Variability across parameter space and mechanisms of DO-like oscillations in a fast Earth System Model

The risk of crossing critical thresholds in the Earth system is continuously increasing due to anthropogenic climate change, potentially leading to accelerated responses. As one of the major tipping elements, the Atlantic Meridional Overturning Circulation (AMOC) can exhibit abrupt, nonlinear shifts between distinct regimes. Evidence of such tipping behavior is found in paleo-climate records, most prominently as Dansgaard-Oeschger (DO) events. Using the Bern3D fast Earth System Model, we investigated DO events driven by AMOC variability under Marine Isotope Stage 3 (MIS3) conditions. The model exhibits unforced, self-sustained oscillations resembling DO events within a narrow parameter space defined by CO₂ concentration, wind stress forcing, and diapycnal diffusivity. We systematically explored this parameter space and its boundaries. Beyond these parameter space boundaries, the AMOC either remains in a weak regime or undergoes an abrupt transition to a stronger state. Within the parameter space, oscillations are stable, with the periodicity being strongly controlled by CO₂. The mechanism underlying DO-like oscillations is primarily oceanic and involves heat accumulation and sea ice changes in the eastern North Atlantic. Sea ice acts as an insulating barrier, allowing subsurface heat to build up until it is rapidly redistributed through the water column, melts the sea ice, is released and triggers deep convection, producing an abrupt strengthening of the AMOC. Freshwater input from sea ice melt, in turn, weakens the circulation. These results indicate that abrupt shifts in the AMOC are an inherent feature of the climate system, although the implications for the AMOC’s future evolution remain unclear due to the vastly different boundary conditions.