On the dynamics of an abrupt climate change

A complex coupled atmosphere-ocean-sea ice climate model with idealized continental configurations has previously been shown to exhibit multiple stable equilibria: a warm, equable, ice-free state; and a cold state with sea ice extending into mid-latitudes (see the companion paper presented by D. Ferreira). Here I focus on the transient adjustment of the global climate system between these very different states. I describe very long integrations (many kyr) of the coupled model with prescribed slowly-varying radiative forcing (± 5 W/m² in the global mean), and the resulting hysteresis associated with large expansions and retreats of sea ice.

The transitions are complex, exhibiting threshold behavior and multiple time scales. The cooling phase is characterized by the initial appearance of a meta-stable small ice cap, followed by an ice-free period lasting several hundred years, followed by a rapid expansion of sea ice into the mid-latitudes. The warming phase begins with a slow retreat of sea ice on the kyr scale, followed by a rapid warming and total sea ice loss within about 100 years. The dynamics of these transitions are governed by complex interactions between the upper ocean stratification (build-up and decay of a halocline), deep ocean convection, and the large-scale convergence of ocean heat transport. I will discuss the extent to which these idealized experiments might shed light on the dynamics of D-O events.