Formation and decay of sea ice pressure ridges: How to enable a GCM sea ice model to represent CyroSat-2 estimates

Abstract:

A sub-grid scale sea ice thickness distribution (ITD) is an important state variable needed to enable a large-scale sea ice model to simulate winter ice growth and sea ice ridging processes realistically. Recent model physics developments, e.g. a melt pond model, a form drag parameterisation, a floe-size distribution model, fundamentally depend on the ITD. In spite of its importance, knowledge is poor about the accuracy of the simulated ITD. Here, we derive the ITD from individual Arctic sea ice thickness estimates available from the CryoSat-2 (CS2) radar altimetry mission during ice growth seasons since 2010. We find that less than 2% of sea ice in the Central Arctic is thicker than 3.6m in October, followed by a strong increase during winter towards a fraction of 15-40% in April. In contrast the annual cycle is weak in historical simulations and future projections with the HadGEM3-GC31 model, as well as forced ocean-sea ice and standalone sea ice simulations with the same model components NEMO v3.6 and CICE v5.1.2. Large modifications to sea ice physics parameters are required to increase both the formation in winter and the decay in summer of the thickest sea ice in a stand-alone sea ice simulation. The effect of these parameter changes is to: weaken the ice, increase the fraction of sea ice participating in ridging, and to introduce a dependency of the ocean and atmospheric turbulent heat fluxes on sea ice thickness. Impact studies on the sea ice mass budget will be presented for coupled simulations, and consequences for the sea ice climate sensitivity are discussed.