Influence of subgrid-scale processes on the sea ice and the polar oceans in a global ocean–sea ice model

From the point of view of an external observer in orbit around Earth, sea ice is an ultra-thin layer capping the polar oceans. This layer regulates the interactions between the atmosphere and the ocean. However, when taking a closer look, sea ice appears as a heterogeneous material governed by complex dynamic and thermodynamic processes on a wide range of scales. Small-scale sea ice may impact large-scale sea ice properties and influence the other components of Earth's climate system. This makes the modelling and study of sea ice challenging for the climate community. In this doctoral thesis, I investigated the impact of unresolved small-scale sea ice processes using the ocean–sea ice modelling framework NEMO-LIM3. I implemented parameterizations to improve the representation of melt ponds, sea ice form drag and landfast ice in NEMO-LIM3 for their importance in the surface energy budget, the turbulent fluxes of momentum and heat and the momentum equation for sea ice. The results of this thesis show that atmospheric forcing uncertainties have a higher impact on the modelled sea ice state than differences in melt pond parameterizations. Sea ice form drag is relevant for modelling the upper Arctic Ocean on annual, interannual and decadal time scales. Landfast ice impacts the properties of the whole Arctic sea ice and Arctic halocline. While these developments may ultimately improve large-scale sea ice models, other unresolved sea ice processes require further examination. In particular, I advise studying the sea ice processes covered in this thesis with Earth System Models to account for the interactions between the ocean, sea ice and the atmosphere.