Abstract
Abstract 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. Analysis focuses on the residual mean Meridional Overturning Circulation (MOC) due to the significant eddy-driven component. The subpolar deepMOCfurther 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 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 timescale 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 carbon-climate 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.
Published Version
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