Comment on egusphere-2022-953

Abstract

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 assess the impact of future Arctic sea ice retreat on occurrence probabilities of wintertime circulation regimes and link these dynamical changes to frequency changes in European winter temperature extremes. For this reason, we analyze ECHAM6 sea ice sensitivity model simulations from the Polar Amplification Intercomparison Project and compare experiments with future sea ice loss prescribed over the entire Arctic, as well as only locally over the Barent/Karasea with a present day reference experiment. We first show how these imposed future Arctic sea ice reductions affect large-scale atmospheric dynamics in terms of occurrence frequency changes of five computed Euro-Atlantic winter circulation regimes. Both sensitivity experiments show similar regime frequency changes, such as more frequent occurrences of a Scandinavian blocking pattern in midwinter under reduced sea ice conditions. Afterwards we demonstrate how the Scandinavian blocking regime, but also a regime that resembles the negative phase of the North Atlantic Oscillation can be linked to favored occurrences of European winter cold extremes. In contrast, winter warm extreme occurrences are typically associated with an anticyclonic regime over the eastern Atlantic and a regime similar to the positive state of the North Atlantic Oscillation. Based on these links between temperature extremes and circulation regimes, as well as on the previously detected regime frequency changes we employ a framework of conditional extreme event attribution. This enables us to decompose sea ice induced frequency changes of European temperature extremes into two different contributions: one term that is related to dynamical changes in regime occurrence frequencies, and another more thermodynamically motivated contribution that assumes fixed atmospheric dynamics in terms of circulation regimes. By employing this decomposition procedure we show how the overall thermodynamical warming effect, but also the previously detected increased Scandinavian blocking pattern frequency under future sea ice reductions can dominate and shape the overall response signal of European cold extremes in midwinter. We also demonstrate how for instance a decreased occurrence frequency of the anticyclonic regime over the eastern Atlantic counteracts the thermodynamical 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, we argue that the detected effects on European temperature extremes related to Arctic sea ice loss are of secondary relevance.