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During the last ice age there were several quasi-periodic abrupt warmingevents. The climatic effects of the so-called Dansgaard-Oeschger (DO) eventswere felt globally, although the North Atlantic experienced the largest andmost abrupt temperature anomalies. Similar but weaker oscillations also tookplace during the interglacial period. This paper proposes an auto-oscillatorymechanism between sea ice and convective deep water formation in the northAtlantic as the source of the persistent cycles. A simple dynamical model isconstructed by coupling and slightly modifying two existing models of oceancirculation and sea ice. The model exhibits mixed mode oscillations, consistingof decadal scale small amplitude oscillations, and a large amplitude relaxationfluctuation. The decadal oscillations occur due to the insulating effect of seaice and leads to periodic ventilation of heat from the polar ocean. Graduallyan instability builds up in the polar column and results in an abruptinitiation of convection and polar warming. The unstable convective staterelaxes back to the small amplitude oscillations from where the process repeatsin a self-sustained manner. Freshwater pulses mimicking Heinrich events causethe oscillations to be grouped into packets of progressively weakerfluctuations, as also observed in the proxy records. Modulation of this stableoscillation mechanism by freshwater and insolation variations could account forthe distribution and pacing of DO and Bond events. Physical aspects of thesystem such as sea ice extent and oceanic advective flow rates could determinethe characteristic 1,500 year timescale of DO events.

Millennial-scale stable oscillations between sea ice and convective deep water formation