Abstract
The Arctic is warming much faster than the global average, a phenomenon referred to as Arctic Amplification (AA). Observations reveal that profound climate change in the Arctic is not a future phenomenon, but is already occurring. AA is evident in both observations and climate models, and multiple mechanisms and climate feedbacks have been proposed to explain the phenomenon. However, which physical mechanisms are paramount in AA, and the relative importance of the feedback mechanisms involved, are still under debate. The uncertainty in Arctic climate projections is larger than in any other region, and a better understanding of the underlying processes is essential to reduce the uncertainty and to produce reliable projections of future Arctic warming. The decrease of sea ice is one of the clearest indicators of the ongoing climate change, and multiple studies have shown a correlation between Arctic warming and sea-ice reduction. However, sea-ice loss is not the only factor causing AA, and sea-ice loss may equally well be the result of Arctic warming, not the cause (or both). The causality of sea-ice loss and AA is an uncertainty in previous studies. In this thesis, the Norwegian Earth System Model (NorESM2) is used to acquire a better understanding of the influence of sea-ice loss on Arctic warming. The Arctic climate response is investigated from extended sets of coupled model experiments, part of the Polar Amplification Model Intercomparison Project (PAMIP). These experiments are nudged to present day and future sea-ice extent. By construction, the difference between the experiments provides the simulated response to future Arctic sea-ice loss, and the causality of reduced sea-ice and AA is assured. The results demonstrate Arctic warming with a pronounced seasonal cycle as a response to sea- ice reduction. The Arctic warming is prominent during autumn and winter, with a maximum November warming above 6◦C in the regions where sea-ice is retreating. The warming is found to be dominated by regional processes, rather than large-scale transports. The Arctic climate change in a future scenario experiment, shared socioeconomic pathway 5-8.5 (SSP5-8.5), is compared to the changes solely due to sea-ice reduction when similar conditions of sea-ice extent are reached. This comparison reveal similar seasonal cycle of near-surface warming, albeit a smaller amplitude is seen in SSP5-8.5. Both regional processes and large scale lateral transports are found to be instrumental for the Arctic warming in SSP5-8.5.