Extreme cyclones entering the Arctic along various tracks in winter and their associated atmospheric circulation features

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.