Suppression of AMOC variability at increased CO2

Multi-decadal variability in the Atlantic Meridional Overturning Circulation (AMOC) is shown to differ significantly between the 4xCO2 and preindustrial control simulations of the GFDL ESM2M general circulation model (GCM). In the preindustrial simulation, this model has a peak in the power spectrum of both AMOC and northwards heat transport between 26°N and 50°N. In the 4xCO2 simulation, the only significant spectral peak is near 60N. Understanding these differences is important for understanding the effect of future climate change on climate variability, as well as for providing insight into the physics underlying AMOC variability. Transfer function analysis demonstrates that the shift is predominantly due to a shift in the internal ocean dynamics rather than a change in stochastic atmospheric forcing. Specifically, the reduction in variance from 26-45N is due to an increased stratification east of Newfoundland that results from the shallower and weaker mean overturning. The reduced AMOC variance that accompanies the reduced mean value of the AMOC at 4xCO2 is in contrast to the predictions of simple box models that predict a weaker circulation to be closer to a stability bifurcation point and therefore be accompanied by amplified variability. The high-latitude variability in the 4xCO2 simulation is related to the advection of anomalies by the subpolar gyre, distinct from the variability mechanism in the control run at lower latitudes. The 4xCO2 variability has only a small effect on meridional heat transport, but does significantly affect sea ice in the northern North Atlantic.