This study analyzes the response of the Atlantic meridional overturning circulation (AMOC) to different CO2 concentrations and two ice sheet configurations in simulations with the coupled climate model MPI-ESM. With preindustrial (PI) ice sheets, there are two different AMOC states within the studied CO2 range: one state with a strong and deep upper overturning cell at high CO2 concentrations and one state with a weak and shallow upper cell at low CO2 concentrations. Changes in AMOC variability with decreasing CO2 indicate two stability thresholds. The strong state is stable above the first threshold near 217 ppm, and the weak state is stable below the second threshold near 190 ppm. Between the two thresholds, both states are marginally unstable, and the AMOC oscillates between them on millennial time scales. The weak AMOC state is stable when Antarctic Bottom Water becomes dense and salty enough to replace North Atlantic Deep Water (NADW) in the deep North Atlantic and when the density gain over the North Atlantic becomes too weak to sustain continuous NADW formation. With Last Glacial Maximum (LGM) ice sheets, the density gain over the North Atlantic and the northward salt transport are enhanced with respect to the PI ice sheet case. This enables active NADW formation and a strong AMOC for the entire range of studied CO2 concentrations. The AMOC variability indicates that the simulated AMOC is far away from a stability threshold with LGM ice sheets. The nonlinear relationship among AMOC, CO2, and prescribed ice sheets provides an explanation for the large intermodel spread of AMOC states found in previous coupled LGM simulations.

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.

We present a mechanism for self-sustained ocean circulation changes that cause abrupt temperature changes over Greenland in a multimillennial climate model simulation with glacial CO 2 concentrations representative of Marine Isotope Stage 3. The Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG) oscillate on millennial time scales. When the AMOC is strong, the SPG is weak and contracted; when the AMOC is weak, the SPG is strong and extensive. The coupling between the two systems via wind-driven and density-driven feedbacks is key to maintaining the oscillations. The SPG controls the transport of heat and salt into the deep-water formation sites and thus controls the AMOC strength. The strength and location of the deep-water formation affect the density-driven part of the SPG and thus control the mean strength and extent of the SPG. This mechanism supports the hypothesis that coupled ocean-ice-atmosphere interactions could have triggered abrupt glacial climate change. Plain Language Summary Between 57.000 and 29.000 years ago, the last glacial period was marked by several abrupt warming and cooling events over Greenland and the North Atlantic. Understanding the mechanism behind these so-called Dansgaard-Oeschger events increases our understanding of possible tipping points that cause abrupt change in the Earth system. The role of the ocean in causing these events is still a topic of debate. We find abrupt changes in the North Atlantic circulation that resemble Dansgaard-Oeschger events in a simulation with a state-of-the-art climate model. These simulated ocean circulation changes are generated without adding external triggers such as meltwater from glaciers. Instead, the events are generated by the interaction of the two large-scale current systems in the North Atlantic-the Atlantic meridional overturning circulation (AMOC) and the North Atlantic subpolar gyre (SPG). Both current systems are affected by changes in surface winds and the density pattern of the North Atlantic. We find that the location where the densest water is formed controls how the SPG interacts with the AMOC. Under favorable conditions, the effects of wind and density combine in such a way that changes in the SPG cause abrupt changes in the AMOC.

Abstract. Heinrich events are among the dominant modes of glacial climate variability. During these events, massive iceberg armadas were released by the Laurentide Ice Sheet and sailed across the Atlantic where they melted and released freshwater, as well as detritus, that formed characteristic layers on the seafloor. Heinrich events are known for cold climates in the North Atlantic region and global climate changes. We study these events in a fully coupled complex ice sheet–climate model with synchronous coupling between ice sheets and oceans. The ice discharges occur as an internal variability of the model with a recurrence period of 5 kyr, an event duration of 1–1.5 kyr, and a peak discharge rate of about 50 mSv, roughly consistent with reconstructions. The climate response shows a two-stage behavior, with freshwater release effects dominating the surge phase and ice sheet elevation effects dominating the post-surge phase. As a direct response to the freshwater discharge during the surge phase, deepwater formation in the North Atlantic decreases and the North Atlantic deepwater cell weakens by 3.5 Sv. With the reduced oceanic heat transport, the surface temperatures across the North Atlantic decrease, and the associated reduction in evaporation causes a drying in Europe. The ice discharge lowers the surface elevation in the Hudson Bay area and thus leads to increased precipitation and accelerated ice sheet regrowth in the post-surge phase. Furthermore, the jet stream widens to the north, which contributes to a weakening of the subpolar gyre and a continued cooling over Europe even after the ice discharge. This two-stage behavior can explain previously contradicting model results and understandings of Heinrich events.