Sea Ice and Climate. Part II: Model Climate Stability to Perturbations of the Hydrological Cycle

Abstract

Abstract The sensitivity of a simple climate model to variations in a global hydrological cycle is studied. The model consists of a zonally averaged single basin, two-hemisphere ocean model coupled to an atmospheric energy balance model and a thermodynamic sea ice model. Land processes are reduced to considerations of the oceanic catchment basins, which serve to amplify or decrease the oceanic freshwater forcing. The model typically admits both symmetric and asymmetric equilibria. For sufficiently strong global freshwater forcing, the symmetric circulation is replaced by asymmetric states. This is accomplished by a supercritical pitchfork bifurcation. It is shown that sea ice enhances climate sensitivity in that larger changes of the hydrological cycle are required to induce instability if sea ice is neglected. This behavior comes from the strong damping of SST anomalies under ice, which is essential for suppressing deep ocean temperature anomalies. Thus, ocean temperature plays an important role in clima...