The authors analyze the global statistics of tropical cyclones undergoing extratropical transition (ET) in the Forecast‐oriented Low Ocean Resolution version of CM2.5 with Flux Adjustment (FLOR‐FA). The cyclone phase space (CPS) is used to diagnose ET. A simulation of the recent historical climate is analyzed and compared with data from the Japanese 55‐year Reanalysis (JRA‐55), and then a simulation of late 21st century climate under Representative Concentration Pathway 4.5 is compared to the historical simulation. When CPS is applied to the FLOR‐FA output in the historical simulation, the results diverge from those obtained from JRA‐55 by having an unrealistic number of ET cases at low latitudes, due to the presence of strong local maxima in the upper‐level geopotential. These features mislead CPS into detecting a cold core where one is not present. The misdiagnosis is largely corrected by replacing the maxima required by CPS with the 95th percentile values, smoothing the CPS trajectories in time, or both. Other climate models may contain grid‐scale structures akin to those in FLOR‐FA and, when used for CPS analysis, require solutions such as those discussed here. Comparisons of ET in the projected future climate with the historical climate show a number of changes that are robust to the details of the ET diagnosis, though few are statistically significant according to standard tests. Among them are an increase in the ET fraction and a reduction in the mean latitude at which ET occurs in the western North Pacific. Plain Language Summary When tropical cyclones move into the midlatitudes, they change their physical structure and sometimes transform into extratropical cyclones. This process is called extratropical transition. One diagnostic tool for defining extratropical transition is the cyclone phase space. In this study, the authors apply the cyclone phase space to the tropical cyclones simulated by a global climate model. The output of this climate model has some features that misguide the cyclone phase space into diagnosing a cyclone as extratropical when in fact it is tropical. The authors examine various adjustments of the cyclone phase space that largely correct this misdiagnosis. They then study the changes in the global occurrence of extratropical transition between two 30‐year time periods simulated by the climate model: a historical period from 1976 to 2005 and a future period from 2071 to 2100, whose simulation is based on a scenario of moderate increases in greenhouse gas emissions. In the western North Pacific, tropical cyclones are found to become more likely to undergo extratropical transition in the future climate. In general, though, the changes in global statistics of extratropical transition (e.g., where and how often it occurs) between the simulated future and historical climate are small.
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