling.The relative strength of surface wind and surface cooling effects depends onthe ice sheet configuration, and the strength of the surface cooling can becomparable to that of surface wind when changes in the extent of ice sheetare prominent.

Impact of mid-glacial ice sheets on deep ocean circulation and global climate