The effect of greenhouse gas concentrations and ice sheets on the glacial AMOC in a coupled climate model

Abstract

Abstract. Simulations with the Max Planck Institute EarthSystem Model (MPI-ESM) are used to study the sensitivity of the AMOC and the deep-ocean water masses during the Last Glacial Maximum to different sets of forcings. Analysing the individual contributions of the glacial forcings reveals that the ice sheets cause an increase in the overturning strength and a deepening of the North Atlantic Deep Water (NADW) cell, while the low greenhouse gas (GHG) concentrations cause a decrease in overturning strength and a shoaling of the NADW cell. The effect of the orbital configuration is negligible. The effects of the ice sheets and the GHG reduction balance each other in the deep ocean so that no shoaling of the NADW cell is simulated in the full glacial state. Experiments in which different GHG concentrations with linearly decreasing radiative forcing are applied to a setup with glacial ice sheets and orbital configuration show that GHG concentrations below the glacial level are necessary to cause a shoaling of the NADW cell with respect to the pre-industrial state in MPI-ESM. For a pCO 2 of 149 ppm, the simulated overturning state and the deep-ocean water masses are in best agreement with the glacial state inferred from proxy data. Sensitivity studies confirm that brine release and shelf convection in the Southern Ocean are key processes for the shoaling of the NADW cell. Shoaling occurs only when Southern Ocean shelf water contributes significantly to the formation of Antarctic Bottom Water.

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

“…The wind shift and cooling appear to drive the AMOC strengthening shown in Fig. 4d, as has previously been observed in other models (Pausata et al 2011;Zhang et al 2014;Zhu et al 2014;Gong et al 2015;Muglia and Schmittner 2015;Klockmann et al 2016;Sherriff-Tadano et al 2017). When compared with the influences of CO 2 and orbital forcing, this suggests that Dansgaard-Oeschger-like AMOC disruptions would be most likely to have occurred in a 'sweet spot' of low CO 2, low obliquity and a relatively small (low-elevation) Laurentide ice sheet.…”

“…As discussed in earlier studies (Keeling and Stephens 2001;Shin et al 2003;Ferrari et al 2014;Watson et al 2015;Jansen and Nadeau 2016;Klockmann et al 2016), low atmospheric temperatures encourage sea ice production and export, extracting freshwater around Antarctica (where freezing dominates) and releasing it further north (where melting dominates). Sea ice also acts as a barrier to precipitation, collecting snowfall and preventing it from entering the ocean until the ice beneath it melts.…”

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

“…Many other coupled climate models, however, show different and widely diverging changes in the deep ocean circulation and stratification between preindustrial and LGM simulations (25,26). Possible reasons for this disagreement include differences between transient and fully equilibrated solutions (27,28), insufficient increase in Antarctic sea-ice cover and formation (29), and/or compensating effects due to other differences in the boundary conditions, such as changes in the ice sheets (26,30). A more detailed analysis of comprehensive LGM climate simulations is needed to better understand differences between them, but is beyond the scope of this study.…”

The influence of the ocean circulation state on ocean carbon storage and CO<sub>2</sub> drawdown potential in an Earth system model

“…Do these changes agree with what we believe actually happened in the climate system during a glacial? When applying PMIP3 boundary conditions for the LGM, the height of the ice sheet in the Northern Hemisphere will tend to intensify both the wind stress over the North Atlantic basin and as a result the AMOC (Muglia and Schmittner, 2015;Klockmann et al, 2016). In a full glacial state, the associated deepening of the AMOC is, however, counteracted by the decrease in pCO atm 2 (Klockmann et al, 2016).…”

“…It is likely that the more sluggish circulation is already allowing for a more efficient biological pump, leading to a higher P * and thus a smaller drawdown potential. Another important mechanism for the global glacial deep ocean circulation is surface buoyancy loss around Antarctica driven by the brine rejection associated with sea ice formation (Klockmann et al, 2016;Marzocchi and Jansen, 2017). This effect is not explored in our simulations.…”