On the basis of the multidecadal component of a Fram Strait sea ice export reconstruction and on a sea surface temperature proxy for changes in the ocean circulation, a bifurcation diagram for two cycles of the Atlantic Ocean multidecadal oscillation is constructed. It suggests a hysteresis behavior that is consistent with nonlinear convective adjustment to changes in deep-water formation in the North Atlantic and with the meridional overturning circulation bistability associated with two distinct configurations of the North Atlantic subpolar gyre, simulated in response to idealized carbon dioxide increase. The nonlinear dynamics of the Atlantic Ocean circulation emphasized here on the basis of observational data adds to the hysteresis behavior shown in theoretical and numerical studies.