Extreme cyclones from the mid-latitudes can transport water vapor and heat to the Arctic, affect the Arctic sea ice extent and the thickness of ocean mixing layer, and bring strong winds, low temperatures, rain and snow. Using ERA-5 reanalysis data at 6 h intervals during 1980—2021, extreme cyclones originating from the mid-latitudes of the North Atlantic and entering the Arctic during winter are objectively identified and tracked. In total 130 extreme cyclones are identified and classified. The generation mechanisms behind extreme cyclones with three different tracks, i.e., west, middle and east, and the reasons for the differences in these tracks after the formation of extreme cyclones are explored. Results show that positive potential vorticity appearing in the lower stratosphere 5—6 d prior to the formation of the extreme cyclones and the downward intrusion of stratospheric positive potential vorticity into the upper troposphere that accelerates the polar front jet provide dynamic conditions favorable for the formation of extreme cyclones. When an extreme cyclone is generated, the upper and lower positive potential vorticity regions are connected, leading to rapid development of the cyclone. Additionally, it is found that the cyclone track after cyclogenesis primarily depends on the transport of warm advection in the lower troposphere.

Temperature inversions are frequently observed in the boundary layer and lower troposphere of polar regions. Future variations of the low-level temperature inversions in these regions, especially the Antarctic, are still poorly understood. Due to the scarcity of observations in the Antarctic, reanalysis data and numerical simulations are often used in the study of Antarctic climate change. Based on ERA-Interim, ERA5, JRA-55, and NCEP–NCAR reanalysis products, this study examines temporal and spatial variations of Antarctic inversion depth in austral autumn and winter during 1979–2020. Deeper inversions are found to occur over the high plateau areas of the Antarctic continent. Based on the Mann–Kendall test, ERA-Interim and ERA5 data reveal that the Antarctic inversion depth in austral autumn and winter increased during 1992–2007, roughly maintained afterwards, and then significantly decreased since around 2016. The decrease trend is more obvious in the last two months of winter. Overall, JRA-55 better represents the spatial distribution of inversion depth, and ERA-Interim has better interannual variability. The Community Earth System Model Large Ensemble (CESM-LE) 30-member simulations in 1979–2005 were first verified against JRA-55, showing reasonable consistency, and were then used to project the future changes of Antarctic low-level inversion depth over 2031–2050 under RCP8.5. The CESM-LE projection results reveal that the temperature inversion will shallow in the Antarctic at the end of the 21st century, and the decrease in depth in autumn will be more pronounced than that in winter. In particular, the temperature inversion will weaken over the ice-free ocean, while it will remain stable over the ice sheet, showing certain spatial heterogeneity and seasonal dependence on the underlying cryospheric surface conditions. In addition, the decrease of inversion depth is found closely linked with the reduction in sea ice, suggesting the strong effect of global warming on the thermal structure change of the Antarctic.