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

Abstract

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 231Pa ∕ 230Th. 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.

Does a difference in ice sheets between Marine Isotope Stages 3 and 5a affect the duration of stadials?

“…3-5. The simulated global air 200 temperatures were 10.6 ˚C in MIS5a, 7.9 ˚C in MIS3 and 8.9 ˚C in MIS3-5aice (Sherriff-Tadano et al 2021). The maximum strength of the AMOC was 18.4 Sv in MIS5a, 15.6 Sv in MIS3 and 15.1 Sv in MIS3-5aice (Fig.…”

“…In particular, the recovery time was 200 to 300 years shorter in PC-MIS3-5aiceH compared with MIS3-5aiceH. This was related to the removal of sub-monthly variations in wind stress (Sherriff-Tadano et al 2021); removal of these variations caused thinning of sea ice in the centre of the subpolar region by reducing sea ice transport in this region (Fig. 12b, c) and created favourable conditions 305 for deepwater to form.…”

“…Previously, it has been shown that large Northern Hemisphere glacial ice sheets increase sea surface salinity over the North Atlantic Deepwater (NADW) formation region by increasing surface winds and decreasing precipitation (Eisenman et al 2009, Smith and Gregory 2012, Brady et al 2013, Zhang et al 2014a, Gong et al 2015, Klockmann et al 2016, Galbraith and de 110 Lavergne 2019, Guo et al 2019. In addition, it has been shown that stronger surface cooling by ice sheets increases the amount of sea ice in the NADW formation region and the Southern Ocean, the latter of which is induced by colder NADW outcropping in the Southern Ocean (Sherriff-Tadano et al 2021). These results imply that differences in glacial ice sheets may play a role in modifying the durations of stadials during glacial periods.…”

“…These results imply that differences in glacial ice sheets may play a role in modifying the durations of stadials during glacial periods. Recently, Sherriff-Tadano et al (2021) performed simulations of MIS3 and MIS5a and explored the impact of ice sheet differences on the AMOC and climate. In their simulations, differences 115 in the ice sheets exerted small impacts on the vigorous mode of the AMOC, because of a compensational balance between the increase in sea surface salinity in the northern North Atlantic (strengthening effect) and the increase in sea ice in the North Atlantic and Southern Ocean (weakening effect).…”

“…In this study, we explored the impacts of differences in the ice sheets between the MIS3 and MIS5a on the recovery time of the AMOC and the durations of stadials. For this purpose, we performed freshwater hosing experiments under three background climates that have been simulated previously, MIS3, MIS5a and MIS3 with the ice sheet forcing of MIS5a 125 (Sherriff-Tadano et al 2021). By comparing the recovery time of the AMOC after the cessation of freshwater hosing in each experiment, we assessed the impact of the ice sheets on the recovery time of the AMOC.…”