Impact of mid-glacial ice sheets on deep ocean circulation and global climate
2021
Abstract. 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 processes
by which the ice sheets modify the AMOC. For this purpose, simulations of
Marine Isotope Stage (MIS) 3 (36 ka) and 5a (80 ka) are performed with an
atmosphere–ocean general circulation model. In the MIS3 and MIS5a
simulations, the global average temperature decreases by 5.0
and 2.2 ∘ C, respectively, compared with the preindustrial climate
simulation. The AMOC weakens by 3 % in MIS3, whereas it strengthens by
16 % in MIS5a, both of which are consistent with an estimate based on
231 Pa ∕ 230 Th. Sensitivity experiments extracting the effect of the
southward 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 two competing effects: surface wind and surface cooling.
The relative strength of surface wind and surface cooling effects 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.
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