Synchronization phenomena observed in glacial-interglacial cycles simulated in an Earth system model of intermediate complexity

The glacial-interglacial cycles of the Quaternary exhibit 41gkyr periodicity before the Mid-Pleistocene Transition (MPT) around 1.2-0.8gMyr ago and g1/4g100gkyr periodicity after that. From the viewpoint of dynamical systems, proposed mechanisms generating these periodicities are broadly divided into two types: (i) nonlinear forced responses of a mono-or multi-stable climate system to the astronomical forcing or (ii) synchronization of internal self-sustained oscillations to the astronomical forcing. In this study, we investigate the dynamics of glacial cycles simulated by the Earth system model of intermediate complexity CLIMBER-2 with a fully interactive carbon cycle, which reproduces the MPT under gradual changes in volcanic-CO2 degassing and regolith cover. We report that, in this model, the dominant frequency of glacial cycles is set in line with the principle of synchronization. It is found that the model exhibits self-sustained oscillations in the absence of astronomical forcing. Before the MPT, glacial cycles synchronize to the 41gkyr obliquity cycles because the self-sustained oscillations have periodicity relatively close to 41gkyr. After the MPT the timescale of internal oscillations becomes too long to follow every 41gkyr obliquity cycle, and the oscillations synchronize to the 100gkyr eccentricity cycles that modulate the amplitude of climatic precession. The latter synchronization occurs with the help of the 41gkyr obliquity forcing, which enables some terminations and glaciations to occur robustly at their right timing. We term this phenomenon vibration-enhanced synchronization because of its similarity to the noise-enhanced synchronization known in nonlinear science. While we interpret the dominant periodicities of glacial cycles as the result of synchronization of internal self-sustained oscillations to the astronomical forcing, the Quaternary glacial cycles show facets of both synchronization and forced response.