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Abstract

There are multiple types of synoptic-scale cyclones over the globe including tropical, subtropical, and extratropical cyclones. As the atmosphere is a continuum, cyclones can form as one type and then evolve to another; for example, it is well known that tropical cyclones change into extratropical cyclones at the midlatitudes (extratropical transition). The working group on “phase transitions” in the 10th WMO International Workshop on Tropical Cyclones (IWTC-10) in 2022 focused not only on extratropical transition but also on other types of transitions comprehensively. Tropical transition (TT) is a phase transition in which subtropical or extratropical cyclones change into tropical cyclones. While many studies have reported TT cases in the North Atlantic, TT is considered to be a universal process. We demonstrate a TT case in the western North Pacific based on observations and simulations. Tropical Storm Kirogi formed through baroclinic processes at relatively high latitude in August 2012. Initially, Kirogi had an asymmetric cloud pattern owing to frontogenesis in an environment with enhanced baroclinicity during this period. It also had a shallow warm core under the influence of an upper cold disturbance, and was classified as a subtropical cyclone based on the cyclone phase space analysis. Then, warm and moist air in the lower troposphere and cold air in the upper troposphere became isolated from the environment and wrapped up around the cyclone center, which were similar to occluding extratropical cyclones. Finally, Kirogi became a symmetric and deep warm-core cyclone. Similar evolution was also found in an idealized experiment.