Self-sustained Multicentennial Oscillation of the Atlantic Meridional Overturning Circulation in Two-hemisphere Box Models

Self-sustained multicentennial variability of the Atlantic Meridional Overturning Circulation (AMOC) has been previously demonstrated in one‐hemisphere box models. In this study, we extend our earlier work by developing a two‐hemisphere box model that incorporates both thermohaline and wind‐driven components. Our analysis reveals that a robust, weakly damped multicentennial eigenmode persists in the two‐hemisphere framework, with the salinity advection feedback in the North Atlantic remaining the dominant control mechanism, while the South Atlantic plays a minor role. Compared to the one‐hemisphere model, the self-sustained multicentennial oscillation in the two-hemisphere box model is much easier to occur and less sensitivity to changes in basin geometry. Moreover, the inclusion of wind‐driven circulation acts to weaken the oscillation amplitude, with negligible impact on the oscillation period. We further demonstrate that the AMOC itself is a necessary and sufficient condition for the multicentennial oscillation, as the mode vanishes when the AMOC is shut down. Finally, stochastic freshwater forcing can excite a sustained oscillation, confirming that the multicentennial mode is intrinsic to the AMOC. We also identify a damped millennial oscillatory mode that deserves further investigation, as it may provide clues to understanding the Dansgaard-Oeschger events that have occurred repeatedly in past climates.