Identification, characteristics, and dynamics of Arctic extreme seasons
Authors: Hartmuth Katharina , Boettcher Maxi , Wernli Heini , Papritz Lukas 
Published: April 2021

Abstract. The Arctic atmosphere is strongly affected by anthropogenic warming leading to long-term trends in, e.g., surface temperature and sea ice extent. In addition, it exhibits a pronounced seasonal cycle and strong variability on time scales from days to seasons. Recent research elucidated processes causing short-term extreme conditions in the Arctic that are typically related to the occurrence of specific weather systems. This study investigates unusual atmospheric conditions in the Arctic on the seasonal time scale, characterized by surface temperature, surface precipitation, and the atmospheric components of the surface energy balance. Based on a principle component analysis in the phase space spanned by the seasonal-mean values of the considered parameters, individual seasons are then objectively identified that deviate strongly from a running-mean climatology, and that we define as extreme seasons. Given the strongly varying surface conditions in the Arctic, this analysis is done separately in Arctic sub-regions that are climatologically characterized by either sea ice, open ocean, or mixed conditions. Using ERA5 reanalyses for the years 1979–2018, our approach identifies 2–3 extreme seasons for winter, spring, summer, and autumn, respectively, with strongly differing characteristics and affecting different Arctic sub-regions. While some show strongly anomalous seasonal-mean values mainly in one parameter, others are characterized by a combination of very unusual seasonal conditions in terms of temperature, precipitation, and the surface energy balance components. Two extreme winters affecting the Kara-Barents Seas are then selected for a detailed investigation of (i) their substructure, (ii) the role of synoptic-scale weather systems that occur during the season, and (iii) potential preconditioning by anomalous sea ice extent and/or sea surface temperature at the beginning of the season. Winter 2011/12 shows the highest surface temperature anomaly in parts of the Kara-Barents Seas (about +5 K), which was due to constantly above-average temperatures during the season related to a strongly enhanced frequency of blocking anticyclones in the Kara-Barents Seas and a strongly reduced frequency of cold air outbreaks. Sea ice coverage was normal at the beginning of the season and then developed a negative anomaly due to the unusually high temperatures. In contrast, winter 2016/17 started with a strongly negative anomaly in sea ice coverage and a strongly positive anomaly in sea surface temperature in the Kara-Barents Seas, which remained during most of the season. The combination of this preconditioning with specific synoptic conditions, i.e., a particularly high frequency of cold air outbreaks and an increased frequency of cyclones, was responsible for the extreme characteristics of this season, reflected in large upward surface heat flux anomalies and strongly increased precipitation. In summary, this study shows that extreme seasonal conditions in the Arctic are spatially heterogeneous, related to different near-surface parameters, and caused by different synoptic-scale weather systems, potentially in combination with surface preconditioning due to anomalous ocean and sea ice conditions at the beginning of the season. The framework developed in this study and the insight gained from analyzing the ERA5 period will be beneficial for addressing the effects of global warming on Arctic extreme seasons.