On the linkage between future Arctic sea ice retreat, Euro-Atlantic circulation regimes and temperature extremes over Europe

The question to what extent Arctic sea ice loss is able to affect atmospheric dynamics and climate extremes over mid-latitudes still remains a highly debated topic. In this study we investigate model experiments from the Polar Amplification Model Intercomparison Project (PAMIP) and compare experiments with future sea ice loss prescribed over the entire Arctic, as well as only locally over the Barents and Kara seas, with a present-day reference experiment. The first step is to perform a regime analysis and analyze the change in occurrence frequencies of five computed Euro-Atlantic winter circulation regimes. Forced by future Arctic sea ice conditions, most models show more frequent occurrences of a Scandinavian blocking pattern in at least 1 winter month, whereas there is an overall disagreement between individual models on the sign of frequency changes of two regimes that, respectively, resemble the negative and positive phase of the North Atlantic Oscillation. Focusing on the ECHAM6 PAMIP experiments, we subsequently employ a framework of conditional extreme-event attribution. It demonstrates how detected regime frequency changes can be used to decompose sea-ice-induced frequency changes of European temperature extremes into two different contributions: one "changed-regime" term that is related to dynamical changes in regime occurrence frequencies and another more thermodynamically motivated "fixed-regime" contribution that is related to increased surface temperatures during a specific circulation regime. We show how the overall fixed-regime warming effect and also an increased Scandinavian blocking pattern frequency under future sea ice reductions can equally contribute to and shape the overall response signal of European cold extremes in midwinter. We also demonstrate how a decreased occurrence frequency of an anticyclonic regime over the eastern Atlantic dynamically counteracts the fixed-regime warming response and results in no significant changes in overall January warm-extreme occurrences. However, when compared to other characteristics of future climate change, such as the thermodynamical impact of globally increased sea surface temperatures, the effects of Arctic sea ice loss on European temperature extremes are of secondary relevance.