Session: Glacial Climates (LGM, Last deglaciation, Ice sheet uncertainties, Glacial-interglacial cycles)
Author: Marlene Klockmann / firstname.lastname@example.org / Max Planck Institute for Meteorology
Co-author: Uwe Mikolajewicz, Max Planck Institute for Meteorology;
Jochem Marotzke, Max Planck Institute for Meteorology;
There is a large intermodel spread regarding the glacial Atlantic Meridional Overturning Circulation (AMOC) within the PMIP ensemble, and many models fail to capture the shoaling of the glacial AMOC indicated by proxy-based reconstructions. The glacial AMOC response in the coupled model MPI-ESM is the sum of two large opposing effects: the glacial ice sheets cause a deepening and a strengthening of the AMOC; the glacial greenhouse gas (GHG) concentrations cause a shoaling and a weakening. The two effects partly compensate for each other. As a result, the glacial AMOC does not shoal with respect to the modern state. The key mechanism for the GHG effect is brine release in the Southern Ocean, which increases the density of Antarctic Bottom Water; the key mechanism for the ice-sheet effect is the salt transport into the North Atlantic, which enhances the density of North Atlantic Deep Water. The magnitude of the respective effects depends on the background climate and likely also on the model-specific implementation of sea-ice dynamics and continental ice sheets. Already small differences in the magnitude of either effect can change the sign of the total AMOC response, which provides a good explanation for the spread within the PMIP ensemble.