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Application of the Cyclone Phase Space to Extratropical Transition in a Global Climate Model

Author(s): Bieli, Melanie; Sobel, Adam H; Camargo, Suzana J; Murakami, Hiroyuki; Vecchi, Gabriel A

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Abstract: 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.
Publication Date: 6-Apr-2020
Citation: Bieli, Melanie, Adam H. Sobel, Suzana J. Camargo, Hiroyuki Murakami, and Gabriel A. Vecchi. "Application of the Cyclone Phase Space to Extratropical Transition in a Global Climate Model." Journal of Advances in Modeling Earth Systems 12, no. 4 (2020). doi:10.1029/2019MS001878.
DOI: doi:10.1029/2019MS001878
EISSN: 1942-2466
Type of Material: Journal Article
Journal/Proceeding Title: Journal of Advances in Modeling Earth Systems
Version: Final published version. This is an open access article.

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