SOLA, 2020, Vol. 16, 57−63, doi:10.2151/sola.2020-010 57 Future Changes of Tropical Cyclones in the Midlatitudes in 4-km-mesh Downscaling Experiments from Large-Ensemble Simulations Sachie Kanada1, Kazuhisa Tsuboki1, and Izuru Takayabu2 1Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan 2Meteorological Research Institute, Tsukuba, Japan precipitation in the future, warmer climate. Abstract In midlatitude regions, TCs undergo an extratropical transition and structural changes as they move poleward into a baroclinic To understand the impacts of global warming on tropical environment characterized by a temperature gradient, increased cyclones (TCs) in midlatitude regions, dynamical downscaling vertical wind shear (VWS), and decreased SST (Evans et al. 2017; experiments were performed using a 4-km-mesh regional model Wada 2016). To simulate the detailed inner-core structure of a with a one-dimensional slab ocean model. Around 100 downscal- TC, it is necessary to use models with a horizontal resolution not ing experiments for midlatitude TCs that traveled over the sea east larger than 5 km (e.g., Gentry and Lackmann 2010; Kanada and of Japan were forced by large-ensemble climate change simula- Wada 2016). tions of both current and warming climates. Mean central pressure Regional TC changes include large uncertainty caused mainly and radius of maximum wind speed of simulated current-climate by SST warming patterns (Knutson et al. 2010; Murakami et al. TCs increased as the TCs moved northward into a baroclinic en- 2012). In addition, the projected TC changes vary greatly among vironment with decreasing sea surface temperature (SST). In the individual storms (Gutmann et al. 2018). Therefore, to understand warming-climate simulations, the mean central pressure of TCs changes in TC activity in the future climate, it is important to use in the analysis regions decreased from 958 hPa to 948 hPa: 12% a large ensemble to model storms. of the warming-climate TCs were of an unusual central pressure The present study aimed to understand the impact of global lower than 925 hPa. In the warming climate, atmospheric condi- warming on TCs likely to affect the large number of people living tions were strongly stabilized, however, the warming-climate TCs in eastern midlatitude coastal regions. Changes in TCs traveling could develope, because the storms developed taller and stronger over the sea east of Japan were investigated by conducting dynam- eyewall updrafts owing to higher SSTs and larger amounts of ical downscaling experiments with a 4-km-mesh regional model. near-surface water vapor. When mean SST and near-surface water The downscaling experiments were forced by large-ensemble vapor were significantly higher and baroclinicity was significantly climate simulations for both current and 4-K-warming climates smaller, unusual intense TCs with extreme wind speeds and large (Mizuta et al. 2017). Around 100 dynamical downscaling experi- amounts of precipitation around a small eye, could develop in ments for both the current and warming climates were conducted midlatitude regions, retaining the axisymetric TC structures. to explore changes in the intensity and structures of midlatitude (Citation: Kanada, S., K. Tsuboki, and I. Takayabu, 2020: TCs. Future changes of tropical cyclones in the midlatitudes in 4-km- mesh downscaling experiments from large-ensemble simulations. SOLA, 16, 57−63, doi:10.2151/sola.2020-010.) 2. Models and methodology Dynamical downscaling experiments of TCs traveling over 1. Introduction the sea east of Japan were performed by using the Policy Deci- sion-Making for Future Climate Change (d4PDF) database (Mizuta Tropical cyclones (TCs) often bring torrential rainfall, gales, et al. 2017). This database comprises results of a large ensemble and storm surges that sometimes cause severe disasters in midlati- of climate change simulations with a 60-km-mesh atmospheric tudinal coastal regions. Sea surface temperature (SST) is projected global circulation model (MRI-AGCM3.2H; Mizuta et al. 2012) to increase as a result of anthropogenic greenhouse warming, and a 20-km-mesh atmospheric regional model (NHRCM; Sasaki and the maximum intensity (the maximum wind speed or central et al. 2011). All TCs that made landfall in eastern Hokkaido in pressure) of future TCs will likely increase as well (e.g., IPCC northern Japan (142°E−146°E and 42°N−46°N) from the western 2012; Mizuta et al. 2014; Murakami et al. 2012), because the TC North Pacific Ocean with no previous landfalls were targeted. intensity generally increases as SST increases (e.g., DeMaria and Only eight TCs met those criteria according to the Regional Spe- Kaplan 1994; Emanuel 1986). cialized Meteorological Center Tokyo (RSMC) best-track dataset In the present climate, TCs that travel to higher latitudes tend from 1951 to 2018, but 98 and 125 storms were selected from the to weaken as SST decreases north of 30°N. However, Kossin et al. 3,000 years of current-climate and 5,400 years of 4-K warming- (2014) reported that the average latitude at which TCs reach their climate runs, respectively, in the d4PDF database. Tracks of the lifetime-maximum wind speed has been shifting poleward over targeted storms are shown in Fig. 1. the past 30 years. A number of future projection studies have im- Downscaling experiments for all targeted storms were con- plied that higher-latitude occurrences of intense TCs will increase ducted with a high-resolution non-hydrostatic regional model, the (e.g., Kanada et al. 2013; Tsuboki et al. 2015; Yoshida et al. 2017) Cloud Resolving Storm Simulator version 3.4 (CReSS; Tsuboki because the projected future increase in SST is larger at higher and Sakakibara 2002), which has a horizontal resolution of 0.04° latitudes (Mizuta et al. 2017). Furthermore, case studies of TCs (approximately 4 km). The computational domain of CReSS spans that caused record-breaking heavy rainfalls in eastern coastal 128°E−152°E and 24°N−48°N (Figs. 1a and 1b). SST cooling regions of northern Japan have shown that precipitation amounts associated with storm passage is considered by a simple thermal associated with a TC’s landfall increased in the warming-climate diffusion model. Initial and lateral boundary conditions were pro- simulations (Kanada et al. 2017a, 2019). These results suggest vided every 6 h from the NHRCM results. Detailed information that large numbers of people living in mid-to-high latitudes may on the models and methodology are given in Supplement 1. be exposed to unusually intense TCs and associated winds and Corresponding author: Sachie Kanada, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan. E-mail: [email protected]. ©The Author(s) 2020. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution 4.0 International (CC BY 4.0) license (http://creativecommons.org/license/by/4.0). 58 Kanada et al., Future Changes of Tropical Cyclones in the Midlatitudes Fig. 1. The downscaling experiment domain showing the tracks of the targeted TCs (thin black lines) in the (a) current- and (b) warming-climate simula- tions. The rectangles outlined in red show the regions used for composite analyses. Box-and-whisker plots of (c) mean sea surface temperature in the TC inner core (SST200: °C), (d) mean water vapor at an altitude of 10 m in the inner core (QVS200: g kg−1), (e) vertical wind shear (VWS: m s−1), and (f) baro- clinicity (BA: K). Outliers are excluded. Pink (cyan) shading indicates that the increase (decrease) in the mean value in the warming climate compared with the current climate is statistically significant at the 95% confidence level (Welch’s two-sided t-test). Boxes with bold outlines indicate that the difference in the mean value between analysis regions is statistically significant at the 95% confidence level elch’s(W two-sided t-test). with latitude increases (Figs. 1 and 2). In the warming-climate 3. Results simulations, mean SST in the inner core of the storms increased in all analysis regions, whereas baroclinicity and VWS decreased 3.1 Changes in midlatitude TCs with increasing latitude (Fig. 1). Mean central pressure decreased in the regions south of In the RSMC best-track dataset for 1951−2018, the lowest 40°N (Fig. 2) were statistically significant at the 95% confidence central pressure over the sea east of Japan between 30°N and level (Welch’s t-test). Mean precipitation amounts and wind 45°N was 925 hPa, during Typhoon Oscar (1995), and that in speeds in the inner core were enhanced in warming-climate the current simulation was 922 hPa. In the warming climate, the storms, but they tended to have a smaller RMW than current- lowest central pressure in the region dropped to 869 hPa. The climate storms (Fig. 2). mean central pressure of TCs in the analysis regions decreased Changes in TC structure were investigated by examining from 958 hPa to 948 hPa in the warming-climate simulation: 12% storm-centered composite structures of all storms whose centers of the warming-climate TCs were of an unusual central pressure were located in 142°E−147°E and 35°N−40°N in the current and lower than 925 hPa. The warming-climate TCs tended to travel warming climates (Fig. 3). Mean central pressures of the storms in northward at slower translation speeds than the current-climate this region were 963 hPa and 955 hPa in the current and warming TCs. Mean translation speed of the current- and warming-climate climates, respectively (Fig. 2a). Areas of high winds appeared TCs between 30°N and 45°N was 9.2 (8.8 in the RSMC TCs) on the right side of the storm center (‘A’ in Fig. 3), because the and 8.0 m s−1, respectively.