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DOI: 10.2151/sola.17B-002.
J-STAGE Advance published date: Aug. 4, 2021
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SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 1
1 Enhanced subtropical anticyclone over the Indo–Pacific Ocean
2 associated with stagnation of the Meiyu–Baiu rainband
3 during summer, 2020
4
5 Hiroaki Ueda1, Mikihiro Yokoi2, and Masaya Kuramochi2
6
7 1 Faculty of Life and Environmental Sciences
8 University of Tsukuba, Tsukuba, Japan
9 2 Graduate School of Science and Technology,
10 University of Tsukuba, Tsukuba, Japan
11
12
13 Submitted to SOLA
14 April 6, 2021
15 Revised June 10, 2021(R1)
16 Revised July 16, 2021(R2)
17
18
19
20 Corresponding author: Hiroaki Ueda, Life and Environmental Sciences,
21 University of Tsukuba, Tsukuba, Ibaraki 305–8572, Japan
22 Email: [email protected]
1 2 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 Abstract
2 During the early summer of 2020, a stagnation of the Meiyu-Baiu front brought torrential
3 rainfall over East Asia. Meanwhile, the anticyclone was much enhanced over the
4 subtropical western Pacific (SWP), which contributed abundant moisture to the Meiyu-
5 Baiu rainband along the western rim of the anticyclone. Based on the sensitivity
6 experiments of the linear baroclinic model by prescribing the observed diabatic heating
7 anomalies, a combination of anomalous convection over the Indian Ocean and reduced
8 rainfall over the western Pacific can account for the maintenance of zonally elongated
9 SWP anticyclone. Interestingly, this period corresponded to the developing stage of La
10 Niña, while the convective activities were notably suppressed over the warmed western
11 Pacific. The sensitivity experiments to SST anomalies using the atmospheric general
12 circulation model shows that the attenuated convection over the western Pacific can be
13 ascribed to the warmed Indian Ocean associated with an atmospheric Kelvin wave wedge
14 from the Indian Ocean. Overall, the suppressant SST effect of the Indian Ocean opposes
15 and is greater than that of tropical Pacific. We issue a caveat regarding the additivity of
16 the remote influence across the maritime continent. These results have important
17 implications for the predictability of early summer rainfall over East Asia.
18 SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 3
1 1. Introduction
2 During the early summer of 2020, a stagnation of the Meiyu-Baiu front was closely
3 associated with catastrophic floods that occurred mainly in the area of Kyushu Island
4 (Hirockawa et al. 2020; Araki et al. 2021) in Japan and central China, particularly in the
5 Yangtze–Huai River Valley; this resulted in record-breaking heavy rainfall in early July.
6 As is shown in Fig. 1a, the seasonal evolution of precipitation averaged over Japan shows
7 continued wet conditions from late June to the end of July 2020. The withdrawal of Baiu
8 season in Japan was delayed by approximately 10 days in comparison with the
9 climatology (Japan Meteorological Agency [JMA] 2020). The monthly rainfall amount
10 in July averaged over Japan was the highest value since 1946, exceeding the long-term
11 mean by a factor of 2.14. The Meiyu rainband (Fig. 1b) and Baiu front rainfall fluctuated
12 to a similar extent, suggesting a common factor for the stagnation of these rainfall fronts.
13 Climatologically, the Meiyu-Baiu front is maintained through moisture transport by
14 the southwesterly winds along the western rim of the subtropical anticyclone over the
15 western North Pacific to the northeast of the Philippines (Ueda and Yasunari 1996; Sampe
16 and Xie 2010). This subtropical anticyclone becomes robust and is associated with severe
17 floods in the Yangtze–Huai River Valley, during the summer following the preceding El
18 Niño (Huang et al. 2004). The subtropical western Pacific (SWP) anticyclone was
19 substantially enhanced during the early summer of 2020 (JMA 2020).
20 Two schools of thought exist regarding the maintenance and fluctuation of this
21 anticyclone. The El Niño has a phase-locked feature to the annual cycle that usually peaks
22 in the boreal winter; this feature is responsible for the anomalous SWP anticyclone
23 anchored with the underlying colder sea surface temperature (SST). It has been widely
24 recognized that the SWP anticyclone persists into the summer following the dissipation
3 4 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 of the colder SST, requiring different explanations. Wang et al. (2000) proposed that the
2 superimposition of an anomalous anticyclone that is induced by negative SST anomalies
3 in the prevailing spring northeast trades partially explains the maintenance of the SWP
4 anticyclone through evaporative cooling over the ocean surface. In addition to this
5 mechanism, delayed warming in the Indian Ocean after the mature phase of El Niño
6 reinforces the SWP anticyclone through atmospheric Kevin wave adjustment (Annamalai
7 et al. 2005). In this context, Xie et al. (2009) discussed the suppressed convection over
8 the SWP in which circulation-convection feedback involved in the ITCZ preferentially
9 amplifies the intensity of anomalous anticyclone to the east of the Philippines. These
10 trans-basin interactions are collectively called the “Indo–western Pacific Ocean Capacitor
11 (IPOC) effect” (Xie et al. 2016).
12 Recently, Takaya et al. (2020) showed that the remote impact of the warmed Indian
13 Ocean can account for the prolonged Meiyu-Baiu rainfall in the early summer 2020.
14 Interestingly, this warmer SST emerged without the preceding El Niño event, rather
15 corresponds to the transition phase to the La Niña condition (World Meteorological
16 Organization [WMO] 2020). Thus, the present study investigates the environmental
17 forcing on the continued SWP anticyclone with focus on the combined effect of warmer
18 SSTs in the tropical Indian–Pacific Ocean and the resultant teleconnections caused by
19 heating anomalies from the trans-basin interaction perspective.
20
21 2. Data and methods
22 For the diagnosis of observed climate conditions in comparison with the climatology,
23 we used the two–month (JJ) average of precipitation data provided by the JMA for SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 5
1 January 1977–July 2020. This includes 63 meteorological observation stations on the
2 main islands of Japan, and the regions of Kyushu, and Shikoku. Reference data for the
3 monthly global precipitation was taken from the Climate Prediction Center (CPC)
4 Merged Analysis of Prediction (CMAP) for the period between 1979–2020 (Xie and
5 Arkin 1997). The regional mean (28°–32° N, 110°–120° E) of daily rainfall that was used
6 for the analysis of the Meiyu front was obtained from the Global Precipitation
7 Climatology Project (GPCP) dataset for the period between 1997–2020 (Adler et
8 al. 2018). We also utilized the atmospheric analysis of the Japanese 55–year Reanalysis
9 dataset (JRA–55; Kobayashi et al. 2015) and SST analysis of Centennial in situ
10 Observation–Based Estimate of SST (COBE-SST) (Ishii et al. 2005) for the period of
11 1979–2020.
12 A linear baroclinic model (LBM) was used to examines the heat-induced atmospheric
13 response. This was established using linearized equations for a basic state. The model has
14 20 sigma levels with a horizontal resolution of T42 and employs Del-forth horizontal
15 diffusion, Raleigh friction, and Newtonian thermal damping. The latter has an e-folding
16 scale of 0.5 day in the lower boundary layer and 1 day in the two uppermost levels, and
17 at 20 days elsewhere. A detailed description is found in Watanabe and Kimoto (2000).
18 The imposed diabatic heat source Q1 is computed as the residual of the thermodynamic
19 equations (Yanai et al. 1973).
20 = + + (1) 𝜕𝜕𝜕𝜕 𝑅𝑅𝑅𝑅 𝜕𝜕𝜕𝜕 𝑄𝑄1 −𝒗𝒗 ∙ ∇𝑇𝑇 𝜔𝜔 � 𝑝𝑝 − � 𝑝𝑝 21 where T is the temperature,𝜕𝜕𝜕𝜕 v is the horizontal𝑐𝑐 𝑃𝑃 wind,𝜕𝜕𝜕𝜕 ω is the𝑐𝑐 vertical p–velocity, cp is the
22 specific heat for the dry air. Q1 is the apparent heat source. As shown by Yanai et al.
5 6 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 (1973), vertically integrating (2) from the tropopause pressure PT to the surface pressure
2 Ps, we obtain
Q1 = QR + LcP + S, (2)
1 Ps = ( ) dp, (3) g ∫ pT 3
4 where Lc, P, S, and g are the latent heat of condensation, precipitation rate, the sensible
5 heat flux, radiative heating rate, and the acceleration of gravity, respectively. In the
6 present study, we computed
7 p–velocity obtained from the JRA–55.
8 We also conducted sensitivity experiments to SST anomalies using ensemble model
9 simulations based on an atmospheric general circulation model (AGCM). This model is
10 identical to the atmospheric part of the global ocean-atmosphere-coupled GCM
11 developed at the Meteorological Research Institute. Readers are referred to Yukimoto et
12 al. (2006) for details. The resolution of the AGCM is T42 spectral truncation (horizontal
13 spacing of ∼300 km) and 30 vertical layers. The model was forced with daily SST
14 interpolated from the monthly SST. The boundary conditions in the control experiment
15 were obtained from the climatological COBE-SST. Except for June and July, the
16 climatological SST was prescribed throughout the year. To account for the atmospheric
17 sensitivity to the initial condition, a 10-member approach was adapted for each forcing
18 scenario. We made changes only in the initial conditions selected from 10 snapshots of
19 the climatological control run preserving the same SST forcing. In this study, we
20 conducted a series of AGCM experiments, in which the global (full experiment) and
21 regional SST anomalies (the Indian Ocean experiment, and the tropical Pacific Ocean SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 7
1 experiment) during June–July 2020 were prescribed to isolate their influence on the
2 circulation fields during boreal summer.
3
4 3. Observed climate anomalies
5 The spatial distribution of rainfall anomalies relative to the climatology during the early
6 (June to July) summer of 2020 showed enhanced precipitation within the Meiyu-Baiu
7 front (Fig. 2a). In contrast, the convective activities were markedly suppressed over the
8 western Pacific, extending from the northern South China Sea to the Bay of Bengal.
9 Another noteworthy feature in the Indo–Pacific domain was the anomalous positive
10 rainfall emerging over the entire Indian Ocean Basin, especially in the western Indian
11 Ocean. Overall, the spatial distributions of these rainfall anomalies were consistent with
12 the vertically integrated apparent heat source < Q1 > between the surface and 1 hPa, which
13 was used for the LBM sensitivity experiments. As for the SST anomalies (Fig. 2b), a La
14 Niña–like pattern was recognizable in the tropical Pacific Ocean. Another important
15 feature of the tropical oceans was the presence of warm SST anomalies in the western
16 Indian Ocean. When compared with the rainfall distribution, the enhanced precipitation
17 in the western Indian Ocean was concurrent with the underlying positive SST anomalies.
18 The convective activity was much suppressed across the western Pacific despite the rise
19 in local SST, which will be discussed later in the section dealing with numerical
20 experiments. These features were recognizable in the circulation field. The presence of a
21 robust SWP anticyclone (Fig. 2c) to the south of Japan resulted in the transport of
22 abundant moisture along the western periphery of the anticyclone. The vertically
23 integrated moisture flux and its convergence (Fig. 2d) show that the anomalous moisture
24 convergence was concurrent with the maintenance of the rainband over East Asia.
7 8 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 Overall, the seasonal evolution of SST in the Indian and Pacific Oceans over the years
2 2018–2020 (Fig. 3) illustrates that the early summer 2020 corresponds to the transition
3 phase from the peak El Niño in November 2018 to the La Niña conditions. It should be
4 noted here that a slight negative SST anomaly temporarily emerged from summer to fall
5 2019 in the eastern tropical Pacific; this was lower than the threshold for La Niña
6 conditions (-0.5 °C). The basin–wide warming in the Indian Ocean during 2019 was
7 statistically consistent with the preceding El Niño (Xie et al. 2009). The strong Indian
8 Ocean dipole (IOD) event occurred in the northern fall of 2019 (Lu and Ren 2020) and
9 then abruptly diminished in the following winter, which could have contributed to the
10 continued warming in the Indian basin until summer 2020. Further investigation of the
11 spatial distribution of SST anomalies in the Indian Ocean (Fig. 2b) showed that a positive
12 IOD index, emerged in June 2020, is reflection of the prominent warming in the western
13 Indian Ocean. Xie et al. (2002) found that the SST rise in the western Indian Ocean is
14 often caused by an oceanic downwelling Rossby wave across the South Indian Ocean;
15 this phenomenon was also observed in 2020 (Takaya et al. 2020). These results indicate
16 that the Meiyu-Baiu front in the early summer 2020 could have been influenced by both
17 salient warming in the Indian Ocean, as well as the events from the developing phase of
18 La Niña in the tropical Pacific Ocean. This study thus incorporated the evaluation using
19 numerical experiments of the impact of these SST anomalies in the Indian Ocean and
20 western Pacific.
21 SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 9
1 4. Numerical experiments
2 4.1 Response to the heat source using LBM
3 This subsection examines the influence of unique patterns of the observed heating
4 anomalies (Fig. 2a) on the global circulation change during the early summer of 2020,
5 with particular focus on the anomalous SWP anticyclone extending westward to the
6 northern part of India (Fig. 2c). This zonally elongated anticyclone appears to be closely
7 tied to the stagnation of the Meiyu-Baiu rainband through the abundant northeastward
8 moisture transport around the western rim of the anticyclone (Kamae et al. 2017; Kosaka
9 et al. 2013). Here, we considered solutions to the LBM, for which the background state
10 is prescribed. The dry version of the LBM is forced by the vertical distribution of the
11 heating profile computed from Eq. (1). In this study, we imposed two types of anomalous
12 heat sources over regions of positive Q1 and the negative Q1 for the Indo–Pacific domain
13 as a whole (20° S–20° N, 30° E–180°). The LBM is integrated for 15 days, attains a
14 steady-state in approximately 7 days, and the response on day 15 is examined.
15 Fig. 4 shows the steady–state response of the circulation fields at 850 hPa. As for the
16 positive < Q1 > experiment (Fig. 4a), anomalous easterly winds are prominent in the
17 tropical Pacific toward the northern Indian Ocean, which can be interpreted as a
18 manifestation of the Kelvin wave response to the anomalous heating over the Indian
19 Ocean. Another important feature of this figure is presence of the anomalous SWP
20 anticyclone, which is consistent with the observational evidence. In contrast, in the
21 negative < Q1 > experiment (Fig. 4b), the anomalous anticyclone is recognizably
22 extending from the SWP close to the Philippines toward the Arabian Sea. These results
23 indicate that the zonally elongated SWP anticyclone is strengthened by a combination of
9 10 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 anomalous heating over the Indian Ocean and negative condensational heating anomalies
2 in the western Pacific Ocean.
3
4 4.2 Sensitivity experiments to the SST anomalies using AGCM
5 To evaluate the impact of SST anomalies on the circulation fields, we conducted
6 sensitivity experiments by prescribing SST anomalies in the Indian Ocean (IO
7 experiment) and western Pacific close to the dateline (TPO experiment). The detailed
8 domains are denoted by a solid line in Fig. 2b. Fig. 5 shows the simulated sea level
9 pressure (SLP) and low-level wind. In response to the positive SST anomalies in the
10 tropical western Indian Ocean (Fig. 5a), an anomalous cyclonic circulation is generated
11 on either side of the equator especially in the western Indian Ocean. In contrast, the SWP
12 anticyclone is greatly enhanced to the northeast of the Philippines. Our results are
13 consistent with those of previous studies (e.g., Annamalai et al. 2005; Xie et al. 2009),
14 suggesting that the Kelvin wave-induced Ekman divergence gives rise to the localized
15 enhancement of the anticyclone. In the TPO experiment (Fig. 5b), an anomalous cyclone
16 emerges over the SWP centered around 25° N, 155° E, which is the opposite situation to
17 the observation. In contrast, the broad Indian Ocean is characterized by increase of the
18 SLP.
19 In Fig. 6, we also show the simulated precipitation and vertically integrated moisture
20 flux. In the full experiment (Fig. 6a), rainfall increases around Japan, especially in the
21 western part. The spatial pattern of the full experiment bears considerable resemblance to
22 that of the IO experiment (Fig. 6b). However, the amplitude of suppressed convection SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 11
1 over the western Pacific is smaller than that of the IO experiments, which could be
2 ascribed to the positive SST anomalies in the western Pacific (Fig. 2b). The influence of
3 the warmed western Pacific on the convective activity over the Indo–Pacific domain (Fig.
4 6c) shows suppressed rainfall emerging across the equator in the Indian Ocean. In general,
5 enhanced tropical convection gives rise to subsidence over the rest of the tropics. That is,
6 the attenuated convection over the Indian Ocean (Fig. 6c) is indicative of the remote
7 impact of the enhanced rainfall in the tropical western Pacific caused by the underlying
8 warm SST anomalies. Xie et al. (2009) indicated that the enhanced rainfall over the Indian
9 Ocean excites a Kelvin-wave wedge penetrating into the western Pacific, contributing to
10 the intensification of the SWP anticyclone. Along this line, it is conceivable that once the
11 convective activities over the Indian Ocean become weak, the above atmospheric Kelvin-
12 wave adjustment will also be obscure. This in turn reduces the intensity of SWP
13 anticyclone through the weakening of the remote suppressant effect of the Indian Ocean.
14 These points of view highlight the need for a careful discussion involved in the inter-
15 basin interactions. During the recent slowdown of the global warming (1999–2013),
16 anomalous warming occurred both in the tropical Indian Ocean and western Pacific,
17 which had a trans-basin offsetting effect on the convective activities across the maritime
18 continent (Ueda et al. 2015). In other words, the anomalous SWP anticyclone cannot
19 simply be understood by the additivity of the response to both the anomalous heating in
20 the Indian Ocean and suppressed rainfall in the tropical western Pacific.
21
11 12 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 5. Summary and discussion
2 The physical mechanisms for the stagnation of the Meiyu-Baiu rainband associated
3 with the torrential rainfall over East Asia during the early summer of 2020 were
4 investigated in connection with the Indo–Pacific SST anomalies and resultant
5 teleconnections caused by regional anomalies in convective heating. This period was
6 characterized by the transition phase to La Niña conditions and the continued warming of
7 the Indian Ocean. Analyses of the circulation field indicate that the anomalous subtropical
8 anticyclone extending over the SWP in the northern Indian Ocean is an important factor
9 for the transport of moisture toward the Meiyu-Baiu rainband.
10 Based on the findings of the LBM sensitivity experiments, it can be concluded that the
11 anomalous SWP anticyclone is strengthened by a combination of heating anomalies over
12 the warmed Indian Ocean and negative condensational heating over the SWP. The warm
13 Kelvin wave wedge that propagated from the Indian Ocean toward the SWP strengthened
14 the SWP anticyclone south of the Baiu rainband. On the other hand, the negative heating
15 anomalies associated with the suppressed convection over the SWP generated an
16 anomalous anticyclone to the southwestward of the Meiyu front resulting from Matsuno–
17 Gill dynamics (Matsuno 1966; Gill 1980).
18 The significantly suppressed convection over the SWP (despite the underlying warmer
19 SST) could be attributed to the vigorous suppressant effect relevant to the IPOC
20 mechanism. Indeed, the AGCM experiment reproduces the anomalous cyclone by
21 prescribing the warm SST anomalies in the TPO. These climate anomalies resemble those
22 observed in the summer of 1988, which was characterized by a wetter climate over Japan,
23 despite the strong La Niña signal in the SWP. The warmer SST in the Indian Ocean (Nitta
24 1990) together with colder SST in the tropical eastern Pacific (Shen et al. 2001) SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 13
1 suppressed the SWP convection by changing the east-west Walker circulation with a
2 westward (toward the Indian Ocean) shift of the convection center.
3 Although we tried to understand the prolonged Meiyu-Baiu rainband from the
4 perspective of teleconnections originating from the tropics, there may be other
5 explanations. For instance, cold air intrusion from higher latitudes can be linked to
6 fluctuations in the Okhotsk High, or meso-synoptic-large-scale interactions. Significant
7 challenges in understanding these phenomena therefore still face the climate research
8 community of East Asia.
9
10 Acknowledgements
11 This work was supported by the Industry-University Collaboration Project (HJH02035)
12 of University of Tsukuba.
13
14
13 14 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 References
2 Annamalai, H., P. Liu, and S.–P. Xie, 2005: Southwest Indian Ocean SST variability: Its
3 local effect and remote influence on Asian monsoons. J. Climate, 18, 4150–4167.
4 Adler, R., et al., 2018: The Global Precipitation Climatology Project (GPCP) monthly
5 analysis (new version 2.3) and a review of 2017 global precipitation. Atmosphere,
6 9, 138.
7 Araki, K., T. Kato, Y. Hirockawa, and W. Mashiko, 2021: Characteristics of atmospheric
8 environments of quasi–stationary convective bands in Kyushu, Japan during the
9 July 2020 heavy rainfall event. SOLA, 17, 8–15.
10 Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J.
11 Roy. Meteor. Soc., 106, 447–462.
12 Hirockawa, Y., T. Kato, K. Araki, and W. Mashiko, 2020: Characteristics of an extreme
13 rainfall event in Kyushu district, southwestern Japan in early July 2020. SOLA, 16,
14 265–270.
15 Huang, R. H., W. Chen, B. L. Yang, and R. H. Zhang, 2004: Recent advances in studies
16 of the interaction between the East Asian winter and summer monsoons and
17 ENSO cycle. Adv. Atmos. Sci., 21, 407–424.
18 JMA, 2020: Climate characteristics of record–heavy rain and record–low sunshine
19 durations in Japan in July 2020 (available online at SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 15
1 http://ds.data.jma.go.jp/tcc/tcc/news/press_20200916.pdf, accessed 31 March
2 2021.
3 Ishii, M., A. Shouji, S. Sugimoto, and T. Matsumoto, 2005: Objective analyses of sea–
4 surface temperature and marine meteorological variables for the 20th century
5 using ICOADS and the Kobe Collection. Int. J. Climatol., 25, 865–879.
6 Kamae, Y., W. Mei, S.–P. Xie, M. Naoi, and H. Ueda, H, 2017: Atmospheric rivers over
7 the northwestern Pacific: Climatology and interannual variability. J. Climate, 30,
8 5605–5619. https://doi.org/10.1175/JCLI–D–16–0875.1
9 Kobayashi, S., Y. Ota, Y. Harada, A. Ebita, M. Moriya, H. Onda, K. Onogi, H. Kamahori,
10 C. Kobayashi, H. Endo, K. Miyaoka, and K. Takahashi, 2015: The JRA–55
11 Reanalysis: General specifications and basic characteristics. J. Meteor. Soc. Japan,
12 93, 5–48.
13 Kosaka, Y., S.–P. Xie, N.–C. Lau, and G. A. Vecchi, 2013: Origin of seasonal
14 predictability for summer climate over the Northwestern Pacific. Proceedings of
15 the National Academy of Sciences, 110, 7574–7579.
16 Lu, B. and H.‐L. Ren, 2020: What caused the extreme Indian Ocean Dipole event in 2019?,
17 Geophys. Res. Lett., 47, e2020GL087768.
18 Matsuno, T., 1966: Quasi-geostrophic motions in the equatorial area. J. Meteor. Soc.
19 Japan, 44, 25–43.
15 16 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 Nitta, T., 1990: Unusual Summer Weather over Japan in 1988. J. Meteor. Soc. Japan, 68,
2 575–588.
3 Sampe, T., and S.–P. Xie, 2010: Large–scale dynamics of the Meiyu-Baiu rain band:
4 Environmental forcing by the westerly jet. J. Climate, 23, 113–134.
5 Shen, X., M. Kimoto, A. Sumi, A. Numaguti, and J. Matsumoto, 2001: Simulation of the
6 1998 East Asian summer monsoon by the CCSR/NIES AGCM. J. Meteor. Soc.
7 Japan, 79, 741–757.
8 Takaya, Y., I. Ishikawa, C. Kobayashi, H. Endo, and T. Ose, 2020: Enhanced Meiyu-Baiu
9 rainfall in early summer 2020: Aftermath of the 2019 super IOD event. Geophys.
10 Res. Lett., 47, e2020GL090671.
11 Yukimoto, S., A. Noda, A. Kitoh, M. Hosaka, H. Yoshimura, T. Uchiyama, K. Shibata,
12 O. Arakawa, and S. Kusunoki, 2006: Present–day climate and climate sensitivity
13 in the Meteorological Research Institute Coupled GCM version 2.3 (MRI–
14 CGCM2.3). J. Meteor. Soc. Japan, 84, 333–363.
15 Ueda, H., and T. Yasunari 1996: Maturing process of summer monsoon over the western
16 North Pacific – A coupled Ocean/Atmosphere system. J. Meteor. Soc. Japan, 74,
17 493–508.
18 Ueda, H., Y. Kamae, M. Hayasaki, A. Kitoh, S. Watanabe, Y. Miki, and A. Kumai, 2015:
19 Combined effects of recent Pacific cooling and Indian Ocean warming on the
20 Asian monsoon. Nature Communications, 6, 8854, doi:10.1038/ncomms9854. SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 17
1 Wang, B., R. Wu, and X. Fu, 2000: Pacific–East Asia teleconnection: How does ENSO
2 affect East Asian climate? J. Climate, 13, 1517–1536.
3 Watanabe, M. and M. Kimoto, 2000: Atmosphere–ocean thermal coupling in the North
4 Atlantic: a positive feedback. Quart. J. Roy. Meteor. Soc., 126, 3343–3369.
5 World Meteorological Organization, 2020: WMO El Niño/La Niña update 28 May 2020
6 (Available at https://public.wmo.int/en/media/news/el–niño–la–niña–update–0,
7 accessed 10 August 2020)
8 Xie, P., and P. A. Arkin, 1997: Global precipitation: A 17–year monthly analysis based
9 on gauge observations, satellite estimates and numerical model outputs. Bull.
10 Amer. Meteor. Soc., 78, 2539–2558.
11 Xie, S.-P., H. Annamalai, F. A. Schott, and J. P. McCreary Jr., 2002: Structure and
12 mechanisms of South IO climate variability. J. Climate, 15, 864–878.
13 Xie, S.–P., K. M. Hu, J. Hafner, H. Tokinaga, Y. Du, G. Huang, and T. Sampe, 2009:
14 Indian Ocean capacitor effect on Indo– western Pacific climate during the summer
15 following El Niño. J. Climate, 22, 730–747.
16 Xie, S.–P., Y. Kosaka, Y. Du, K. Hu, J. S. Chowdary, and G. Huang, 2016: Indo–Western
17 Pacific Ocean capacitor and coherent climate anomalies in post–ENSO summer:
18 A Review. Adv. Atmos. Sci., 33, 411–432.
17 18 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1 Yanai, M., S. Esbensen, and J.–H. Chu, 1973: Determination of bulk properties of tropical
2 cloud clusters from large–scale heat and moisture budgets. J. Atmos. Sci., 30, 611–
3 627.
4 SOLA, 2021, Vol. 17B, 14-17(TBA), doi:10.2151/sola.17B-002 19
1 List of Figure Captions
2 Fig. 1. Seasonal evolution of precipitation averaged over Japan based on the
3 meteorological observation station (upper panel) and the Yangtze–Huai River
4 Basins (28°–32° N, 110°–120° E) derived from Global Precipitation Climatology
5 Project (GPCP) daily archive (lower panel) between 1 May and 31 August 2020
6 (unit; mm day–1). Gray bars show the climatological mean.
7
8 Fig. 2. Enhanced Meiyu-Baiu front and anomalous climate conditions in the tropics
9 during early summer 2020 (1 June–31 July). (a) Anomalies in precipitation (shading
10 shown in the color bar) and vertically integrated diabatic heating < Q1 > (contoured
11 for every 50 W m–2). (b) Anomalies in SST (shading using contours). (c) Sea level
12 pressure (hPa) and horizontal wind at 1000 hPa. The horizontal winds less than 0.5
13 ms-1 are masked. (d) Vertically integrated water vapor flux between 1000 hPa and
14 200 hPa (kg m-1 s-1) (Shading denotes the flux divergence (mm day-1). The moisture
15 flux less than 20 (kg m-1 s-1) are masked. Note that the red box depicts in (a) the area
16 of Meiyu rainband (28°–32° N, 110°–120° E) shown in Fig. 1b; and in (b) the
17 prescribed anomalies of SST for the AGCM experiment.
18
19 Fig. 3. Time series of SST indices in the tropical Indo–Pacific domain from January
20 2018 to September 2020 (unit; K). The indices are defined as SST anomalies
21 averaged as follows: in Nino3 as 10° S–10° N and 150° W–90° W; in IOBW as 20°
22 S–20° N, 40° E–100° E; and in IOD as 5° S–5° N, 50° E–70° E minus 10° S–Equator,
23 90° E–110° E).
19 20 Ueda et al., Robust subtropical anticyclone and Meiyu-Baiu front during summer 2020
1
2 Fig. 4. Sensitivity experiment for the linear baroclinic model (LBM) for anomalous
3 heating field during early summer 2020. Contours denote the streamfunction at 850
4 hPa (1.0×106 m2s-1) and vectors are horizontal winds at 850 hPa (m s-1). The
5 atmospheric response to (a) positive (> 50 Wm-2), and (b) negative (< -50 W m-2)
6 diabatic heating anomalies in the tropical Indo–Pacific domain (20° S–20° N, 30°
7 E–180°) is shown. The prescribed heating fields are depicted using shading. The
8 horizontal winds less than 0.1 ms-1 are masked.
9
10 Fig. 5. Sensitivity experiment of MRI–AGCM for Indo–Pacific SST anomalies during
11 early summer 2020. Contours and shading denote sea level pressure anomalies from
12 the control experiment. Horizontal wind anomalies at 1000 hPa are superimposed.
13 The effect of (a) the tropical Indian Ocean (IO experiment) and (b) the tropical
14 Pacific Ocean (TPO experiment) is shown, respectively (also see section 2 and Fig.
15 2b). The horizontal winds less than 0.5 ms-1 are masked.
16
17 Fig. 6. Sensitivity experiment of MRI-AGCM for anomalous Indo-Pacific SST during
18 early summer in 2020. The effects of (a) the global ocean (full experiment), (b) the
19 tropical Indian Ocean (IO experiment) and (c) the tropical Pacific Ocean (TPO
20 experiment), respectively, are shown. Shadings denote precipitation anomalies from
21 the control experiment. Vectors show anomalies in vertically integrated water vapor
22 flux between 1000 hPa and 200 hPa (kg m-1 s-1) for the control experiment. The
23 moisture flux less than 10 (kg m-1 s-1) are masked. a) Japan (baiu) 35 baiu 2020 30 baiu Clim 25
20
15
10 Precip(mm/day) 5
0 01 May 01 Jun 01 Jul 01 Aug
b) China (meiyu) 30 meiyu 2020 25 meiyu Clim
20
15
10 Precip(mm/day) 5
000 01 May 01 Jun 01 Jul 01 Aug
Figure 1. Seasonal evolution of precipitation averaged over Japan based on the meteorological observation station (upper panel) and the Yangtze–Huai River Basins (28°–32° N, 110°–120° E) derived from Global Precipitation Climatology Project (GPCP) daily archive (lower panel) between 1 May and 31 August 2020 (unit; mm day–1). Gray bars show the climatological mean. (a) Precip., anomay June and July 2020 1
[mm/day] (b) SST anomaly
TPO
IO
[℃] (c) SLP, UV(1000hPa)
[hPa]
(d) Moisture flux and divergence
[mm/day]
Figure 2. Enhanced Meiyu-Baiu front and anomalous climate conditions in the tropics during early summer 2020 (1 June–31 July). (a) Anomalies in precipitation (shading shown in the color bar) and vertically integrated diabatic heating < Q1 > (contoured for every 50 W m–2). (b) Anomalies in SST (shading using contours). (c) Sea level pressure (hPa) and horizontal wind at 1000 hPa. The horizontal winds less than 0.5 ms-1 are masked. (d) Vertically integrated water vapor flux between 1000 hPa and 200 hPa (kg m-1 s-1) (Shading denotes the flux divergence (mm day-1). The moisture flux less than 20 (kg m-1 s-1) are masked. Note that the red box depicts in (a) the area of Meiyu rainband (28°–32° N, 110°–120° E) shown in Fig. 1b; and in (b) the prescribed anomalies of SST for the AGCM experiment. SST anomaly 1.5 2.5 Nino3 1.0 IOBW 2.0 IOD 0.5 1.5 1.0 0 0.5 -0.5
0 IOD index (K) Nino3, IOBW (K) -1.0 -0.5
-1.5 J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S -1.0 2018 2019 2020
Figure 3. Time series of SST indices in the tropical Indo–Pacific domain from January 2018 to September 2020 (unit; K). The indices are defined as SST anomalies averaged as follows: in Nino3 as 10° S–10° N and 150° W–90° W; in IOBW as 20° S–20° N, 40° E–100° E; and in IOD as 5° S–5° N, 50° E–70° E minus 10° S–Equator, 90° E–110° E). a) Positive LBM Exp. 1
[W m -2 ]
b) Negative LBM Exp. 1
Figure 4. Sensitivity experiment for the linear baroclinic model (LBM) for anomalous heating field during early summer 2020. Contours denote the streamfunction at 850 hPa (1.0×106 m2s-1) and vectors are horizontal winds at 850 hPa (m s-1). The atmospheric response to (a) positive (> 50 Wm-2), and (b) negative (< -50 W m-2) diabatic heating anomalies in the tropical Indo–Pacific domain (20° S–20° N, 30° E–180°) is shown. The prescribed heating fields are depicted using shading. The horizontal winds less than 0.1 ms-1 are masked. a) IO SLP AGCM Exp. June and July 2020
AC
b) TPO SLP
C
[hPa]
Figure 5. Sensitivity experiment of MRI–AGCM for Indo–Pacific SST anomalies during early summer 2020. Contours and shading denote sea level pressure anomalies from the control experiment. Horizontal wind anomalies at 1000 hPa are superimposed. The effect of (a) the tropical Indian Ocean (IO experiment) and (b) the tropical Pacific Ocean (TPO experiment) is shown, respectively (also see section 2 and Fig. 2b). The horizontal winds less than 0.5 ms-1 are masked. a) Full AGCM Exp. Precip., Moisture Flux and divergence June and July, 2020
b) IO AGCM Exp.
C
c) TPO AGCM Exp.
[ mm day -1 ]
Figure 6. Sensitivity experiment of MRI-AGCM for anomalous Indo-Pacific SST during early summer in 2020. The effects of (a) the global ocean (full experiment), (b) the tropical Indian Ocean (IO experiment) and (c) the tropical Pacific Ocean (TPO experiment), respectively, are shown. Shadings denote precipitation anomalies from the control experiment. Vectors show anomalies in vertically integrated water vapor flux between 1000 hPa and 200 hPa (kg m-1 s-1) for the control experiment. The moisture flux less than 10 (kg m-1 s-1) are masked.