Atmospheric Response to a Collapse of the North Atlantic Circulation Under a Mid-Range Future Scenario: A Regime Shift in Northern Hemisphere Dynamics

Climate models project a future weakening of the Atlantic Meridional Overturning Circulation (AMOC), but the impacts of this weakening on climate remain highly uncertain. A key challenge in quantifying the impact of an AMOC decline is in isolating its influence on climate, relative to other changes associated with increased greenhouse gases. Here we isolate the climate impacts of a weakened AMOC in the broader context of a warming climate using a unique ensemble of Shared Socioeconomic Pathway (SSP) 2-4.5 integrations that was performed using the Climate Model Intercomparison Project Phase 6 (CMIP6) version of the NASA Goddard Institute for Space Studies ModelE (E2.1). In these runs internal variability alone results in a spontaneous bifurcation of the ocean flow, wherein two out of ten ensemble members exhibit an entire AMOC collapse, while the other eight recover at various stages despite identical forcing of each ensemble member and with no externally prescribed freshwater perturbation. We show that an AMOC collapse results in an abrupt northward shift and strengthening of the Northern Hemisphere (NH) Hadley Cell (HC) and intensification of the northern midlatitude eddy-driven jet. We then use a set of coupled atmosphere-ocean abrupt CO₂ experiments spanning the range 1-5xCO₂ to show that this response to an AMOC collapse results in a nonlinear shift in the NH circulation moving from 2xCO₂ to 3xCO₂. Slab-ocean versions of these experiments, by comparison, do not capture this nonlinear behavior. Our results suggest that changes in ocean heat flux convergences associated with an AMOC collapse — while highly uncertain — can result in profound changes in the NH circulation and continued efforts to constrain the AMOC response to future climate change are needed.