Bifurcations in simple models of Arctic sea ice

Thesis

The observed rapid decline of Arctic sea ice has raised concerns that anthropogenic warming could cause the abrupt loss of sea ice by passing a saddle-node bifurcation. A variety of simple models exhibit this behaviour, however most GCM studies simulate a smooth loss of sea ice without passing a bifurcation point. In this thesis I attempt to bridge the gap between these types of models by adding additional features to simple models and assessing their impact on the models’ bifurcation structures.

I use the ice thickness distribution to resolve the effect of variability and mechanical processes within sea ice. My results indicate that, with an appropriate choice of parameterisation, mechanical processes reduce bistability in the system – likely induced by positive fluxes into open water being translated into an increased bottom heat flux.

I also use a single column model of the Arctic atmosphere coupled to a slab sea ice model to explore the impact of atmospheric processes. In the absence of heat transport feedbacks, the system exhibits broad perennially ice-covered and ice-free states separated by bifurcation points in clear sky and cloudy configurations. I find that the resolution of temperature inversions appears to destabilise the ice-covered state. I also find stabilising negative feedbacks from low clouds when ice-covered, and elevated clouds when ice-free, that strengthen bistability in the system.

Feedbacks in atmospheric heat transport significantly alter the bifurcation structure of the system – introducing a smooth loss of summer sea ice and significantly reducing the size of the bistable region. The bistable region shrinks further for stronger meridional heat transport and stronger warming of lower latitudes.

Authors: Derby, E

• DOI: 10.5287/ora-ga57banme
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English