  Paleoclimate proxies reveal abrupt transitions of the North Atlantic climate
during past glacial intervals known as Dansgaard--Oeschger (DO) events. A
central feature of DO events is a sudden warming of about 10$^{\circ}$C in
Greenland marking the beginning of relatively mild phases termed interstadials.
These exhibit gradual cooling over several hundred to a few thousand years
until a final abrupt decline brings the temperatures back to cold stadial
levels. As of now, the exact mechanism behind this millennia-scale variability
remains inconclusive. Here, we propose an excitable model to explain
Dansgaard-Oeschger cycles, where interstadials occur as noise-induced state
space excursions. Our model comprises the mutual multi-scale interactions
between four dynamical variables representing Arctic atmospheric temperatures,
Nordic Seas' temperatures and sea ice cover, and the Atlantic Meridional
Overturning Circulation. The model's atmosphere-ocean heat flux is moderated by
the sea ice, which in turn is subject to large perturbations dynamically
generated by fast-evolving intermittent noise. If supercritical, perturbations
trigger interstadial-like state space excursions during which all four model
variables undergo qualitative changes that consistently resemble the signature
of interstadials in corresponding proxy records. As a physical intermittent
process generating the noise we propose convective events in the ocean or
atmospheric blocking events. Our model accurately reproduces the DO cycle
shape, return times and the dependence of the interstadial and stadial
durations on the background conditions. In contrast to the prevailing
understanding that DO variability is based on bistability in the underlying
dynamics, we show that multi-scale, monostable excitable dynamics provide a
promising alternative to explain millennial-scale climate variability
associated with DO events.
