Unforced Millennial-Scale Oscillations in a Coupled Climate–Carbon System

We identify and investigate spontaneous unforced millennial-scale oscillations in an idealized global coupled climate–carbon model under nearly ice-free conditions. The oscillations first appear due to gradual loss of deep polar ocean stratification in a global warming scenario, which triggers the onset of a subpolar deep circulation, with important implications for ocean carbon storage. The analysis focuses on the residual mean meridional overturning circulation (MOC) due to the significant eddy-driven component. The subpolar deep MOC further invigorates the subtropical deep MOC, increasing global air–sea CO₂ fluxes and atmospheric CO₂ through upwelling of deep dissolved inorganic carbon (DIC). The increasing atmospheric CO₂ leads to global surface warming that further warms and stratifies the subsurface polar ocean, shutting down the subpolar deep MOC and completing a delayed negative feedback which reverses the oscillation phase. Mechanism-denial experiments reveal that the CO₂ radiative feedback and time-varying MOC and polar stratification are all essential components of the oscillator mechanism, while winter sea ice and time-varying biological nutrients are not. The millennial time scale is determined by the delayed response of atmospheric CO₂ to variations in polar stratification and subpolar deep MOC. This study sheds light on the coupled climate–carbon dynamics behind the internal millennial-scale variability of an ice-free warm climate. We discuss the relevance to understanding observed millennial-scale climate variability during the last glacial period.