  During the last ice age several quasi-periodic abrupt warming events took
place. Known as Dansgaard-Oeschger (DO) events their effects were felt
globally, although the North Atlantic experienced the largest temperature
anomalies. Paleoclimate data shows that the fluctuations often occurred right
after massive glacial meltwater releases in the North Atlantic and in bursts of
three or four with progressively decreasing strengths. In this study a simple
dynamical model of an overturning circulation and sea ice is developed with the
goal of understanding the fundamental mechanisms that could have caused the DO
events. Interaction between sea ice and the overturning circulation in the
model produces self-sustained oscillations. Analysis and numerical experiments
reveal that the insulating effect of sea ice causes the ocean to periodically
vent out accumulated heat in the deep ocean into the atmosphere. Subjecting the
model to idealized freshwater forcing mimicking Heinrich events causes
modulation of the natural periodicity and produces burst patterns very similar
to what is observed in temperature proxy data. Numerical experiments with the
model also suggests that the characteristic period of 1,500 years is due to the
geometry, or the effective heat capacity, of the ocean that comes under sea ice
cover.
