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Chi-Cherng Hong
,
Chih-Hua Tsou
,
Pang-Chi Hsu
,
Kuan-Chieh Chen
,
Hsin-Chien Liang
,
Huang-Hsiung Hsu
,
Chia-Ying Tu
, and
Akio Kitoh

Abstract

The future changes in tropical cyclone (TC) intensity and frequency over the western North Pacific (WNP) under global warming remain uncertain. In this study, we investigated such changes using 20-km resolution HiRAM and Meteorological Research Institute (MRI) models, which can realistically simulate the TC activity in the present climate. We found that the mean intensity of TCs in the future (2075–99) would increase by approximately 15%, along with an eastward shift of TC genesis location in response to the El Niño–like warming. However, the lifetime of future TCs would be shortened because the TCs tend to have more poleward genesis locations and move faster due to a stronger steering flow related to the strengthened WNP subtropical high in a warmer climate. In other words, the enhancement of TC intensity in the future is not attributable to the duration of TC lifetime. To understand the processes responsible for the change in TC intensity in a warmer climate, we applied the budget equation of synoptic-scale eddy kinetic energy along the TC tracks in model simulations. The diagnostic results suggested that both the upper-level baroclinic energy conversion (CE) and lower-level barotropic energy conversion (CK) contribute to the intensified TCs under global warming. The increased CE results from the enhancement of TC-related perturbations of temperature and vertical velocity over the subtropical WNP, whereas the increased CK mainly comes from synoptic-scale eddies interacting with enhanced zonal-wind convergence associated with seasonal-mean and intraseasonal flows over Southeast China and the northwestern sector of WNP.

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Matthew J. Widlansky
,
H. Annamalai
,
Stephen B. Gingerich
,
Curt D. Storlazzi
,
John J. Marra
,
Kevin I. Hodges
,
Barry Choy
, and
Akio Kitoh

Abstract

Potential changing climate threats in the tropical and subtropical North Pacific Ocean were assessed, using coupled ocean–atmosphere and atmosphere-only general circulation models, to explore their response to projected increasing greenhouse gas emissions. Tropical cyclone occurrence, described by frequency and intensity, near islands housing major U.S. defense installations was the primary focus. Four island regions—Guam and Kwajalein Atoll in the tropical northwestern Pacific, Okinawa in the subtropical northwestern Pacific, and Oahu in the tropical north-central Pacific—were considered, as they provide unique climate and geographical characteristics that either enhance or reduce the tropical cyclone risk. Guam experiences the most frequent and severe tropical cyclones, which often originate as weak systems close to the equator near Kwajalein and sometimes track far enough north to affect Okinawa, whereas intense storms are the least frequent around Oahu. From assessments of models that simulate well the tropical Pacific climate, it was determined that, with a projected warming climate, the number of tropical cyclones is likely to decrease for Guam and Kwajalein but remain about the same near Okinawa and Oahu; however, the maximum intensity of the strongest storms may increase in most regions. The likelihood of fewer but stronger storms will necessitate new localized assessments of the risk and vulnerabilities to tropical cyclones in the North Pacific.

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Ping Liu
,
Yoshiyuki Kajikawa
,
Bin Wang
,
Akio Kitoh
,
Tetsuzo Yasunari
,
Tim Li
,
H. Annamalai
,
Xiouhua Fu
,
Kazuyoshi Kikuchi
,
Ryo Mizuta
,
Kavirajan Rajendran
,
Duane E. Waliser
, and
Daehyun Kim

Abstract

This study documents the detailed characteristics of the tropical intraseasonal variability (TISV) in the MRI-20km60L AGCM that uses a variant of the Arakawa–Schubert cumulus parameterization. Mean states, power spectra, propagation features, leading EOF modes, horizontal and vertical structures, and seasonality associated with the TISV are analyzed. Results show that the model reproduces the mean states in winds realistically and in convection comparable to that of the observations. However, the simulated TISV is less realistic. It shows low amplitudes in convection and low-level winds in the 30–60-day band. Filtered anomalies have standing structures. Power spectra and lag correlation of the signals do not propagate dominantly either in the eastward direction during boreal winter or in the northward direction during boreal summer. A combined EOF (CEOF) analysis shows that winds and convection have a loose coupling that cannot sustain the simulated TISV as realistically as that observed. In the composited mature phase of the simulated MJO, the low-level convergence does not lead convection clearly so that the moisture anomalies do not tilt westward in the vertical, indicating that the low-level convergence does not favor the eastward propagation. The less realistic TISV suggests that the representation of cumulus convection needs to be improved in this model.

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Bin Wang
,
Michela Biasutti
,
Michael P. Byrne
,
Christopher Castro
,
Chih-Pei Chang
,
Kerry Cook
,
Rong Fu
,
Alice M. Grimm
,
Kyung-Ja Ha
,
Harry Hendon
,
Akio Kitoh
,
R. Krishnan
,
June-Yi Lee
,
Jianping Li
,
Jian Liu
,
Aurel Moise
,
Salvatore Pascale
,
M. K. Roxy
,
Anji Seth
,
Chung-Hsiung Sui
,
Andrew Turner
,
Song Yang
,
Kyung-Sook Yun
,
Lixia Zhang
, and
Tianjun Zhou

Abstract

Monsoon rainfall has profound economic and societal impacts for more than two-thirds of the global population. Here we provide a review on past monsoon changes and their primary drivers, the projected future changes, and key physical processes, and discuss challenges of the present and future modeling and outlooks. Continued global warming and urbanization over the past century has already caused a significant rise in the intensity and frequency of extreme rainfall events in all monsoon regions (high confidence). Observed changes in the mean monsoon rainfall vary by region with significant decadal variations. Northern Hemisphere land monsoon rainfall as a whole declined from 1950 to 1980 and rebounded after the 1980s, due to the competing influences of internal climate variability and radiative forcing from greenhouse gases and aerosol forcing (high confidence); however, it remains a challenge to quantify their relative contributions. The CMIP6 models simulate better global monsoon intensity and precipitation over CMIP5 models, but common biases and large intermodal spreads persist. Nevertheless, there is high confidence that the frequency and intensity of monsoon extreme rainfall events will increase, alongside an increasing risk of drought over some regions. Also, land monsoon rainfall will increase in South Asia and East Asia (high confidence) and northern Africa (medium confidence), decrease in North America, and be unchanged in the Southern Hemisphere. Over the Asian–Australian monsoon region, the rainfall variability is projected to increase on daily to decadal scales. The rainy season will likely be lengthened in the Northern Hemisphere due to late retreat (especially over East Asia), but shortened in the Southern Hemisphere due to delayed onset.

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Ryo Mizuta
,
Akihiko Murata
,
Masayoshi Ishii
,
Hideo Shiogama
,
Kenshi Hibino
,
Nobuhito Mori
,
Osamu Arakawa
,
Yukiko Imada
,
Kohei Yoshida
,
Toshinori Aoyagi
,
Hiroaki Kawase
,
Masato Mori
,
Yasuko Okada
,
Tomoya Shimura
,
Toshiharu Nagatomo
,
Mikiko Ikeda
,
Hirokazu Endo
,
Masaya Nosaka
,
Miki Arai
,
Chiharu Takahashi
,
Kenji Tanaka
,
Tetsuya Takemi
,
Yasuto Tachikawa
,
Khujanazarov Temur
,
Youichi Kamae
,
Masahiro Watanabe
,
Hidetaka Sasaki
,
Akio Kitoh
,
Izuru Takayabu
,
Eiichi Nakakita
, and
Masahide Kimoto

Abstract

An unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the assessment of probabilistic change in localized severe events that have large uncertainty from internal variability. The simulation outputs are open to the public as a database called “Database for Policy Decision Making for Future Climate Change” (d4PDF), which is intended to be utilized for impact assessment studies and adaptation planning for global warming.

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Toru Terao
,
Shinjiro Kanae
,
Hatsuki Fujinami
,
Someshwar Das
,
A. P. Dimri
,
Subashisa Dutta
,
Koji Fujita
,
Azusa Fukushima
,
Kyung-Ja Ha
,
Masafumi Hirose
,
Jinkyu Hong
,
Hideyuki Kamimera
,
Rijan Bhakta Kayastha
,
Masashi Kiguchi
,
Kazuyoshi Kikuchi
,
Hyun Mee Kim
,
Akio Kitoh
,
Hisayuki Kubota
,
Weiqiang Ma
,
Yaoming Ma
,
Milind Mujumdar
,
Masato I. Nodzu
,
Tomonori Sato
,
Z. Su
,
Shiori Sugimoto
,
Hiroshi G. Takahashi
,
Yuhei Takaya
,
Shuyu Wang
,
Kun Yang
,
Satoru Yokoi
,
Peter van Oevelen,
, and
Jun Matsumoto

Abstract

The Asian Precipitation Experiment (AsiaPEX) was initiated in 2019 to understand terrestrial precipitation over diverse hydroclimatological conditions for improved predictions, disaster reduction, and sustainable development across Asia under the framework of the Global Hydroclimatology Panel (GHP)/Global Energy and Water Exchanges (GEWEX). AsiaPEX is the successor to GEWEX Asian Monsoon Experiment (GAME; 1995–2005) and Monsoon Asian Hydro-Atmosphere Scientific Research and Prediction Initiative (MAHASRI; 2006–16). While retaining the key objectives of the aforementioned projects, the scientific targets of AsiaPEX focus on land–atmosphere coupling and improvements to the predictability of the Asian hydroclimatological system. AsiaPEX was designed for both fine-scale hydroclimatological processes occurring at the land surface and the integrated Asian hydroclimatological system characterized by multiscale interactions. We adopt six approaches including observation, process studies, scale interactions, high-resolution hydrological modeling, field campaigns, and climate projection, which bridge gaps in research activities conducted in different regions. Collaboration with mesoscale and global modeling researchers is one of the core methods in AsiaPEX. We review these strategies based on the literature and our initial outcomes. These include the estimation and validation of high-resolution satellite precipitation, investigations of extreme rainfall mechanisms, field campaigns over the Maritime Continent and Tibetan Plateau, areas of significant impact on the entire AsiaPEX region, process studies on diurnal- to interdecadal-scale interactions, and evaluation of the predictabilities of climate models for long-term variabilities. We will conduct integrated observational and modeling initiative, the Asian Monsoon Year (AMY)-II around 2025–28, whose strategies are the subregional observation platforms and integrated global analysis.

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