Impact of anomalous Eurasian blocking activities on the East Asian Meiyu rainfall

An extreme East Asian Meiyu rainfall in both amount and duration occurred along Yangtze River valley during June–July of 2020, however, possible mid-high latitude signatures causing this super Meiyu have not been well identified. This study explores the cause of the Meiyu rainfall from the aspect of anomalous Eurasian blocking activities with a two-dimensional blocking index, using the Japanese 55-yr Reanalysis for 1979–2020. The major findings are as follows. Variabilities of the Eurasian blocking activities are primarily characterized by a tripole pattern with three centers over the Baltic Sea, the Ural Mountains, and the Sea of Okhotsk, respectively. The tripole pattern is associated with two zonally-oriented Rossby wavetrains which may originate from the rainfall anomaly in central Europe. Corresponding to a positive phase of the tripole pattern, the northern wavetrain through energy dispersion tends to induce an anomalous anticyclone (i.e., enhanced blocking) over the Sea of Okhotsk, while the southern wavetrain tends to induce an anomalous cyclone over the Sea of Japan which is conducive to a southward intrusion of more high-latitude cold airs. Consequently, a meridional dipole anomalous circulation pattern over the northeastern Asia is formed, favoring more East Asian Meiyu rainfall. In 2020, the Eurasian blocking activities exhibit a significantly positive phase of the tripole pattern and considerably contribute to the super Meiyu. The results identified in this study highlight the importance of the Eurasian blocking activities in the East Asian Meiyu and provide a new perspective for the prediction of the Meiyu rainfall with mid-high latitude signatures.


Introduction
The unique East Asian rainy season from mid-June to mid-July is identified as Meiyu in China (Ding et al. 2020), Baiu in Japan (Ninomiya and Shibagaki 2007), and Changma in South Korea (Oh et al. 1997), which has attracted great attention from the international community in the past few decades Li et al. 2008). The 2020 Meiyu rainfall is recognized as a 'super Meiyu' due to the strongest rainfall since the 1960s as well as an anomalous long duration (Liu et al. 2020a;Ding et al. 2021), In 2020, the severe flood exerts enormous impacts on the economic development and human life along the middle-lower reaches of the Yangtze River valley (YRV) in eastern China. Meanwhile, many regions throughout East Asia also experienced this historically extreme rainfall event (Takaya et al. 2020;Park et al. 2021;Wang et al. 2021). However, recent studies have shown that the commonly used signatures for the prediction of Meiyu are not significant in 2020 and could only explain part of this super Meiyu (Takaya et al. 2020;Zhou et al. 2021). Therefore, the physical mechanism responsible for the Meiyu rainfall is still a noteworthy research issue.
Previous studies highlighted that the interannual variation of Meiyu rainfall is directly modulated by low latitude components of the East Asian summer monsoon (EASM), including the western Pacific subtropical high (WPSH) and the South Asian high (Zhou and Yu 2005;Wang et al. 2008;Ding et al. 2020). In particular, the intensified WPSH can enhance the northward transportation of water vapor from the tropical ocean to the YRV region. Moreover, the interannual variation of Meiyu rainfall is affected by sea surface temperature (SST) anomalies in the tropical Pacific and the Indian Ocean Feng et al. 2011;Kosaka et al. 2011;Xie et al. 2016;Takaya et al. 2020). Specifically, the impact of El Niño-Southern Oscillation (ENSO) on the East Asian Meiyu, which is associated with the developing or decaying phase of an ENSO event, also attracts extensive attention (Huang and Wu 1989;Chang et al. 2000;Fowler et al. 2008). As a significant predictor of the Meiyu rainfall, El Niño events generally lead to positive rainfall anomalies along around YRV in El Niño decaying summer . According to previous studies, the WPSH plays an important role in conveying the El Niño impact on the East Asian Meiyu rainfall (Li et al. 2017). In addition, the preceding winter El Niño has a close relationship with the warmer Indian ocean and leads to a positive rainfall anomaly in YRV through the impact on WPSH (Xie et al. 2016).
Although the above tropical anomalies can affect the Meiyu rainfall, those signatures are not in significant anomalous phases in 2020 and could only explain part of this super Meiyu (Takaya et al. 2020;Zhou et al. 2021). Compared with the tropical signatures, the extratropical signatures, which also play an important role in the EASM system, have been paid relatively little attention in previous studies. The Eurasian mid-to-high latitude large-scale circulation includes the Eurasian blocking activities, of which the Sea of Okhotsk blocking high can favor the southward intrusions of high-latitude cold-dry air into East Asia and thus enhance the Meiyu rainfall (Wang 1992;Li et al. 2001Li et al. , 2010Niu and Jin 2009;Chen and Zhai 2014;Park and Ahn 2014). In 2020, the high-latitude blocking activities over East Siberia exert an important effect on the Meiyu-Baiu rainfall and result in this super Meiyu according to the recent studies (Chen et al. 2021;Ding et al. 2021).
Many previous studies explored the relationship between East Asian Meiyu and Eurasian blocking activities based on case studies (Wang 1992;Chen et al. 2007). Furthermore, to reflect the climatic effect of anomalous blocking activities on East Asian climate variabilities, several onedimensional blocking indices are defined (Rex 1950;Dole 1978;Hartmann and Ghan 1980;Dole and Gordon 1983;Lejenäs and Økland 1983;Shukla and Mo 1983;Tibaldi and Molteni 1990;Lupo and Smith 1995). Among them, the T&M blocking index proposed by Tibaldi and Molteni (1990) has been widely used to reveal the zonal distribution of blocking activities. Two-dimensional blocking indices are further developed to reflect the two-dimensional spatial distribution of blocking highs (Pelly and Hoskins 2003;Diao et al. 2006;Davini et al. 2012;Masato et al. 2012;Kim and Ha 2015;Nakamura and Huang 2018), while they are seldom used to demonstrate the impact of blocking highs on the Meiyu rainfall in previous studies.
In this study, the effect of anomalous Eurasian blocking activities on the Meiyu rainfall on the interannual timescale is firstly explored with the two-dimensional blocking index proposed by Davini et al. (2012). Then, the atmospheric circulation anomalies associated with the anomalous Eurasian blocking activities are investigated from the view of quasi-stationary Rossby waves. In particular, the impact of anomalous Eurasian blocking activities on the Meiyu rainfall is used to explain one of the causes of the super Meiyu in 2020. The rest of the paper is organized as follows. Section 2 describes the data and methods used in this study. Section 3 shows the characteristics of the leading mode of anomalous Eurasian blocking activities and the significant effect of this mode on the Meiyu rainfall. Section 4 illustrates the physical mechanisms responsible for the Eurasian blocking mode and its climate effect on East Asian precipitation. Section 5 analyzes the 2020 'super Meiyu' as a typical case to highlight the important impact of blocking high on the Meiyu rainfall. The final section is devoted to summary and discussion.

Data
To obtain the characteristic of precipitation during the Meiyu period, daily precipitation data with a 0.5° × 0.5° horizontal resolution provided by NOAA Climate Prediction Center (CPC) is used in this study. The climatological time-latitude section (averaged between 110 and 123 °E) of summer (June, July, August, JJA in short) rainfall is shown in Fig. 1. The precipitation in eastern China mainly concentrates south of 28 °N in early-June, and shifts northward gradually to YRV afterward (area between the two red lines in Fig. 1). The rainband mainly locates in YRV from mid-June to late-July. In August, the precipitation is decreased and the rainband becomes obscure. Therefore, based on the climatological evolution of the precipitation, the Meiyu period is defined as the months of June and July in this study. The time period is from 1979 to 2020, 42 years in total.
Daily atmospheric variables, including zonal and meridional winds, geopotential height, air temperature, and specific humidity at 27 standard pressure levels from 1000 to 100 hPa, as well as sea level pressure (SLP), are taken from the Japanese 55-yr Reanalysis (JRA-55), the second global atmospheric reanalysis conducted by the Japan Meteorological Agency with horizontal resolution at 1.25° × 1.25° (Kobayashi et al. 2015;Harada et al. 2016). NCEP-NCAR Reanalysis dataset at a 2.5° spatial resolution (Kalnay et al. 1996) is used to confirm the robustness of results. Daily solar radiative heating rate, longwave radiative heating rate, large-scale condensation heating rate, convective heating rate, and vertical diffusion heating rate from JRA-55 with the same resolution are used to calculate the total diabatic heating. Besides, daily radiative and surface turbulent heat flux, skin temperature, and air temperature at 2 m from JRA-55 with the same resolution are used to diagnose the surface energy balance. The latest generation of ECMWF atmospheric reanalysis (ERA5) with a horizontal resolution of 1° × 1° (Hersbach et al. 2020) is also used to diagnose the surface energy balance for comparison. In this study, the climatological state is averaged from 1981 to 2010 during Meiyu period. To reduce the effect of global warming, the linear trends are removed for all the data.

Method
The indices of blocking activities can be defined based on the type, lifetime, and other properties (Tibaldi and Molteni 1990;Scherrer et al. 2006). In this study, we focus on the frequency of blocking activities and choose to use the two-dimensional blocking index calculation developed by Scherrer et al. (2006) and Davini et al. (2012). This index is mainly defined based on the meridional gradient of 500 hPa geopotential height. For a given grid point ( 0, Φ 0 ) , the meridional gradient of 500 hPa geopotential height in its southern (GHGS) and northern (GHGN) flank is defined by: and where λ 0 ( Φ 0 ) denotes the longitude (latitude), Φ S and Φ N are the southern and northern boundary latitude defined as When both GHGS 0, Φ 0 > 0 and GHGN 0, Φ 0 < −10 , a local instantaneous blocking index (IB) is defined and set as one on that grid point, otherwise set as zero. The two-dimensional blocking index proposed by Diao et al. (2006) is also used to check the robustness of results. The empirical orthogonal function (EOF) analysis (Lorenz 1956) is applied to extract the leading mode of the blocking frequency variability over the Eurasian continent (10-160 °E, 45-75 °N) during Meiyu period. Linear regression method is used to obtain the Eurasian blocking-related rainfall and atmospheric circulation anomalies. The statistical significance is assessed with the Student's t-test. The Rossby wave source (Sardeshmukh and Hoskins 1988) and wave activity flux (WAF, Takaya and Nakamura 2001) are used to analyze the wave energy propagation-related processes.

The leading mode of anomalous Eurasian blocking activities and its impact on Meiyu rainfall
In climatology, spatial distributions of blocking frequencies over the Eurasian continent display significant blocking activities over three centers: the Baltic Sea, the Ural Mountains, and the Sea of Okhotsk (Fig. 2a), which is also pointed by other studies (Li and Ding 2004;Park and Ahn 2014). The blocking frequencies in these blocking centers reach about 10 days per Meiyu period. Corresponding to 1 3 the spatial distributions of blocking activities, the climatological atmospheric geopotential height over the Eurasian continent is characterized by a "two-ridge and one-trough" pattern during Meiyu period. Following Davini et al. (2012), more strict criteria on special scale (grids at least 15 continuous longitudes are blocked) and time persistence (last more than 5 days) are applied on IB to test the sensitivity. As shown in Fig. 2b, the spatial distribution of the strict blocking frequency basically is consistent with that of the pure IB index, albeit the amplitude is relatively weaker. To reduce possible discontinuity, the pure IB index is used here. Similar results (not shown) can be obtained based on the definition of blocking frequency by Diao et al. (2006). Over the mid-to-high latitudes of the Eurasian continent, the leading EOF mode of blocking frequency anomalies is characterized by a tripole pattern during Meiyu period (Fig. 3a). Compared with the climatology (Fig. 2a), the Baltic Sea blocking and the Sea of Okhotsk blocking are significantly enhanced. The northern part of the Ural blocking is weakened and the southern part of it is enhanced, indicating a slight southward movement of the Ural blocking. The corresponding time series (tripole pattern index, TPI) explains 20.6% of the total variance and exhibits obvious year-to-year differences, indicating a large variation of blocking frequency during Meiyu period on interannual timescale (bar in Fig. 3b). The second and third EOF modes of blocking frequency anomalies account for 13.4% and 11.8% of the total variation, respectively. Only the first EOF mode can pass the North test (North et al. 1982), which is mainly analyzed in this study.
Previous studies indicated that the blockings over the Eurasian continent may exert a significant influence on the East Asian Meiyu rainfall (Li and Ding 2004;Niu and Jin 2009;Chen et al. 2021;Ding et al. 2021). As shown in Fig. 4, corresponding to a positive phase of the tripole pattern of anomalous Eurasian blocking activities, the rainfall anomalies linearly regressed upon TPI display a dipole pattern in East Asia with the decreased precipitation along the Yellow River and northern Korean Peninsula, but increased precipitation in YRV and its southern area, southern Korean Peninsula, and southern Japan. A Meiyu rainfall index (black lines in Fig. 4b) is defined by the standardized area-averaged (red box in Fig. 4a) precipitation anomalies during Meiyu period. The year-to-year variation of Meiyu rainfall anomaly resembles to that of the tripole blocking frequency anomalies, especially in recent three decades (black line vs. bar in Fig. 4b). The 42-year correlation coefficient between the Meiyu index and TPI is 0.40 (passing the Student's t-test at a 0.05 significance level), showing that there exists a significant relationship between the tripole pattern of Eurasian blocking activities and the Meiyu rainfall anomaly. Noticeably, the magnitude of the Meiyu index in 2020 is outstanding and exceeds 3.5 standard deviations, which is the largest in the past 40 years. Meanwhile, TPI reaches 0.88 standard deviation, indicating a strong positive phase of the tripole pattern of Eurasian blocking activities occurs in the year 2020.
Similarly, through linear regression upon TPI, the atmospheric anomalies that are related to the positive phase of the tripole pattern of anomalous Eurasian blocking activities during Meiyu period are obtained (Fig. 5). During the positive phase of the tripole pattern, there is an atmospheric cyclonic wind anomaly over around the Yellow Sea and an anticyclonic wind anomaly over the South China Sea on 850 hPa (vectors in Fig. 5b). Consequently, the moisture flux anomaly is convergent between 20 and 30 °N over eastern Asia (shaded in Fig. 5a), favoring an increased rainfall anomaly there (Fig. 4a). Meanwhile, the Yellow River and its northern area are controlled by 850 hPa cold temperature advection anomaly, and the area south of 30 °N in the East Asian continent is controlled by warm temperature advection anomaly (shaded in Fig. 5b). The cold air from mid-to-high latitudes and the warm air from lower latitudes are against to each other over YRV, enhancing the meridional temperature Fig. 3 a Horizontal distribution of the first EOF mode of Eurasian instantaneous blocking frequency anomalies (units: day 60 day −1 ) for the period 1979-2020 during Meiyu period, represented by regression upon (b) the standardized time series of the EOF mode (PC1). Black contours in (a) are the same as in Fig. 2, denoting the climatology of 500 hPa geopotential height (units: gpm) during Meiyu period gradient and favoring the persistent rainfall there. Therefore, influenced by signatures from both mid-to-high latitudes and the lower latitudes, the rainfall in southern East Asia is increased during Meiyu Period.

Dynamical analysis on the relationship between Eurasian blocking activities and Meiyu rainfall
Over the midlatitude Eurasian continent, the energy propagation of Rossby waves plays an important role in connecting remote atmospheric anomalies (Enomoto 2004;Chowdary et al. 2019;Li et al. 2021b;Wang et al. 2017). Accordingly, the TPI-related upper-level WAF, Rossby wave source, and corresponding geopotential height anomalies are shown in Fig. 6. Corresponding to a positive phase of the tripole pattern of anomalous Eurasian blocking activities, the  Fig. 6a), while there is an important source of the Rossby waves over central Europe (shaded in Fig. 6b). The energy propagations of the Rossby wavetrains associated with the tripole pattern of anomalous Eurasian blocking activities are basically along two zonally-oriented pathways: the northern branch along which the energy propagates northeastward to the Ural area and the Kara Sea, then southeastward to the Far East, and finally to the northwest of North Pacific, and the southern branch along which the energy propagates to the south of the Ural area, then southeastward to central Asia and Mongolian area, and finally to the Korean Peninsula. For both pathways, the amplified WAF over the Ural area is important. The northern pathway locates in the mid-to-high latitudes, and tends to enhance the Sea of Okhotsk blocking. Meanwhile, the southern pathway locates mainly in the midlatitudes, and tends to induce a negative geopotential height anomaly over the Sea of Japan (Fig. 6a). The corresponding northerly wind anomalies associated with the anomalous cyclonic circulation over East Asia help the transportation of cold/dry air from high to lower latitudes (Fig. 5), which is in favor of the formation of rainfall increase along YRV. Some recent studies also pointed out that the Rossby wavetrains from Europe such as the Silk Road pattern Liu et al. 2020b) and the British-Okhotsk Corridor pattern (Li et al. 2021c;Xu et al. 2022) could affect the climate in East Asia.
To explore the causation of the anomalous Rossby wave source over central Europe, Fig. 7 shows the total diabatic heating anomaly over central Europe in vertical direction and its components, including vertical diffusion, latent heats from convective processes and large-scale condensation, longwave and shortwave radiations. The diabatic heating anomaly over central Europe is positive between 700-300 hPa and negative below 700 hPa (Fig. 7a). The vertical distribution of diabatic heating anomaly is mainly induced by the moisture condensation latent heat released  The crosses indicate the regions exceeding 95% confidence level with the Student's t-test from convective processes (Fig. 7c), albeit the longwave radiation partly contributes to the low-level diabatic cooling anomaly (Fig. 7e). From the view of horizontal distribution (Fig. 8), during the positive phase of the tripole pattern of anomalous Eurasian blocking activities, the precipitation anomaly as well as the diabatic heating anomaly is increased over around central Europe (Fig. 8a, b). The similar horizontal distributions of regressed precipitation and vertically integrated diabatic heating anomalies confirm again that the diabatic heating anomaly is mainly determined by moisture condensation. Resultantly, the atmospheric circulation anomalies display a divergent field in the upper troposphere, conducive to a formation of the Rossby wave source there.
In order to further investigate the role of the central Europe rainfall anomalies, a central Europe rainfall index is defined with the area-averaged precipitation in central Europe (red box in Fig. 8a). As shown in Fig. 4b, the correlation coefficient between the Meiyu rainfall index and the central Europe precipitation index (black vs. purple lines in Fig. 4b) is 0.31, which passes the Student's t-test at a 0.05 significance level. Similarly, linear regressions of atmospheric variables upon the standardized central Europe precipitation index are performed. When the rainfall anomaly in central Europe is increased (Fig. 9a), the atmospheric diabatic heating anomaly is positive (Fig. 9b) and leads to an anomalous upper-level divergence (Fig. 9c), which tends to generate a Rossby wave source (Fig. 10c). Correspondingly, the Rossby wave propagations present as a wavetrain over the European area and split into two pathways downstream of the Ural area (Fig. 10b, c), which are similar to those two Rossby wavetrains that are associated with the tripole pattern of anomalous Eurasian blocking activities (Fig. 6b). The northern Rossby wavetrain strengthens the blocking highs over the Europe area and north of the Sea of Okhotsk (Fig. 10a), resembling that associated with the tripole pattern (Fig. 3a). Meanwhile, the southern Rossby wavetrain tends to induce a negative geopotential height anomaly over the Korean Peninsula and East China Sea, which tends to transport cold/dry to the north of YRV (Fig. 11c) and leads to a convergent atmospheric moisture flux anomaly over YRV (Fig. 11b). Resultantly, the rainfall in YRV and south of Japan is anomalously increased during Meiyu Period.
Except for the formation of the Rossby wave source over central Europe, the largely amplified WAF over the Ural Fig. 11 As in Fig. 10, but for a precipitation (shading; units: mm day −1 ), b vertically integrated water vapor fluxes (vectors; units: kg m −1 s −1 ) and their divergence (shading; units: 10 -5 kg m −2 s −1 ), and c 850 hPa temperature advection (shading; units: 10 -6 K s −1 ) and horizontal wind (vector; units: m s −1 ) anomalies over the East Asian domain. The red box denotes the key region of eastern China ▸ area is also essential for both pathways as mentioned before. Previous studies noticed that the local surface feedback processes are important in the formation and maintenance of the Ural blocking activities (Seneviratne et al. 2010;Zhang et al. 2018b). Figure 12 illustrates the TPI-regressed anomalies for variables in the surface energy balance equation, including surface shortwave and longwave radiations, surface turbulent sensible and latent heat fluxes, air-land temperature differences (skin temperature minus 2 m temperature), and skin temperature. Taking the Ural area as an example (the leftmost red box in Fig. 12), when the geopotential height over the Ural area is anomalously high, the downward net shortwave radiation is increased, which warms the surface (Fig. 12a, f). The resultant upward net longwave radiation and sensible and latent heat flux (Fig. 12c, d) anomalies induce a positive diabatic heating anomaly in the boundary layer, which leads to a negative vertical gradient anomaly of diabatic heating. According to the quasi-geostrophic potential vorticity equation (Fang and Yang 2016;Tao et al. 2019Tao et al. , 2020, the negative vertical gradient anomaly of diabatic heating tends to produce a positive geopotential height tendency anomaly near the surface, thus forming a local positive feedback. Similar to the positive feedback over the Ural area, the geopotential anomalies over Mongolia, Siberian Plateau, and the Far East also tend to be strengthened by the air-land interactions, which can amplify WAF. On this regard, the local air-land interaction tends to act as a "gas station" during the propagation of the Rossby wavetrains. The existence of "gas stations" may explain why the energy of atmospheric anomalies over central Europe could efficiently propagate to the North Pacific region and influence the Meiyu rainfall.

Impact of anomalous Eurasian blocking activities on 2020 super Meiyu rainfall
In Sects. 3 and 4, the impact of anomalous Eurasian blocking activities on Meiyu rainfall and associated physical processes are investigated by regression analysis. As mentioned before, the Meiyu rainfall in YRV reaches its highest value in 2020. In this section, possible contribution of anomalous Eurasian blocking activities to 2020 super Meiyu is further analyzed.
During the 2020 Meiyu period, rainfall is anomalously increased along YRV and in southern Japan, of which the maximum value exceeds 20 mm day −1 . The distribution of precipitation anomaly in 2020 (Fig. 13a) resembles to that of the regression upon TPI (Fig. 4a). Although the cyclonic wind anomaly in the Korean Peninsula is relatively weak, the anomalous northerly wind associated with the cyclonic wind anomaly favors the atmospheric moisture flux convergence over YRV (Fig. 13b) and the cold air temperature advection north of YRV (Fig. 13c), helping the Meiyu rainfall stay along YRV.
During the 2020 Meiyu period, TPI and the central Europe precipitation index are 0.88 standard deviation and 1.30 standard deviations, respectively. The Eurasian blocking activities in 2020 are anomalously strengthened over the Baltic Sea and the north of the Sea of Okhotsk and weakened over the north of the Ural Mountains (Fig. 14a), basically displaying a tripole distribution (Fig. 3a). The precipitation is anomalously increased over central Europe (Fig. 15a), inducing a positive diabatic heating anomaly (Fig. 15b). Consequently, the atmospheric circulations are divergent in high-level tropopause (Fig. 15c), which leads to a Rossby wave source there (Fig. 14c). The Rossby wave energy originates from central Europe and propagates eastward through northern and southern branches (Fig. 14b, c). Associated with the two wavetrains, a negative geopotential height anomaly occurs over northeastern Asia and the anomalous negative geopotential height extends to the Korean Peninsula (Fig. 14b). The corresponding anomalous northerly wind of the anomalous cyclonic circulation over the Korean Peninsula is conducive to a southward intrusion of more high-latitude cold airs and thus in favor of the super Meiyu in 2020. Therefore, although the tropical influence may be the primary cause of the record-breaking Meiyu rainfall anomaly in 2020, the mid-to-high latitude anomalies also provide a favorable condition for the large amount and long persistence of the heavy rainfall.

Summary and discussion
A number of the existing studies indicate that tropical air-sea anomalies play an important role in the formation of extreme Meiyu rainfall. However, the impact of mid-to-high latitude anomalies on this super Meiyu rainfall is still an open question. In this study, the cause of anomalous Meiyu rainfall is explored from the aspect of anomalous Eurasian blocking activities. Based on the JRA-55 reanalysis dataset from 1979 to 2020, the anomalous Eurasian blocking activities are represented by a two-dimensional blocking index. The leading EOF mode of the anomalous Eurasian blocking activities during Meiyu period shows a tripole pattern. During a positive phase of the tripole pattern, two positive centers located over the Baltic Sea and the Sea of Okhotsk, and one negative center located over the north of the Ural Mountains. As illustrated in a schematic diagram (Fig. 16), the tripole pattern is associated with two zonally-oriented Rossby wavetrains that may originate from the rainfall anomaly in central Europe. Corresponding to a positive phase of the tripole pattern, the northern wavetrain through energy dispersion tends to induce an anomalous geopotential high over the Sea of Okhotsk, and the southern wavetrain tends to induce an anomalous geopotential low over the Sea of Japan. The anomalous cyclonic circulation over the Sea of Japan leads to a southward intrusion of more high-latitude cold airs, and helps increase East Asian Meiyu rainfall.
During June-July of 2020, a record-breaking East Asian Meiyu rainfall occurred along the Yangtze River valley. The anomalous Eurasian blocking activities also exhibit a positive phase of the tripole pattern in 2020. Associated with two zonally-oriented Rossby wavetrains, more high-latitude cold-dry airs are transported southward to lower latitudes, which is conducive to an increase of the Meiyu rainfall in East Asia. However, the Meiyu index in 2020 exceeds 3.5 standard deviations, while TPI in 2020 is 0.88. Previous studies pointed out that the warming Indian Ocean makes a considerable contribution to the increased Meiyu rainfall in 2020 through the intensification and westward extension of WPSH (Takaya et al. 2020;Ding et al. 2021;Fang et al. 2021;Li et al. 2021a;Liang et al. 2021;Tang et al. 2021;Wang et al. 2021;Zhao et al. Fig. 15 As in Fig. 13, but for a precipitation (shading; units: mm day −1 ), b 1000~100 hPa integrated diabatic heating (shading; units: W m −2 ), and c divergence (units: shading; 10 -7 s −1 ) anomalies during Meiyu Period in 2020 over the European domain. The red boxes denote the central Europe 2021; Zheng and Wang 2021;Cai et al. 2022). The relative contributions of mid-to-high latitude and tropical signals to the Meiyu rainfall anomaly need to be further evaluated in the future work. Although the tropical influence may be the primary reason for the record-breaking Meiyu rainfall anomaly, the anomalous Eurasian blocking activities also make a positive contribution to the super Meiyu. The mid-to-high latitude anomalies provide a favorable condition for the large amount and long persistence of the heavy Meiyu rainfall. In this regard, the results identified in this study provide a new perspective for the prediction of Meiyu rainfall by using mid-to-high latitude signals.
In this study, the impact of the Eurasian blocking activities on East Asian Meiyu rainfall is highlighted. Previous studies indicated that the anomalous Eurasian blocking activities may be related to the early spring Arctic sea ice cover (Wu et al. 2013;Petrie et al. 2015;Zhang et al. 2018aZhang et al. , 2018b, the subseasonal phase transition of the North Atlantic Oscillation (Luo et al. 2015(Luo et al. , 2016Liu et al. 2020a), and a tripole sea surface temperature anomaly pattern in the North Atlantic (Wu et al. 2009(Wu et al. , 2011Zuo et al. 2013). A lead-lag analysis needs to be applied to explore the factors that contribute to the formation of the tripole pattern of the anomalous Eurasian blocking activities during Meiyu period in the future study.
In addition, our results show that the precipitation in central Europe can stimulate Rossby wavetrains. Some recent studies also pointed out that the European anomalies could affect the climate in East Asia by long-distance Rossby wavetrains (Liu et al. 2020b;Li et al. 2021c;Wang et al. 2017;Xu et al. 2022). Based on this study, the Rossby wavetrains may be enhanced by local air-land processes. The WAF is amplified over northeastern Europe, Mongolia, Siberian Plateau, and the Far East. The possible positive feedbacks tend to act as "gas stations", which favors the maintenance of the local atmospheric anomalies and supports the long-distance downstream energy dispersion efficiently. Furthermore, Krishnan and Sugi (2001) proposed a teleconnection, namely the Asian continental pattern (ACP). The ACP index has a high correlation of 0.59 with Meiyu rainfall and a correlation of − 0.64 with precipitation over India. The spatial distribution of ACP is basically consistent with the southern Rossby wavetrain identified in this study. Therefore, the southern Rossby wavetrain may be stimulated by both mid-to-high and low latitude atmospheric anomalies. The formation and maintenance of the wavetrains over the Eurasian continent still need further investigation by designing a series of model experiments in the future study.
Author contributions All authors contributed to the conception and design of the study. The main idea of the study was put forward by X-QY. Material preparation, data collection and analysis were performed by ZX. The initial manuscript was drafted by ZX and improved by X-QY, LT, and LS. All authors reviewed and approved the final manuscript.
Funding This study is jointly supported by the National Key Research and Development Program of China (2022YFE0106600) and the National Natural Science Foundation of China (41621005).

Availability of data and material
The Japanese 55-year Reanalysis (JRA-55) data was provided by the Japan Meteorological Agency and is available at https:// rda. ucar. edu. The daily precipitation data was produced by NOAA Climate Prediction Center (CPC) and can be obtained from https:// psl. noaa. gov/ data/ gridd ed/ data. cpc. globa lprec ip. html. The fifth generation ECMWF reanalysis (ERA-5) data was produced by ECMWF and is available from https:// www. ecmwf. int/ en/ forec asts/