Unforced Millennial Oscillations of the Coupled Climate-Carbon System

The paleoclimate record contains abundant evidence for millennial-scale climate variability, such as Dangaard-Oeschger events. However there are few examples of unforced millennial oscillations in climate models, and a satisfying physical understanding remains elusive. We find unforced oscillations at a period of 1100 years in long simulations of an idealized but fully coupled atmosphere-ocean model with an oceanic carbon cycle and prognostic atmospheric pCO2. The oscillations occur spontaneously within a narrow range of global climate as the model undergoes multi-millennial drift toward a warm, ice-free climate, and result in variations of roughly 20 ppm pCO2 and 0.3 K global mean surface temperature. We investigate key physical and biogeochemical processes that initiate, maintain, and terminate the free oscillations through composite analyses and targeted mechanism-denial experiments.

The oscillations are strongly associated with the deep stratification of the high-latitude oceans and abrupt changes in the eddy-driven component of the deep Meridional Overturning Circulation (MOC). Onset of oscillations is triggered by gradual stratification loss due to slow warming of the deep ocean. Loss of stratification is in turn associated with onset of a deep MOC that preferentially transports stored Dissolved Inorganic Carbon (DIC) from the high latitude net-sequestration regions toward the equatorial net-outgassing region. Radiative feedback from the consequent pCO2 increase causes global warming of near-surface waters, and anomalously warm, salty water is in turn advected by the anomalous MOC to intermediate depths of the high-latitude oceans. The high-latitude oceans are thus temporarily re-stratified, abruptly terminating the deep MOC and reversing the oscillation phase. The millennial timescale results from equatorward advection of DIC and subsequent poleward advection of anomalous heat and salt. We summarize our physical interpretation via a low-order delayed-oscillator model. We stress the importance of considering climate-carbon feedbacks for understanding millennial-scale climate dynamics.