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

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. Plain Language SummaryWhen 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.

Historical Variability and Lifecycles of North Atlantic Midlatitude Cyclones Originating in the Tropics

“…We used three levels—900, 600, and 300 hPa—for all reanalyses except JRA25, for which 925, 600, and 300 hPa were used. Our use of three vertical levels is consistent with previous studies (e.g., Bieli et al., 2019, 2020; Liu et al., 2017; Studholme et al., 2015), and is preferred because it will allow for future comparisons with climate model output, where the number of available vertical levels is typically restricted.…”

“…Cyclone phase-space parameters are also used to identify extratropical transition, which is defined herein as the occurrence of B ≥ 10 m and T L < 0 for at least 1 day. Use of these thresholds follows previous studies (e.g., Bieli et al, 2019Bieli et al, , 2020Studholme et al, 2015), but systematic sensitivity analysis would be a useful future contribution.…”

“…Recent studies on the evolution of tropical cyclones in the midlatitudes have focused on extratropical transition events, both case studies (e.g., Laurila et al., 2019) and climatological studies of reanalysis (e.g., Bieli et al., 2019) and climate model (e.g., Bieli et al., 2020; Zarzycki et al., 2017) data. Based on European Centre for Medium‐Range Weather Forecasts' operational analysis data, 47–61% of Northern Hemisphere tropical cyclones between 2008 and 2012 underwent extratropical transition (Studholme et al., 2015), although estimates are sensitive to the criteria used to identify extratropical transition.…”

European Windstorm Risk of Post‐Tropical Cyclones and the Impact of Climate Change

“…In the present climate, the north eastern part of the tropical Atlantic is too cool for the genesis of TCs, but will attain SSTs warmer than 26.5°C in a warmer climate (see Haarsma et al., 2013; Figure 3), and thereby induce a north‐eastern expansion of the MDR (Bieli et al., 2020). TCs that originate in the middle and western part of the tropical Atlantic, usually follow a C‐shape curve along the east coast of North America, circumventing the Atlantic subtropical‐high before entering Europe.…”

“…Three different mechanisms might contribute to an increased risk of European storm risk of post‐TCs: Most studies suggest a shift to more intense TCs (T. Knutson et al., 2020; T. R. Knutson et al., 2010) in a warmer climate, although this is also debated (Roberts et al., 2020b). Stronger TCs will have a larger chance to survive over the cooler midlatitude SSTs before being able to reintensify after encountering favorable conditions to initiate phase‐locking with the upper‐level flow when entering Europe The region of SSTs warmer than 26.5°C will shift poleward in a warmer climate, inducing a poleward shift of the main development region (MDR) for TCs and reducing the distance a TC must travel over a hostile environment before reaching Europe In the present climate, the north eastern part of the tropical Atlantic is too cool for the genesis of TCs, but will attain SSTs warmer than 26.5°C in a warmer climate (see Haarsma et al., 2013; Figure 3), and thereby induce a north‐eastern expansion of the MDR (Bieli et al., 2020). TCs that originate in the middle and western part of the tropical Atlantic, usually follow a C‐shape curve along the east coast of North America, circumventing the Atlantic subtropical‐high before entering Europe.…”