Kuramochi et al.
Progress in Earth and Planetary Science(2023) 10:69
https://doi.org/10.1186/s40645-023-00598-4

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Coherent amplification of the Okhotsk
high, Korean trough, and northwestern
Pacific subtropical high during heavy rainfall
over Japan in August 2021
Masaya Kuramochi1* , Hiroaki Ueda2, Tomoshige  Inoue2, Meiji Honda3 and Koutarou Takaya4

Abstract
In August 2021, rain front stagnation in Japan resulted in prolonged and disastrous rainfall across the entire country.
During the heavy rainfall period, the large-scale atmospheric field over the East Asian–western North Pacific region
was characterized by meridional tripolar circulation anomalies: the Okhotsk high (OH), the trough over the Korean
Peninsula (Korean trough), and the northwestern Pacific subtropical high (NWPSH). Simultaneously, tropical convective activity was enhanced over the eastern Indian Ocean and suppressed over the tropical western–central Pacific.
This study investigates the dynamic mechanism of linkage of the extratropical tripolar anomalies and the effectsof tropical convective modulation using a reanalysis dataset, a cutoff low detection scheme, the potential vorticity inversion method, and numerical experiments. Upper-tropospheric blocking over eastern Siberia connectedto the surface OH is conducive to the stagnation of synoptic depressions, including cutoff lows and troughs,
over the Korean Peninsula, contributing to the development and maintenance of the quasi-stationary Korean
trough. Rossby waves emanating from the Korean trough excite an anticyclonic anomaly over the northwestern
Pacific. This upper-level anomalous anticyclone acts to enhance the surface NWPSH through zonal heat transport,
accompanied by a northward tilting structure with height. Simultaneously, the tropical intraseasonal oscillationis amplified over the Indo–western Pacific Ocean sector under the negative-phase Indian Ocean dipole and multiyear La Niña conditions. The combination of enhanced convection over the eastern Indian Ocean and suppressed
convection across the tropical western–central Pacific reinforces the NWPSH. The anomalous circulation associatedwith the extratropical tripolar pattern and concurrent tropical heat forcing causes more moisture transport, convergence, and anomalous ascent, which contribute to heavy rainfall in Japan. These results suggest that the dynamicallycorrelated amplification of tropical and extratropical circulation anomalies plays a crucial role in precipitation variability in East Asia.
Keywords Heavy rainfall, Climate variability, East Asian summer monsoon, Rossby wave propagation, Intraseasonal
oscillation, Northwestern Pacific subtropical high, Korean trough, Okhotsk high, Cutoff low, Tropical–extratropicalinteraction*Correspondence:
Masaya Kuramochi
kuramochi.masaya.sp@alumni.tsukuba.ac.jp
Full list of author information is available at the end of the article© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
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Kuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

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1 Introduction
During the late boreal summer of 2021, the Japaneseislands recorded extremely heavy rainfall associated withthe stagnation of a rain front, which resulted in disastrous floods in central Japan as well as the Kyushu region(western Japan) (Japan Meteorological Agency [JMA]2021). As shown in Fig. 1a, the time series of observed
precipitation in Japan indicates continued wet conditions in August 2021. Although the early summer rainyseason in mainland Japan, known as Baiu, is generally inJune–July (e.g., Yoshino 1965; Akiyama 1973; Ueda et al.
2009; Sampe and Xie 2010), precipitation in August 2021
was remarkable in comparison with that during Baiu in
the same year (Fig. 1a). Figure 1b and 1c shows the spatial
distributions of the precipitation anomalies relative tothe climatological mean during the heavy rainfall period(August 5–20, 2021). Enhanced precipitation extended
over South China, the East China Sea, and Japan. This
zonally elongated precipitation anomaly appeared to be
similar to the Meiyu–Baiu rain front, which is climatologically more prominent in early summer than in latesummer. The western side of Japan recorded four times
more precipitation than the climatological average in
mid-August (Fig. 1a and c). The occurrence of mesoscale quasi-stationary linear precipitation systems wasreported in the Kyushu region during this heavy rainfall
event (JMA 2021).
From the viewpoint of the global climate system, several atmospheric and oceanic anomalies have been identified as contributing factors to recent heavy rainfall
events in East Asia during summer (e.g., Shimpo et al.
2019; Takemura et al. 2019; Takaya et al. 2020; Haradaet al. 2020; Horinouchi et al. 2021; Ueda et al. 2021;
Park et al. 2021a; Chen et al. 2022). They can be broadlyclassified into two aspects: effects from the tropics andextratropics. With regard to the effect of tropical modulation, anomalous sea surface temperatures (SSTs) and/or convective heating over the Indo–western Pacific sector induce an enhancement of the northwestern Pacificsubtropical high (NWPSH) in the lower troposphere(Lu 2001; Terao and Kubota 2005; Wu et al. 2010; Wanget al. 2013; Takaya et al. 2020; Ueda et al. 2021), thereby(a)Timeseries of Precip. in summer 2021(b)

Precip(Heavy rain period)

Heavy rainfall period(c)[mm day−1]

Fig. 1 Precipitation in East Asia during the summer of 2021. a Time series of observed precipitation (blue bar; unit: mm d
­ ay−1) in Kanto (toppanel), Kansai (middle panel), and Kyushu (bottom panel), Japan by AMeDAS. Color lines indicate their climatological mean for 1991–2020. Grayand black two-way arrows in each panel denote the Baiu period defined by the JMA and the heavy rainfall period in this study [August 5–20, 2021],
respectively. b, c Spatial distributions of period-averaged precipitation anomaly (shading; unit: mm d
­ ay−1) as deviation from the climatologicalmean during the heavy rainfall period. Red rectangle in b is the region of area-averaged precipitation over Japan [30°–37°N, 127°–142°E]. In c, white
contours indicate ratios of precipitation to its climatological mean (unit: %), and color dots are plotted at the location of the AMeDAS observation
stations in Kanto (yellow), Kansai (blue), and Kyushu (red) used for showing the time series in (a)Kuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

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allowing more moisture to be transported from the tropics to East Asia (Chowdary et al. 2013; Kamae et al. 2017;
Sekizawa et al. 2019). The El Niño–Southern oscillation(ENSO), particularly its phase transition from mature El
Niño in the preceding boreal winter, affects the excitation of the anomalous NWPSH and the resultant precipitation variability in East Asia (Chang et al. 2000; Wanget al. 2000, 2001; Tomita et al. 2004; Naoi et al. 2020).
Wang et al. (2000) indicated a local effect of concurrent
SST cooling in the tropical western Pacific associatedwith El Niño in the preceding winter on the intensified
NWPSH. In addition, subsequent studies showed thatthe sequential process and its remote effect involve the
Ekman divergence associated with the eastward-propagating Kelvin wave forced by the warm Indian Ocean
capacitor (Yang et al. 2007; Xie et al. 2009) and positive feedback from the anomalous NWPSH through thewind-evaporation-SST feedback mechanism (Kawamura
et al. 2001; Du et al. 2009; Kosaka et al. 2013). This transbasin air–sea interaction related to the transition from El
Niño in the boreal summer is understood as the Indo–western Pacific Ocean Capacitor (IPOC) effect (Xie et al.
2016). However, Wang et al. (2013) suggested that SST
cooling in the equatorial central Pacific, i.e., summer La
Niña-like conditions, can intensify the NWPSH. Observational evidence has consistently shown that La Niña
events are concurrent with extremely wetter conditions
in East Asia, such as in summer 1988 (Nitta 1990), June–
July 2020 (Ueda et al. 2021; Park et al. 2021a), and latesummer 2021 (see Sect. 3.1).
In the meanwhile, the Rossby wave trains associatedwith the meandering of the extratropical westerly jet
have also been interpreted as significant contributors toprecipitation variability in East Asia (Wakabayashi and
Kawamura 2004; Kosaka et al. 2011; Hirota and Takahashi 2012; Chen and Huang 2012; Harada et al. 2020;
Horinouchi et al. 2021). For example, the quasi-stationary wave train along the Asian jet centered at approximately 40°N, called the Silk Road pattern (Enomoto et al.
2003), is crucial for enhancing and maintaining perturbation vortices through energy propagation and conversion (Enomoto 2004; Sato and Takahashi 2006; Kosaka
et al. 2009). The deepened upper-level trough near the
Korean Peninsula acts to dynamically produce vertical motion accompanied by heavy rainfall in Japan and
Korea (Takemura et al. 2019; Tsuji et al. 2020; Yokoyama
et al. 2020; Park et al. 2021b). Moreover, a cold cutoff low
or trough emerges around East Asia simultaneously withthe occurrence of heavy rainfall (Ninomiya and Shibagaki
2007; Zhao and Sun 2007; Shimpo et al. 2019). Haradaet al. (2020) showed that the formation of an upper-level
trough over the Korean Peninsula during a heavy rain
event in July 2018 was dominated by high-frequency(shorter than 8 days) variability. Furthermore, theOkhotsk high (OH), which is one of the major climate
systems during the Baiu season, is an essential factor
in maintaining the rain front over East Asia because it
transports cold air masses (Ninomiya and Mizuno 1985;
Yim et al. 2014). The development of the surface OH isclosely related to the meandered polar front jet centeredat ~ 60°N in the upper troposphere as well as the land–sea thermal contrast (Nakamura and Fukamachi 2004;
Tachibana et al. 2004; Sato and Takahashi 2007). Kawasaki et al. (2021) suggested the role of the cold Okhotsk
Sea in the enhancement of the NWPSH and Baiu rain
front by increasing the baroclinic instability controlled by
the meridional temperature gradient.
Although the primary factors causing heavy and prolonged rainfall over East Asia have been extensively
investigated from the tropical and extratropical perspectives, their dynamic relationships have not been
sufficiently discussed. Kosaka et al. (2011) showed thatseveral teleconnection patterns, such as ENSO, thePacific–Japan pattern (Nitta 1987), the Silk Road pattern,
and the wave train along the polar front jet accompaniedby the surface OH, coherently contribute to summertime
precipitation variability in East Asia. Moreover, by performing singular value decomposition analysis, Hirota
and Takahashi (2012) showed that a meridional tripolar
pattern of circulation anomalies appears over the Philippines, China/Japan, and eastern Siberia, causing anomalous precipitation in East Asia. They indicated that the
tripolar anomaly pattern was accompanied by internal
wave propagation and tropical diabatic heating. These
pioneering studies suggest the presence of a dynamicallylinked tropical–extratropical covariability that causes
heavy rainfall in the northwestern Pacific during summer.
This study investigates the anomalous circulation fields
associated with heavy rainfall over Japan that occurredin August 2021, focusing on the tropical–extratropical relationship. To explain the atmospheric circulation
anomalies in August 2021, various aspects of the circulation anomalies must be considered, including Rossby
wave-packet propagation, Rossby wave breaking, interactions between the upper- and lower-level circulations,and tropical heat forcing. The remainder of this paper is
structured as follows: In Sect. 2, the data and methods
used in our analyses are explained. Section 3.1 presents
the observed climate anomalies in the tropics and extratropics in August 2021. Section 3.2 presents analyses of
the relationships between (1) the Rossby wave breaking
over eastern Siberia and trough over the Korean Peninsula, and (2) the upper- and lower-level circulationsin the NWPSH region. Section 3.3 discusses the rolesof tropical heat forcing. In Sect. 4, the conclusions arepresented.

Kuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

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2 Data and methods
This section presents the data and tools used for the analyses from various tropical and extratropical perspectives,
including Rossby wave propagation/breaking, movementof upper-level depressions, vertical circulation interactions, and tropical heat forcing.

where the overbar and prime denote the climatological-mean state and anomaly, respectively, u is the zonal
wind, v is the meridional wind, ψ = φ/f0 is the geostrophic streamfunction, with φ as the geopotential andf0 as the reference Coriolis parameter, and p∗ =(pressure/1000 hPa). Subscripts x and y denote the partial
derivatives with respect to longitude and latitude, respectively. The flux W is proportional to the group velocity ofstationary Rossby waves in the plain wave limit; therefore, it is a useful diagnostic tool for depicting the wavepacket propagation of Rossby waves in applications to thedata. The wave activity is also transported by the phasespeed of the wave (Takaya and Nakamura 2001), but thephase-speed term was omitted here under the assumption of quasi-stationary Rossby waves.

2.1 Observational data

The atmospheric data used in this study were 6-hourly
data from the Japanese 55-year Reanalysis (JRA-55) dataset with a horizontal resolution of 1.25° and 37 pressurelevels (Kobayashi et al. 2015) from 1991 to 2021. Additionally, monthly-mean SST data from the Centennialin situ Observation-Based Estimate-SST (COBE-SST)with a horizontal resolution of 1.0° (Ishii et al. 2005) wereused for the same period. For daily precipitation data,
we utilized the Global Precipitation Climatology Project (GPCP) dataset with a horizontal resolution of 1.0°
(Huffman et al. 2001) for the period of 1997–2021 fromthe National Oceanic and Atmospheric Administration (NOAA). Precipitation observations in Japan wereobtained from a data source based on the automatedmeteorological data acquisition system (AMeDAS)
maintained by the JMA. Tropical convective activity wasquantified using the daily interpolated outgoing longwave radiation (OLR) data with a horizontal resolutionof 2.5° (Liebmann and Smith 1996) provided by NOAA
for the period of 1991–2021. We also used the Madden–Julian oscillation (MJO) daily index provided by theAustralian Bureau of Meteorology as the real-time multivariate MJO (RMM) index (Wheeler and Hendon 2004)
and the boreal summer intraseasonal oscillation (BSISO)
daily index (Lee et al. 2013) provided by the Asia–Pacific
Economic Cooperation (APEC) Climate Center.
We mainly analyzed the anomalous climate fields during the heavy rainfall period (August 5–20, 2021; Fig. 1a).
The 6-hourly and daily data from JRA-55 and GPCP,
respectively, were temporally averaged for the period, andthe climatological-mean state was defined as the study
period average for the same calendar day of each dataset.
Anomalies were defined as deviations from the climatological mean. No long-term detrending was applied.
2.2 Wave‑activity flux

The propagation of stationary Rossby waves was analyzed
using the quasi-geostrophic (QG) wave-activity flux
defined by Takaya and Nakamura (2001). The horizontal
component of flux W is defined as follows:
�
�
�
 �
′
′
u ψ ′ 2x − ψ ′ ψxx+ v ψx′ ψy′ − ψ ′ ψxy
 �
�
�
� ,
W = �
′
′
+ v ψ ′ 2y − ψ ′ ψyy
2 u2 + v2 u ψx′ ψy′ − ψ ′ ψxyp∗(1)

2.3 Cutoff low index

To detect synoptic depressions in the mid- to upper troposphere including cutoff lows and preexisting troughs,
the cutoff low index (COL index; Kasuga et al. 2021)
was utilized, which is a useful tool for seamless detection of isobaric depressions. This scheme was based on
the geometric horizontal slope of the geopotential height
field. The average slope (AS) function representing thetwo-dimensional average of the four-directional slope isdefined as follows:

1
Z x + r, y + Z x − r, y
4r
+Z x, y + r + Z x, y − r − 4Z x, y ,(2)where x and y denote the longitudinal and latitudinal grid
points, respectively, r is the radial searching variable, andZ is the geopotential height at any isobaric level. Here,the AS maximum against variable r is denoted as A
­ S +,
that is AS+ x, y ≡ max AS x, y; r ,(3)rAS x, y; r =and the bottom points of a geometric depression( xb , yb) are specified by the local maximum of A
­ S+ asfollows:






So ≡ AS+ xb , yb = AS xb , yb ; ro ≥ AS x, y; r ,(4)where So is the optimal slope, and ro is the optimal
radius. Hence, A­ S+ represents the depression of the geopotential height field, So reflects the strength and position of the cutoff lows or toughs, and ro is interpreted
as their depression area. When a local height minimum
point is located within the optimal radius of a detected
depression, it is regarded as a cutoff low; otherwise, it isKuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

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regarded as a trough. Based on the definition of the AS
function [Eq. (2)], the AS minimum (­AS−) may detect
anticyclonic disturbances including blocking highs andridges (Kasuga et al. 2021). The COL index was calculatedusing the 6-hourly geopotential height data from JRA-55.

thermal damping. The e-folding decay time of the diffusion coefficient was set to 6 h for the maximum horizontal wavenumber. The damping coefficient was set to half a
day for the lowest three levels, one day for the uppermost
two levels, and 20 days elsewhere. The LBM was forced
with externally imposed heating and time integrated to asteady state, which was day 15 when the model reacheda quasi-steady state in this study. Details regarding theexperimental settings, including the prescribed heating
and background states, are presented in Sect. 3.3.1.

2.4 Quasi‑geostrophic potential vorticity inversion

The dynamic relationship between the upper- and lowertropospheric circulation anomalies was determined using
the potential vorticity (PV) inversion technique. In the
QG framework, PV (q ; QGPV) is given by:


2 ∂ψ
f
∂
0
,
q = f + ∇ 2ψ +(5)∂p σ ∂pand anomalous QGPV (q ′ ) is written as: ∂ f02 ∂ψ ′′2 ′q =∇ ψ += Lg ψ ′ ,∂p σ ∂p(6)

where f is the planetary vorticity, σ ≡ −RT0 p−1 d ln θ0 /dp
is the static stability, with R as the gas constant and θ0 as thepotential temperature corresponding to the basic statetemperature T0, and Lg = ∇ 2 + (∂/∂p) f02 /σ (∂/∂p) is
the linear operator. Using the PV invertibility (Charney
and Stern 1962; Hoskins et al. 1985), the distribution of
the anomalous streamfunction (ψ ′ ) is uniquely determined from a specified perturbation PV (q ′ ) by invert ′
ing the Laplacian-like operator (Lg ), i.e., ψ ′ = L−1
g q .
Appropriate boundary conditions are required to solve
the Laplacian inversion problem (Bretherton 1966;
Hoskins et al. 1985). To invert upper-tropospheric QGPV
anomalies, Nakamura and Fukamachi (2004) and Takaya
and Nakamura (2005) performed an inversion with
an imaginary flat surface much deeper than the actual
ground surface, which allowed the immediate effect of
the upper level on the ground surface to be extracted.
The climatological Brunt–Väisälä frequency-averaged
zonally at each pressure surface was assigned, and theperturbation potential temperature (θ ′ ) was set to zero at
the imaginary surface as the lower-boundary condition.
The same measures were adopted in this study. We refer
to Takaya and Nakamura (2005) for details on our QGPV
inversion method.
2.5 Linear baroclinic model

To assess the effects of tropical diabatic heating on theanomalous NWPSH, we used the linear baroclinic model(LBM; Watanabe and Kimoto 2000), which is a spectral model based on linearized primitive equations. The
LBM used in this study comprised 20 sigma levels witha horizontal resolution of T42 and employed del-forthhorizontal diffusion, Rayleigh friction, and Newtonian

3 Results and discussion
3.1 Anomalous fields during the heavy rainfall periodin August 2021
3.1.1 Climate anomalies in the atmosphere and ocean

The anomalous atmospheric circulation fields during the heavy rainfall period in August 2021 are shownin Fig. 2. The sea-level pressure (SLP) field exhibited
positive anomalies over the subtropical western Pacificand Okhotsk Sea (Fig. 2a), which were identified as theintensified NWPSH and OH, respectively. A zonallyelongated negative SLP anomaly was found over Japanbetween the two intensified anticyclones, corresponding to an enhanced rain band (Fig. 1b). The anomalous
moisture flux in the lower troposphere indicated that theintensified NWPSH transported more moisture to theextratropics and converged over Japan (Fig. 2b), resulting in heavy rainfall. The anomalous OH and associatednortherly winds can contribute to moisture flux convergence anomaly, which favors the frontal zone. In the
mid-troposphere (500 hPa), a meridional tripolar structure comprising an anticyclonic anomaly over eastern
Siberia, an anomalous trough over the Korean Peninsula (hereinafter referred to as the Korean trough), andan anticyclone appearing southeast of the Japan region
was evident (Fig. 2a). The northern anticyclonic anomaly
extended over eastern Siberia toward the Okhotsk Sea,
where blocking events occur frequently during boreal
summer (e.g., Barriopedro et al. 2006). It is important
to note that the Korean trough as well as the blockinglike anticyclonic anomaly over eastern Siberia werequasi-stationary systems, as discussed in Sect. 3.2.1. The
other anticyclonic circulation anomaly over the subtropical northwestern Pacific appeared to correspond to theenhancement of the surface NWPSH, whereas the anomalous circulation at 500 hPa was located slightly northeast of the surface anomalies. The vertical structure of thenorthward tilting with height is discussed in Sect. 3.2.2.
The upper-tropospheric circulation anomalies show
a wave-train pattern from northern Europe via Central
Asia to Japan (Fig. 2c). The wave train over the Eurasiancontinent was accompanied by an eastward wave-activity
flux along the Asian jet, indicating the propagation of

Kuramochi et al. Progress in Earth and Planetary Science(a) SLP & Z500

(2023) 10:69(Heavy rain period)

Page 6 of 19(b) Moisture flux & divergence

[hPa]

(c)

[10−7 s−1]

Z200 & WAF[m][m2 s−2]

Fig. 2 Anomalous atmospheric fields during the heavy rainfall period in August 2021. a Anomalies of SLP (shading; unit: hPa) and geopotential
height at 500 hPa (contours; unit: m). b Moisture flux anomaly (vectors; unit: kg k­ g−1 m ­s−1) at 925 hPa and its divergence (shading; unit: ­s−1). SLP
anomaly (unit: hPa) is superimposed by thin contours. c Geopotential height anomaly at 200 hPa (shading; unit: m) and wave-activity flux (vectors;
unit: ­m2 ­s−2). Green contours indicate 20 and 30 m ­s−1 isolines of the climatological-mean zonal winds at 200 hPa

stationary Rossby waves. The Korean trough appeared
to be the easternmost wave train over the continent;
however, the eastward propagation was obscure aroundsouthern China (100°E). Moreover, in the western North
Pacific, the wave-activity flux appeared to emanate fromthe deepened Korean trough to the anticyclone over thesubtropical western Pacific in a southeastward direction.
These extratropical atmospheric features, which indicated the occurrence of the OH/blocking, Korean trough,and NWPSH, resembled to the modulated circulation
fields associated with the previous heavy rainfall events
over East Asia in July 2018 (e.g., Shimpo et al. 2019;
Harada et al. 2020) and July 2020 (Park et al. 2021a).
Figure 3 shows the SST anomalies in August 2021 andthe temporal evolution of the tropical oceanic indicesbased on the area-averaged SST anomalies. The negative
SST anomalies in the eastern side of the equatorial Pacificand the positive anomalies in the tropical western Pacificindicate La Niña conditions in August 2021 (Fig. 3a).
Simultaneously, SST in the eastern Indian Ocean was
higher than normal, which coincided with cooling in thewestern Indian Ocean. This SST pattern in the Indian
Ocean was identified as a negative phase of the Indian
Ocean dipole (IOD). The time series of the Niño3.4 index
shows continued La Niña conditions from the previous
winter of 2020/21 through the summer of 2021 to thenext winter of 2021/22 (Fig. 3b; World MeteorologicalOrganization 2022), whereas the negative-phase IOD
emerged from May 2021 (Fig. 3b).

Kuramochi et al. Progress in Earth and Planetary Science(a)(2023) 10:69

SST

Page 7 of 19(August 2021)

[K]

(b)

Fig. 3 a SST anomalies (unit: K) in August 2021. b Monthly time series of the tropical oceanic indices from 2019 to 2021: Niño3.4 index (blue line)
as [5°S–5°N, 170°–120°W] (Trenberth 1997), Indian Ocean dipole (IOD) index (green line) as the difference between [5°S–5°N, 50°–70°E] and [10°S–
Equator, 90°–110°E] (Saji et al. 1999), and Indian Ocean basin warming (IOBW) index (red line) as [20°S–20°N, 40°–100°E]. The units are K. The areasof the indices are shown by rectangles in (a)Regarding the relationship between tropical SST andconvection anomalies (Fig. 1b), the anomalously high
SST around the Maritime Continent was consistent with
the in situ enhancement of convective activity. Several
studies have shown that significantly suppressed convection over the equatorial central Pacific appears in La Niña
summers associated with the modulated Walker circulation (e.g., Webster and Yang 1992; Ju and Slingo 1995).
The thermodynamic relationship between the anomalous
NWPSH and diabatic heating over the Indo–Pacific sector is discussed in Sect. 3.3.
3.1.2 Synchronized amplification of climate anomalies

The previous subsection presented the time-mean
anomalies in the atmosphere and ocean during theheavy rainfall period in August 2021. This subsection focuses on the daily evolution of these key factors. Figure 4 shows the daily time series of the indicesrepresenting the intensity of the anomalous circulation and precipitation in August 2021. The upperlevel blocking high over eastern Siberia (pink lines)
and surface OH (red lines) varied coherently, indicating their strong linkage as shown by Nakamura andFukamachi (2004). The intensity of the blocking highand OH reached a maximum on August 8–10, 2021,
prior to the maximum precipitation in Japan (August
13, 2021; navy bars). The Korean trough (yellow lines)
deepened toward the peak of precipitation in Japanand maintained its intensity for several days. Similarly,
the NWPSH (blue lines) intensified in early Augustand reached its maximum intensity on August 13,
2021. Furthermore, convection over the eastern Indian
Ocean was much enhanced in early August and continued during the heavy rainfall period. This synchronized amplification of the OH/blocking, Korean trough,
NWPSH, and convective activity over the Indian Ocean

Kuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

Timeseries of indices

Page 8 of 19

AS+, Z, U200, & Omega500(Aug 13 ±48 h)[m (100 km)−1]

Fig. 4 Daily time series of the indices of the blocking highover eastern Siberia (pink), Okhotsk high (OH; red), Korean
trough (yellow), northwestern Pacific subtropical high (NWPSH;
blue), convective activity over the eastern Indian Ocean (green),
and precipitation in Japan (heavy blue bar) in August 2021.
Dotted and solid lines indicate the daily and 5-day running means,respectively. The bottom bars also indicate the 5-day running mean.
Vertical dotted lines denote August 13, 2021. The indices were
defined by area average as follows; Blocking high: Geopotential
height anomaly at 500 hPa (unit: m) over eastern Siberia [50°–60°N,
125°–140°E], OH: SLP anomaly (unit: hPa) over the Okhotsk Sea[45°–60°N, 140°–155°E], Korean trough: Geopotential heightanomaly at 500 hPa (unit: m) over the Korean Peninsula [30°–45°N,
120°–135°E], NWPSH: SLP anomaly (unit: hPa) over the subtropicalnorthwestern Pacific [15°–30°N, 130°–155°E], Convective activity:
Precipitation anomaly (unit: mm d
­ ay−1) over the eastern Indian Ocean[5°S–10°N, 85°–100°E], Japan precipitation: Precipitation (unit: mm­day−1) over Japan [30°–37°N, 127°–142°E]. Precipitation was basedon the GPCP data

implies a dynamic linkage in the tropical–extratropicalatmosphere. Therefore, we attempt to explain the synchronicity in the following sections.
3.2 Dynamical mechanisms of tripolar circulationsynchronicity
3.2.1 Trough over the Korean Peninsula and blocking highover eastern Siberia

The previous section described the coherent amplification of the tripolar circulation anomalies. In thissubsection, the relationship between the quasi-stationary Korean trough and the eastern Siberian blocking
high is analyzed using the COL index. Although theFig. 5 Spatial distributions of the mean atmospheric fields in August
11–15, 2021. ­AS+ (shading; unit: m [100 ­km]−1) and geopotential
height (black contours; unit: m) at 200 hPa. Green contours indicate
vertical p-velocity at 500 hPa from − 0.5 to − 0.1 Pa ­s−1 with an intervalof 0.1 Pa ­s−1. The 200-hPa horizontal wind velocity more than 20 m ­s−1
is superimposed by red contours with an interval of 10 m ­s−1

stationary cyclonic anomaly and related high-frequency
variability over the Korean Peninsula have been suggested as important contributors to a heavy rainfall
event in western Japan (Harada et al. 2020), the specific behavior of the shorter-scale eddies has not been
explicitly shown. Therefore, the detection of synopticscale isobaric depressions using the COL index might
be advantageous for capturing the motions of upperlevel transient eddies and clarifying the formation of
the quasi-stationary Korean trough.
The spatial distributions of the upper-troposphericgeopotential height and A­ S+ around the day of the rainfall peak are shown in Fig. 5. Positive values of ­AS+ (grayshading) represent the presence of isobaric depressions
corresponding to a deepened trough over northeastern
China toward the southern Okhotsk Sea. Moreover, thenorthward positive gradient of the geopotential heightindicates that the trough was accompanied by anticyclonic wave breaking and a blocking high over eastern
Siberia. In the southeast of the wave breaking, the salient ascent was found over Japan, south of the jet streak
entrance. This strong vertical motion over Japan may bepartly attributed to a dynamically induced ascent underthe QG balance, as diagnosed using the Q-vector proposed by Hoskins et al. (1978) (Additional file 1: Fig. S1).
In general, from the perspective of the QG omega equation, the ascent is dynamically required in the south of
the upper-tropospheric jet entrance (Hoskins et al. 1987;

Kuramochi et al. Progress in Earth and Planetary Science(a)AS+200 & 500(2023) 10:69(August; clim)

Page 9 of 19(b)(Heavy rain period)

N
40

N
40
N
20

N
20

180

180
90E

120E

150E

90E

120E

150E

[m (100 km) ]−1(c)(Heavy rain period; anom)[m (100 km)−1](d)

COL tracking; So & ro 200

40

N

18UTC 2

20

N

N

40

20

12UTC 13

N

So

180

20–
15–20
10–15
5–10
0–5

[m (100 km)−1]

90E

120E

150E

90E

120E

150E

[m (100 km)−1]

Fig. 6 Spatial distributions of the COL index. Mean ­AS+ at 200 hPa (shading) and 500 hPa (contours) in the a August climatology and b heavy
rainfall period in August 2021. c Anomalies of ­AS+ at 200 hPa (shading) and 500 hPa (contours) during the heavy rainfall period. d Trackingof the extracted depression at 200 hPa from development (18 UTC on August 2, 2021) to disappearance (12 UTC on August 13, 2021) with 6-hourlyintervals. Color dots represent positions and optimal slope So of the depression. Green and blue circles denote optimal radius ro (unit: km)
when the depression was trough and cutoff low, respectively. The units of ­AS+ and optimal slope are m (100 km)−1

Park et al. 2021b). Consistent with this fact, Fig. 5 actually shows that the distribution of the intense ascent overJapan was located southwest to the upper-level jet core(> 50 m ­s−1). Moreover, the equatorward Q-vectors were
zonally convergent along the jet entrance (Additional
file 1: Fig. S1), indicating the presence of the dynamically induced ascent corresponding to the heavy rainfall
region.
Figure 6a and b shows the time-mean COL index in
the August climatology and the heavy rainfall period in
August 2021, respectively. In the August climatology,
the ­AS+ maxima appeared around Central Asia, Northeast Asia, and the Aleutian Islands (Fig. 6a). The southwestward extension of ­AS+ to the subtropical Pacific at
200 hPa corresponds to the so-called tropical uppertropospheric trough or mid-Pacific trough associatedwith Rossby wave breaking (Postel and Hitchman 1999;
Abatzoglou and Magnusdottir 2006). The climatologicalmean ­AS+ may imply the main routes and generation
areas of cutoff lows from western Siberia via northern

East Asia to the Aleutian Islands in the boreal summer.
During the rainy period in August 2021 (Fig. 6b), intense­AS+ maxima were found around over Lake Balkhash(45°N, 75°E), the northwest of the Korean Peninsula,and the Aleutian Islands both at 500 and 200 hPa, whichwere almost the same areas as those of the climatological maxima and exhibited corresponding positive anomalies (Fig. 6c). The geographical locations of the maximaof ­AS+ and its anomalies differed slightly. A positive A­ S+
anomaly indicated an increase in the frequency and/or
intensity of depression. Notably, positive A­ S+ anoma+lies were accompanied by negative A­ S anomalies in thenorth. The anomaly of ­AS+ corresponded well with thatof the upper-level geopotential height (Fig. 2c).
Figure 7 shows distributions of A
­ S− provided in the
COL index denoting bumps on the isobaric surface during the heavy rain period. Overall, the mean A­ S− and+its anomaly were opposite to those of ­AS (comparison between Figs. 6 and 7). The maximum and positive
anomaly of ­AS− over the northern part of eastern Siberia

Kuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

Page 10 of 19

Fig. 7 a, b, c Same as Fig. 6a, b, c, respectively, but for ­AS−. d Spatial distribution of correlation coefficient between monthly-mean A­ S−
and area-averaged ­AS+ near the Korean Peninsula [30°–45°N, 120°–135°E] (green box). Gradational red and blue shadings denote, respectively,
positive and negative correlations exceeding the 95, 99, and 99.9% confidence levels. The statistical period was July and August for 1991–2020

represents the occurrence of blocking (Fig. 7b, c). Interestingly, we can find an A
­ S− maximum and a blocking
structure over eastern Siberia also in the climatologicalmean field in August (Fig. 7a), indicating the frequent
occurrence and favorable conditions for blocking. The
north–south pair of ­AS− over eastern Siberia and ­AS+
over the Korean Peninsula implies a classical relationship between blocking highs and lows; in other words,
blocking highs are occasionally accompanied by cyclones
on the equator side (e.g., Rex 1950; Nakamura et al.
1997; Barriopedro et al. 2006). Even if the cyclone does
not completely separate from the background stream,
troughs intrude south of the blocking highs, as shown in
Fig. 5 (e.g., Lejenäs and Økland 1983; Barriopedro et al.
2006). On the basis of this finding, we show the correlation coefficients between the monthly-mean ­AS− and+area-averaged ­AS in the vicinity of the Korean Peninsula
in Fig. 7d for the statistical period of July–August from1991 to 2020. The significant positive correlation overeastern Siberia indicates a strong linkage between theeastern Siberian blocking high and the Korean trough.
This result implies a significant coherent amplification

between the Siberian blocking high and the Korean
trough during the heavy rainfall period in August 2021.
From a Lagrangian perspective, the COL index can
track upper-tropospheric depressions and indicate their
movements (Kasuga et al. 2021). Using the 6-hourly COL
index, we visually performed a back-trajectory analysis of
the depression at 200 hPa that appeared over the Korean
Peninsula on August 13, 2021; its trajectory is shownin Fig. 6d. The target depression emerged at 18 UTC
on August 2, 2021, as a cutoff low over western Siberia,
which is a region where cutoff lows develop frequently(Fig. 6a). Subsequently, it moved southeastward over East
Asia for several days and remained near the Korean Peninsula for approximately four days. When it remained
there, some other small depressions were separated fromand also merged with it. This was characterized by aneastward movement toward the cyclonic anomaly over
the Aleutian Islands (Additional file 1: Fig. S2). The target depression disappeared at 12 UTC (21 Japan Standard Time) on August 13, 2021, and merged with anothercutoff low over Sakhalin. These results indicate that thelong-lived cutoff low, which was generated ~ 10 days

Kuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

Page 11 of 19

earlier in western Siberia, traveled southeastward overEast Asia and finally contributed to the heavy rainfall inJapan.
Furthermore, the movements of the synoptic depressions may be associated with the blocking over eastern
Siberia and may indicate the formation and deepening processes of the stationary Korean trough. From avortex–vortex interaction perspective, Yamazaki and
Itoh (2013a, b) proposed that relatively small perturbations (synoptic eddies) move along the waveguideto a same-polarity large-scale eddy (blocking highs
and lows), thereby contributing to the maintenance ofblocking by supplying PV. Based on this idea, the synoptic depressions may have selectively traveled andstagnated south of the blocking high over eastern Siberia (Fig. 6d), thereby deepening the preexisting stationary Korean trough during the heavy rainfall periodin August 2021. This suggests an interaction between
the stationary cyclonic anomaly and transient cycloniceddies over the Korean Peninsula.

advection of surface temperature by the induced zonal
geostrophic winds is shown in Fig. 8c. Since the summer background air temperature over the subtropical western Pacific has a westward positive gradient in
the lower troposphere (Fig. 8b), the induced anomalous
easterly wind advects cold air temperatures from the sea
side. The cold air advection may contribute to the maintenance and enhancement of the NWPSH because surface cold anomalies behave as anticyclonic PV (Hoskins
et al. 1985). This can be confirmed from Fig. 8d, whichshows the streamfunction tendency at 1000 hPa only due
to the upper-level-induced zonal temperature advection.
The region of the positive streamfunction tendency coincided locally with that of the enhanced NWPSH (Fig. 2a),accompanied by a poleward tilting structure with heightassociated with the zonal heat transport. This structure
is suitable for circulation anomalies over the summer
subtropical western Pacific due to the westward positive
air temperature gradient in the climatology, like as midlatitude baroclinic instability waves tilting westward withheight due to the basic southward positive thermal gradient. In fact, the northward tilting structure of the circulation anomalies over the northwestern Pacific exhibited
effective energy conversion to the perturbations (Kosaka
and Nakamura 2006). These results suggest that the anticyclonic circulation anomaly in the upper troposphere
over the western North Pacific acted to enhance andmaintain the surface NWPSH through a baroclinic connection characterized by the northward-tilting structure.
Similarly, the strong relationship between the surface
OH and upper-tropospheric blocking high over eastern
Siberia can be found in Fig. 8c and d, which is consistent with Nakamura and Fukamachi (2004). This can also
be identified in the anomalous circulation fields, in which
the height axis tilted poleward with altitude around the
Okhotsk Sea and eastern Siberia (Fig. 2a, c).

3.2.2 Relationship between the surface NWPSH
and upper‑level anticyclone

The preceding subsection suggested a correlation
between the Siberian blocking and Korean trough. In
this section, we further discuss the possible relationshipbetween extratropical wave propagation in the upper
troposphere and anomalous NWPSH in the lower troposphere. As mentioned in Sect. 3.1, stationary Rossby
waves emanated from the Korean trough toward theanticyclonic anomaly over the western North Pacific in
the upper troposphere (Fig. 2c). This indicates that theupper-tropospheric anomalous anticyclone was excited
and maintained by wave propagation from upstream.
The resultant negative QGPV anomaly over the western North Pacific (Fig. 8a) may have induced anomalous
circulation in the lower troposphere (e.g., Hoskins et al.
1985).
To examine the upper-level impacts on surface circulation, the QGPV inversion was conducted as shownin Fig. 8. Figure 8b shows the anomalous geostrophic
winds at 1000 hPa induced solely by the 200-hPa QGPV
anomaly in the Northern Hemisphere averaged during
the heavy rainfall period. Clockwise circulation was indicated over the Pacific Ocean east of Japan owing to theupper-tropospheric low QGPV anomaly. Moreover, thelower-level cyclonic circulation over the Korean Peninsula was induced by the notably high QGPV anomaly inthe upper troposphere (Fig. 8a), which may have partly
contributed to the northeastward transport of moisture
to Japan in the lower troposphere (Fig. 2b).
It is well known that surface heat transport is vital tobaroclinic wave amplification and maintenance. The

3.3 Role of tropical modulation in August 2021
3.3.1 Tropical heat forcing on the surface NWPSH

It is widely accepted that the modulation of tropical convective activity has great impacts on the NWPSH variability and subsequent precipitation variability in East
Asia (e.g., Wang et al. 2000, 2013; Kosaka et al. 2013;
Xie et al. 2016); however, previous studies have mainly
focused on the Meiyu–Baiu season or the JJA mean
fields. Therefore, using the LBM, we first confirm thearea-wise contribution of the tropical heat forcing to
the anomalous NWPSH in the basic state in late summer (August). Figure 9a shows the tropical heating contributions to the anomalous NWPSH as area-averaged
surface pressure response to the solely imposed heatingin each 10° × 10° grid point over the Indo–Pacific sector, where the imposed heating has an idealized vertical

Kuramochi et al. Progress in Earth and Planetary Science(a) PV200(2023) 10:69(Heavy rain period)

Page 12 of 19(b) INV PSI1000[10−5 s−1]

[m s−1](c) Tadv by INV U1000(d) PSI1000 tendency[10−2 K d−1][105 m2 s−1 d−1]

Fig. 8 QGPV inversion results. a QGPV anomaly at 200 hPa (unit: ­s−1) during the heavy rainfall period. b Anomalous geostrophic winds at 1000 hPa(vectors; unit: m ­s−1) solely induced by the PV anomalies at 200 hPa in the Northern Hemisphere. Climatological-mean air temperature at 1000 hPa(unit: K) is superimposed by red contours. c Air temperature advection (unit: K d­ ay−1) by the induced anomalous zonal geostrophic winds
at 1000 hPa. Blue contours secondarily indicate its − 0.7, − 0.5, and − 0.1 × ­10−2 K ­day−1 isolines in the south of 30°N. d Tendency of streamfunction
at 1000 hPa (unit: ­m2 ­s−1 ­day−1) solely due to the air temperature advection by the induced anomalous zonal winds. Red contours secondarily
indicate its 0.02, 0.05, and 0.1 × ­105 ­m2 ­s−1 ­day−1 isolines in the south of 30°N

structure with a maximum of ~ 1.7 K ­day−1 at σ = 0.45,
and the background state is the climatological-mean fieldin August derived from JRA-55. Overall, positive contributions were dominant over the Indian Ocean, whereas
negative contributions were dominant over the tropical Pacific Ocean. The positive values over the tropicalIndian Ocean indicate that anomalous heating intensified the NWPSH. This may be consistent with the Kelvinwave-induced Ekman divergence mechanism proposedby Xie et al. (2009), who showed that a warm equatorial
Kelvin wave forced by Indian Ocean warming is responsible for the anomalous anticyclone and weakened rainfall over the subtropical northwestern Pacific by inducing
atmospheric surface Ekman divergence. The contributions of the western Indian Ocean were slightly stronger
than those of the eastern Indian Ocean. These results
are suggestive of great potential to cause the enhanced

NWPSH and resultant heavy rainfall in East Asia not
only in the Meiyu–Baiu season but also in August fromthe viewpoint of the tropical Indian Ocean forcing. For
the Pacific Ocean, negative values indicate the contribution of tropical anomalous cooling to the reinforcementof the NWPSH (Fig. 9a). In the equatorial lower troposphere, external atmospheric cooling may excite a pairof anticyclonic circulations on the western side, which
is well known as the Rossby wave response in the Matsuno–Gill type heat-induced atmospheric response(Matsuno 1966; Gill 1980). Based on this idea, the diabatic cooling associated with suppressed convective activity over the equatorial central Pacific around the dateline may remotely induce the enhancement of NWPSH.
This mechanism appears to be consistent with theexperimental results of Wang et al. (2013), who showedthat La Niña-like SST cooling in the central Pacific mayKuramochi et al. Progress in Earth and Planetary Science(a)

NWPSH response to point-wise forcing(2023) 10:69

Aug

Page 13 of 19(b)

ps response

Heavy rainfall period[W m−2]

Fig. 9 The LBM experiment results. a Anomalous heating contribution to the development of NWPSH. Plotted values are area-averaged
surface pressure anomaly in the subtropical northwestern Pacific [15°–30°N, 130°–155°E] (black rectangle) as a response to the idealized heatingin each grid with the background state in August. Positive values in each grid indicate that the enhancement of convective activity strengthens
the anomalous anticyclone (AC′) over the northwestern Pacific, and vice versa. b Steady response of surface pressure (contours; unit: hPa)
to the prescribed heating anomalies over 20°S–20°N, 30°E–150°W during the heavy rainfall period. Red, blue, and black contours denote positive,
negative, and zero values, respectively. The vertically integrated heat source anomaly from the surface to 10 hPa is indicated by shaded grid boxes(unit: W ­m−2)

be responsible for the stronger-than-normal NWPSH.
Moreover, the striking local cooling contribution over thenorthwestern Pacific (10°N, 140°–170°E) is indicative ofmoist feedback between the atmospheric circulation andconvective activity in the development and maintenanceof the NWPSH. The locally suppressed convective activities strengthen the NWPSH; simultaneously, anomalous
surface easterly winds on the southern flank of the anomalous anticyclone induce atmospheric Ekman divergenceand suppress convection over the northwestern Pacific(Xie et al. 2009, 2016; Hu et al. 2019).
In August 2021, tropical convective modulation wascharacterized by enhancement over the eastern Indian
Ocean and suppression over the tropical western–central Pacific (Fig. 1b). To assess the contribution of thetropical convection anomalies to the intensification of
the NWPSH, we performed another set of sensitivity
experiments for realistic external heat forcing using the
LBM. In the experiments, the diabatic heating anomalies during the heavy rainfall period of August 2021,
derived from JRA-55, were prescribed for the model.
The imposed diabatic heat source (Q ) was calculated asa residual of the thermodynamic equation, as follows(Yanai et al. 1973):


∂T
Q
∂T
RT
,
=
+ v · ∇T + ω
−
(7)
cp
∂t
∂p
cp p
where v is the horizontal wind vector, ω is the vertical p-velocity, cp is the specific heat for the dry air, andQ is the apparent heat source. The calculation was conducted based on the 6-hourly time step. As shown in
Fig. 9b, the anomaly of the vertically integrated Q in thetropics was in good agreement with the observed precipitation anomaly (Fig. 1b), indicating the dominantrole of precipitation in the heat source over the tropics.
The heat source anomaly only across the region from thetropical Indian Ocean to the central Pacific [20°S–20°N,
30°E–150°W] was prescribed in the model as external
forcing with the basic state as the climatological-mean
field in August derived from JRA-55. The data were interpolated into the LBM grid.
Figure 9b shows the atmospheric response in the LBM
to the diabatic heat source anomaly during the heavy
rainfall period in August 2021. The surface pressure
over the subtropical northwestern Pacific was anomalously high, indicating strengthening of the NWPSH
due to the tropical heat forcing. Relatively low pressure
over Japan was also represented in the model response.
The full heating anomaly over the tropical Indo–western Pacific sector almost explains the intensificationof the NWPSH, while one may wonder which is more
important, the heating anomaly or the cooling anomaly.
We further compared the contributions of the anomalous heating and cooling to the anomalous NWPSH byperforming sensitivity experiments only for the positive and negative anomalies of the diabatic heat source,
respectively. In the model responses (Additional file 1:
Fig. S3), the anomalous heating (cooling) increased thesurface pressure over the northern (southern) portionof the NWPSH region, which means that the spatial
distributions of the atmospheric responses only to theanomalous heating/cooling were not entirely consistent
with that of the reanalysis and full heating experiment.
Therefore, these experimental results indicate that it isKuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

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a combination of the anomalous heating in the eastern
Indian Ocean and the anomalous cooling in the tropicalwestern Pacific that plays a crucial role in the intensification and localization of the surface NWPSH.
Meanwhile, we should note that the contribution of thetropical heat forcing during the August 2021 event may
be evident only in the lower troposphere. The uppertropospheric circulation response in the LBM experiment against the realistic heating anomalies exhibited acyclonic (not anticyclonic) circulation anomaly over thesubtropical northwestern Pacific due to the baroclinicity of the Matsuno–Gill type Rossby wave response (notshown), which completely differs from the observed
tripolar structure of the anomalous circulation field.
Thus, the tropical heat forcing mainly induced the SLP
anomaly, and the extratropical variations also contributed to the surface circulation anomalies through the
Rossby wave propagation in the upper troposphere andsubsequent PV response. The coherent occurrence of thetropical heat forcing and extratropical variations was theessential factor for the occurrence of the heavy rainfall
event in August 2021 from a climate system perspective.

clearly in the positive anomaly of the velocity potential,
began to propagate eastward from the western hemisphere in mid-July and subsequently reached the western Pacific (~ 150°E) during the heavy rainfall period in
August. Moreover, the suppression was accompanied by
the enhanced convective activity over the Indian Ocean(~ 60°–100°E), which appeared to propagate eastward
faster than the suppressed convective activity. The phasetransition of the MJO can also be found in the phasespace diagram (Fig. 10b), indicating its approximately
40-day cycle. The MJO moved from weak phase 8 prior
to the heavy rainfall event. Subsequently, it was amplified and remained in phase 2 during mid-August. Moreover, the ISO of convective activity can be identified as
the BSISO, which dominates the ISO in the boreal summer and is characterized by a northeastward propagation over the northern Indian Ocean and western North
Pacific (e.g., Wang and Xie 1997; Kemball-Cook andWang 2001; Kikuchi 2020). The BSISO quickly changed
from phase 8 to phase 3 in early August 2021 and maintained phase 3 during the heavy rainfall period (Fig. 10c),showing rapid amplification of the first principal component of the BSISO. Phase 2 of the MJO and phase 3 of the
BSISO are characterized by enhanced convection overthe eastern Indian Ocean paired with suppressed convection over the northwestern Pacific, accompanied by theanomalous anticyclone over the subtropical northwestern Pacific (Donald et al. 2006; Pai et al. 2011; Lee et al.
2013; Kikuchi 2021). This is consistent with our resultson the August 2021 event. Thus, it is suggested that the
ISO amplification and phase transition toward the tropical Indo–western Pacific Ocean sector may be responsible for the pair of convective activity anomalies over theeastern Indian Ocean and tropical western Pacific, which
resulted in the intensification of the surface NWPSH andheavy rainfall in late summer.

3.3.2 Possible formation process of convective heating
anomalies

The numerical experiment described in the previous subsection indicated the important roles of the combination
effect of enhanced convective activity over the eastern
Indian Ocean and suppression over the tropical Pacificin reinforcing the NWPSH. The tropical SST anomalies
might contribute to the formation of the localized heating anomalies by promoting the anomalous convection
because the distributions of the precipitation and SST
anomalies within the tropics were regionally consistent
as mentioned in Sect. 3.1 (Figs. 1b and 3a). However, thefact that the heavy rainfall prolonged only in August eventhough the tropical oceanic anomalies persisted throughout the summer of 2021 (Fig. 3b) suggests the importanceof the intraseasonal variations inherent in the tropical–
extratropical atmosphere. That is, the influence of theshorter timescale variability (< 1 month) in the atmosphere overlapped with that of the tropical oceanic anomalies varying in the longer timescale of more than several
months, which resulted in the extremely heavy rainfall
event. We finally provide a brief discussion about the formation process of the heat source anomalies in view of
the tropical intraseasonal oscillation (ISO) of convective
activity in this subsection.
Figure 10a shows the time–longitude cross section of
the OLR and velocity potential anomalies in the tropics, which implies the eastward propagation of anomalous convective activity like the MJO (Madden andJulian 1972). The suppressed convective activity, found4 Concluding remarks
In this study, large-scale atmospheric and oceanic fields
during heavy rainfall over Japan in August 2021 and therelated dynamic mechanisms were examined from a teleconnection perspective. The essential tropical and extratropical anomalies that resulted in the prolonged heavy
rainfall are illustrated schematically in Fig. 11. During
the heavy rainfall period, the atmospheric circulation
field was characterized by the enhanced OH, Korean
trough, and NWPSH, which were identified as a threedimensional meridional tripolar structure of anomalous
circulations. They have been proposed to be dynamicallylinked with each other through the following sequential mechanism: The surface OH was closely associatedwith the upper-level wave breaking over eastern Siberia and exhibited a northward tilt with height. South of

Kuramochi et al. Progress in Earth and Planetary Science(2023) 10:69

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Fig. 10 a Time-longitude cross section of anomalies of OLR (shading; unit: W ­m−2) and 200-hPa velocity potential (contours; unit: ­106 ­m2 ­s−1)
averaged in the tropics [10°S–10°N] from June 1 to August 31, 2021. A 5-day running mean was applied. The positive (negative) anomaly of velocity
potential indicates anomalous convergence (divergence) of winds in the upper troposphere and the zero contours were omitted. b, c Phase space
diagrams of MJO and BSISO, respectively, during the period of July 11–August 31, 2021 with an interval of 1 day. The dots enclosed by the blackborder line represent the heavy rainfall period [August 5–20, 2021]

the blocking high, synoptic depressions detected as cutoff lows or troughs intermittently moved and remained
over the Korean Peninsula, which was important forstrengthening the stationary Korean trough. Depression tracking analysis using the COL index revealed that

one of the upper-level depressions was generated overcentral Siberia 11 days before the heavy rainfall peak.
The stationary Korean trough and stagnated synoptic
depressions were concurrent with the blocking high overeastern Siberia. Simultaneously, stationary Robby wave

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sp

he

re

Synoptic
depressions
KoreanRossby wave
Troughpropagation

Blocking
High

Wave Breaking

ISO

rfa

ce

+Q

Nor
thw
a

rd ti
l

t

Up

pe

rt

ro

po

AC

infallHeavy raMoisture

NWPSH

OH

Su

Indian Ocean

SST+
Maritime Continent

20N
EQ

Warm Kelvin wave
SST−

40N

−Q
Matsuno-Gill
response

SST−
W. PacificDateline

Fig. 11 Schematic illustration of predominant anomalies and mechanisms brought the prolonged heavy rainfall to Japan in August 2021

propagation from upstream may have contributed to the
Korean trough. The southeastward Rossby wave emanating from the Korean trough further excited the anticyclonic circulation anomaly over the northwestern Pacificin the upper troposphere. This anomalous anticyclone
may have afforded the enhancement and maintenance ofthe surface NWPSH, which exhibited a northward-tilting
structure with height due to the background zonal temperature gradient over the northwestern Pacific. These
results suggest that the meridional tripolar circulation
anomalies were dynamically correlated, and their coherent amplification brought record heavy rainfall to Japanthrough increased moisture transport, convergence, anddynamically induced ascent.
Furthermore, the pair of reinforced tropical convection over the eastern Indian Ocean and suppressed
convection over the tropical western–central Pacificintensified the NWPSH (Fig. 11). The diabatic heating contribution over the eastern Indian Ocean may
be associated with the warm equatorial Kelvin wave
accompanied by the surface Ekman divergence off theequator, whereas the anomalous cooling over the tropical central Pacific can be understood as the Matsuno–
Gill type heat-induced atmospheric response. The
suppressed convective activity over the tropical western Pacific may involve the local convection–circulation feedback with the NWPSH. The tropical oceanic
anomalies characterized by the negative-phase IOD

and multi-year La Niña, that can promote the anomalous convection, persisted throughout the summer of2021. Moreover, the anomalous convective activity
was closely associated with the phase transition andamplification of the tropical ISO over the Indo–western Pacific Ocean sector, resulting in the intensified
NWPSH and heavy rainfall. The simultaneous occurrence of intraseasonal convective modulations in thetropics and the dynamically correlated tripolar structure of anomalous circulations in the extratropics canbe regarded as key factors resulting in the heavy andprolonged rainfall in August 2021.
From the ENSO and IPOC perspectives, it is controversial that heavy rainfall in August 2021 occurredunder multi-year La Niña conditions. The NWPSH
tends to be stronger in the post-El Niño summer, whichis associated with the capacitor effect of the Indian
Ocean (e.g., Xie et al. 2009, 2016). Indeed, Naoi et al.
(2020) showed the weakening of the NWPSH and theresultant decrease in the occurrence frequency of
atmospheric rivers in the vicinity of Japan during the
La Niña summer continued from the preceding-winter La Niña. Furthermore, previous studies indicatedthe warmer conditions in Japan and dry conditions incentral-eastern China during the summer between thewintertime maturity of multi-year La Niña (Ueda andKawamura 2004; Iwakiri and Watanabe 2020; Huang
et al. 2022). These previous indications, as well as

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our results, suggest that the highlighted effects of thetropical ISO, IOD, and extratropical tripolar structure
consisting of the OH/blocking, Korean trough, and
NWPSH were prioritized over the tropical Pacific SST
forcing in the August 2021 event.
Although this study attempted to clarify the dynamic
relationship of the circulation pattern by focusing on an
extreme event, we plan to verify whether the synchronicity mechanism shown in this study occurs in other heavy
and prolonged rainfall events as well. In recent summers,
disastrous heavy rainfall associated with stagnated rain
fronts has occurred over East Asia approximately every
year, such as in July 2018 (e.g., Shimpo et al. 2019), June–
July 2020 (e.g., Ueda et al. 2021; Horinouchi et al. 2021),
August 2021 (this study), and August 2022 (JMA 2022).
This may be related to the 2–3- and 3–5-year oscillations in the NWPSH (Sui et al. 2007; Wu and Zhou
2008; Chung et al. 2011) as well as to the anthropogenic
global warming. The heavy rainfall event in August 2021,
which was associated with the stagnated Baiu-like rain
front (Fig. 1), can be referred to as a “re-appearance” ofthe Baiu front in late summer. Baiu has been interpreted
in view of the seasonal evolution of the Asian monsoon(Kawamura and Murakami 1998; Ueda et al. 2009), andthe transition of climatological fields from the preceding
season to the fall rainfall season (Akisame in Japanese)should be analyzed to further understand the re-appearance of the Baiu front and its relationship/difference with
the Akisame front.

Acknowledgements
The authors are grateful to Dr. Satoru Kasuga for providing the COL index dataand exciting discussion, and to Mr. Shunsuke Hashimoto for his assistancein our LBM experiments. We would also like to thank the two anonymous
reviewers for providing constructive comments on an earlier version of thispaper.

Abbreviations
AS	Average slope
BSISO	Boreal summer intraseasonal oscillation
COL	Cutoff low
ENSO	El Niño–Southern oscillation
IOD	Indian ocean dipole
IPOC	Indo–western Pacific Ocean capacitorISO	Intraseasonal oscillation
JMA	Japan Meteorological Agency
LBM	Linear baroclinic model
MJO	Madden–Julian oscillation
NWPSH	Northwestern Pacific subtropical highOH	Okhotsk highPV	Potential vorticity
QG	Quasi-geostrophic
SST	Sea surface temperature

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​023-​00598-4.
Additional file 1. Fig. S1. Vertically integrated Q-vectors and their divergence in August 11–15, 2021. Fig. S2. Snapshots of spatial distributionsof ­AS+ at 200 and 500 hPa from 00 UTC 10 to 12 UTC on August 14, 2021
with 12-h intervals. Fig. S3. Atmospheric responses to the tropical positive
and negative heat-source anomalies in the LBM.

Author contributions
MK, HU, MH, and KT conceived and designed the study. MK analyzed the data,
performed the experimental study, created the figures, and wrote the manuscript. MH provided the COL index data and KT provided the code for QGPV
inversion, respectively, to the corresponding author. HU, TI, MH, and KT helpedin the interpretation of the results and the construction of the manuscript. All
the authors have read and approved the final version of the manuscript.
Funding
This work was supported by JSPS KAKENHI (grant number 21H00626).
Availability of data and materials
The JRA-55 dataset, COBE-SST, and AMeDAS are provided by the JMA (https://​jra.​kishou.​go.​jp/​JRA-​55/​index_​en.​html; https://​ds.​data.​jma.​go.​jp/​tcc/​tcc/​index.​
html; https://​www.​jma.​go.​jp/​jma/​index.​html). The GPCP data are provided at
NOAA NCEI (https://​www.​ncei.​noaa.​gov/) and OLR data are provided at NOAAPSL (https://​psl.​noaa.​gov/). The RMM data are available at http://​www.​bom.​gov.​au/​clima​te/​enso/#​tabs=​Tropi​cs&​tropi​cs=​Histo​r y-​to-​now, and the BSISO
index is available at https://​www.​apcc21.​org/​ser/​moni.​do?​lang=​en#​grap1.
Documents and codes for the LBM are available at https://​ccsr.​aori.u-​tokyo.​ac.​jp/​~lbm/​sub/​lbm.​html.

Declarations
Competing interests
The authors declare that they have no competing interest.
Author details
1
Graduate School of Science and Technology, University of Tsukuba, 1‑1‑1
Tennodai, Tsukuba, Ibaraki 305‑8572, Japan. 2 Faculty of Life and Environmental
Sciences, University of Tsukuba, 1‑1‑1 Tennodai, Tsukuba, Ibaraki 305‑8572,
Japan. 3 Faculty of Science, Niigata University, 8050 Ikarashi 2‑no‑cho, Nishi‑Ku,
Niigata 950‑2181, Japan. 4 Department of Astrophysics and Atmospheric Sciences, Faculty of Science, Kyoto Sangyo University, Motoyama, Kita‑Ku, Kyoto,
Kamigamo 603‑8555, Japan.
Received: 22 June 2023 Accepted: 7 November 2023

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