Constraining and predicting Arctic amplification and relevant climate feedbacks

The Arctic region shows a particularly high susceptibility to climate change, which historically manifests in an amplification of the near-surface warming in the Arctic relative to the global mean. This Arctic amplification (AA) has impacts on the climate system also beyond the northern polar regions, which highlights the importance to adequately represent it in numerical models. While state-of-the-art climate models widely agree on the presence of AA, they simulate a large spread in the magnitude of Arctic-amplified warming. This thesis addresses the need to evaluate the performance of global climate models in projecting AA and its most important drivers. For the latter, the focus is on the three amplifying climate feedbacks (ACFs) that largely drive the meridional warming structure leading to AA. The ACFs include the sea-ice-albedo feedback (SIAF), the Planck feedback, and the lapse-rate feedback (LRF). These feedbacks arise from the relevant changes in Arctic sea ice, near-surface temperatures, and the deviation from the near-surface temperature change through the atmosphere, respectively. In the thesis, two observational constraints are presented to narrow the range of climate models of the sixth Coupled Model Intercomparison Project (CMIP6) regarding their projection of AA and the ACFs in both past and future climate. While for the past, the models representation of near-surface processes can often be directly evaluated against observations, it is particularly the LRF that is difficult to constrain as it incorporates the entire atmospheric warming structure. As a consequence, the historical constraint focuses on the LRF, while the future constraint gives a prediction range for the evolution of AA and all three ACFs through the 21st century. The main results are highlighted in the view of the changing atmospheric energy budget (AEB) of the Arctic under anthropogenic climate forcing. The AEB provides a framework to address Arctic climate change at large scales, a