A theory for self-sustained multicentennial AMOC oscillation and its evidence in CESM1

Multicentennial North Atlantic climate variability revealed by paleoclimate reconstruction has been linked to the Atlantic meridional overturning circulation (AMOC) variability. However, mechanisms of multicentennial AMOC variability in coupled models have yet to reach a consensus, reflecting a necessity of more fundamental theoretical studies. To this end, we propose an ocean-only North Atlantic 4-box theoretical model. A self-sustained AMOC oscillation with a typical period of 300-400 years exhibits. The timescale is largely set by rate of AMOC advection but also modulated by thermal processes, while the self-sustained oscillation mechanism can be generalized as a combination of a linear growing oscillation and a nonlinear restraining. The linear growing oscillation is energized by the salinity advection feedback and stabilized by the temperature advection feedback, while the latter is hampered by surface temperature restoring. Nonlinear restraining processes restrict the runaway tendency of the linear growing oscillation and finally turn it into a self-sustained one. We further identify a 300-400-year AMOC oscillation in a CESM1 control simulation, which can be well explained by the self-sustained oscillation mechanism of the theoretical model. Our work demonstrates that internal variability plays a vital role in multicentennial AMOC variability, while the dominating processes primarily lie in the North Atlantic.