Anomalous Features of Extreme Meiyu in 2020 over the Yangtze–Huai River Basin and Attribution to Large-Scale Circulations

2020年江淮流域梅雨极端异常特征及大尺度环流成因

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  • Extremely anomalous features of Meiyu in 2020 over the Yangtze–Huai River basin (YHRB) and associated causes in perspective of the large-scale circulation are investigated in this study, based on the Meiyu operational monitoring information and daily data of precipitation, global atmospheric reanalysis, and sea surface temperature (SST). The main results are as follows. (1) The 2020 YHRB Meiyu exhibits extremely anomalous characteristics, which are the most prominent since the 1980s. The 2020 Meiyu season features the fourth earliest onset, the third latest retreat, the longest duration, the maximum Meiyu rainfall, the strongest mean rainfall intensity, and the maximum number of stations/days with rainstorm. (2) The extremely long duration of the 2020 Meiyu season lies in the farily early onset and late retreat of Meiyu in this particular year. The early onset of Meiyu is due to the earlier-than-normal first northward shift and migration of the key influential systems including the northwestern Pacific subtropical high (NWPSH) and the South Asian high (SAH) along with the East Asian summer monsoon, induced by weak cold air activities from late May to early mid-June. However, the extremely late retreat of Meiyu is because of later-than-normal second northward shift of the associated large-scale circulation systems accompanied with strong cold air activities, and extremely weak and southward located ITCZ over Northwest Pacific in July. (3) The extremely more than normal Meiyu rainfall is represented by its long duration and strong rainfall intensity. The latter is likely attributed to extreme anomalies of water vapor convergence and vertical ascending motion over the YHRB, resulting from the compound effects of the westward extended and enlarged NWPSH, the eastward extended and expanded SAH, and the strong water vapor transport associated with the low-level southerly wind. The extremely warm SST in the tropical Indian Ocean seems to be the key factor to induce the above-mentioned anomalous large-scale circulations. The results from this study serve to improve understanding of formation mechanisms of the extreme Meiyu in China and may help forecasters to extract useful large-scale circulation features from numerical model products to improve medium-extended-range operational forecasts.
    本文分析了2020年江淮流域梅雨极端异常特征及其大尺度环流成因。2020年江淮流域梅雨季具有入梅早、出梅晚,长度长、梅雨量多、降雨强度强等极端异常特征。梅雨季极端异常偏长, 体现在入梅极端异常偏早和出梅极端异常偏晚。梅雨量极端异常偏多,是受梅雨季极端异常偏长和降雨强度极端异常偏强的共同影响。梅雨季降雨强度极端异常偏强,是在副高西部脊极端异常偏西和范围极端异常偏大、南亚高压东部脊极端异常偏东和范围极端异常偏大、低层偏南气流水汽输送极端异常偏强的共同影响下形成的;而热带印度洋海温极端异常偏暖是诱发上述环流极端异常特征的关键因子。本文所得结论有助于提高对极端梅雨形成机制的认识,有助于预报员从数值模式预报中提取大尺度环流特征来制作中期和延伸期梅雨预报。
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  • Fig. 1.  (a) The western RLP of the NWPSH at 500 hPa and the eastern RLP of the SAH at 200 hPa at the different percentile in the Meiyu season and (b) correlation coefficient (r) between the number of days of the RLP falling in the latitude range corresponding to each percentile interval during maximum possible duration period of Meiyu and the duration days of the Meiyu season over the YHRB from 1981 to 2020.

    Fig. 2.  (a, b) Normalized and (c, d) original time series of the extreme properties of heavy rain in the Meiyu season over the YHRB from 1981 to 2020: (a) Meiyu onset date, retreat date, and duration days, (b) Meiyu rainfall, mean rainfall intensity, and duration days, (c) sum of the daily number of stations with rainstorm (blue) or heavy rainstorm (red) during the Meiyu season, and (d) the number of stations with daily precipitation ranking the top five. Note that r in (a) and (b) denotes correlation coefficient.

    Fig. 3.  (a) Time–latitude evolution of daily precipitation averaged over 110°–123°E from May to August, and distributions of (b) accumulated precipitation and (c) its percentage anomalies during the Meiyu season over the YHRB in 2020.

    Fig. 4.  Evolution of the daily (a) western RLP of the NWPSH at 500 hPa and (b) eastern RLP of the SAH at 200 hPa from May to August in 2020. Red lines denote the original time series, black lines represent the 5-day sliding average, and green lines represent the climatological mean.

    Fig. 5.  Time–latitude cross-sections averaged over 110°–123°E of the daily (a) wind vector (arrow), meridional wind (brown line and shading), and θse isoline of 340 K (bold black line) at 850 hPa; (b) water vapor flux vector (arrow), and its meridional component (brown line) and divergence (shading) at 850 hPa; and (c) geopotential height at 500 hPa (bold black line denotes the 588-dagpm isoline of the NWPSH while black dot–dashed line shows the ridge line of the NWPSH) and temperature anomalies at 850 hPa (shading) from May to August in 2020. In (a)–(c), green dashed lines denote the latitude range of the YHRB.

    Fig. 6.  Time–latitude cross-sections averaged over 110°–140°E of daily geopotential height at 500 hPa above 586 dagpm (bold dark-red solid line shows the 588-dagpm isoline of NWPSH, dark-red dot–dashed line shows the ridge line of NWPSH, and black lines show the climatic NWPSH and its ridge line) and OLR below 220 W m−2 (blue shading; gray line denotes the climatic isoline of 220 W m−2) from May to August: (a) 2020, (b) composite for the Meiyu season with anomalously late retreat date and long duration, and (c) composite for the Meiyu season with anomalously early retreat date and short duration.

    Fig. 7.  Normalized time series of the daily (a) ITCZ intensity index over Northwest Pacific and (b) Philippines CEF index (red line) and New Guinea CEF index (green line) from May to August in 2020.

    Fig. 8.  Normalized time series of (a) ITCZ intensity index over Northwest Pacific, (b) Philippines CEF index, and (c) New Guinea CEF index, in July from 1981 to 2010.

    Fig. 9.  Mean large-scale circulations for the Meiyu season of 2020: (a) geopotential height (black solid lines) and its anomalies (shading) at 500 hPa, (b) geopotential height (black solid lines), zonal wind greater than 30 m s−1 (blue lines), and divergence (shading) at 200 hPa, (c) water vapor flux (vector) and its divergence (shading) at 850 hPa, (d) anomalies of water vapor flux (vector) and its divergence (shading) at 850 hPa. In (a) and (b), the bold black solid lines show the range of the subtropical highs at 500 hPa and the SAH at 200 hPa denoted by the 588- and 1252-dagpm isolines respectively; the black dashed lines show the ridge line of the highs; and the green lines show the climatic range and ridge line of the highs. The rectangle in (c) and (d) denotes the region 20°–33°N, 105°–123°E.

    Fig. 10.  Standardized time series of the averaged circulation characteristic indices, relative physical quantities, and the IOBW index in the Meiyu season of the YHRB from 1981 to 2020: (a) the WRP and area index of the NWPSH at 500 hPa, (b) the ERP and area index of the SAH at 200 hPa, (c) the IOBW index and area averaged vertical motion (omega) over the YHRB at 500 hPa, and (d) area averaged divergence over the YHRB and meridional component of the water vapor flux in the region 20°–33°N, 105°–123°E at 850 hPa.

    Fig. 11.  Schematic diagram showing large-scale circulation anomalies responsible for the extremely anomalous features of Meiyu in 2020 over the YHRB.

    Fig. 12.  Time–latitude cross-sections averaged over 110°–123°E of geopotential height at 500 hPa (bold black line shows the 588-dagpm isoline of NWPSH and black dot–dashed line shows the NWPSH ridge line) and temperature anomalies at 850 hPa (shading) based on the ensemble mean forecast of ECMWF initialized at 1200 UTC of (a) 22 May and (b) 20 July with lead time of 15 days and initial fields in the previous 15 days, respectively, in 2020 (purple vertical line indicates initial time; green dashed lines show the latitude range of the YHRB).

    Table 1.  Correlation coefficients between the mean rainfall intensity and averaged circulation indices, related physical quantities, and the IOBW index in the Meiyu season from 1981 to 2020

    NWPSH SAH at 200 hPaIOBW indexAverage over the YHRB
    WRPArea indexERPArea indexWater vapor flux divergence at 850 hPaOmega at 500 hPa
    −0.49#0.46# 0.31*0.33#0.44#−0.60#−0.67#
    Note: # (*) denotes the value passing the t-test at the significance level of α = 0.05 (0.10).
    Download: Download as CSV

    Table 2.  Correlation coefficients between averaged circulation characteristic indices and IOBW index in the Meiyu season from 1981 to 2020

    SAH at 200 hPaIOBW index
    ERPArea index
    WRP of NWPSH−0.59#−0.62#−0.53#
    Area index of NWPSH 0.75# 0.81# 0.63#
    Note: # denotes the value passing the t-test at the significance level of α = 0.05.
    Download: Download as CSV
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Anomalous Features of Extreme Meiyu in 2020 over the Yangtze–Huai River Basin and Attribution to Large-Scale Circulations

    Corresponding author: Panmao ZHAI, pmzhai@cma.gov.cn
  • 1. National Meteorological Center, China Meteorological Administration, Beijing 100081
  • 2. Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081
  • 3. Nanjing University of Information Science & Technology, Nanjing 210044
Funds: Supported by the National Key Research and Development Program of China (2018YFC1507703)

Abstract: Extremely anomalous features of Meiyu in 2020 over the Yangtze–Huai River basin (YHRB) and associated causes in perspective of the large-scale circulation are investigated in this study, based on the Meiyu operational monitoring information and daily data of precipitation, global atmospheric reanalysis, and sea surface temperature (SST). The main results are as follows. (1) The 2020 YHRB Meiyu exhibits extremely anomalous characteristics, which are the most prominent since the 1980s. The 2020 Meiyu season features the fourth earliest onset, the third latest retreat, the longest duration, the maximum Meiyu rainfall, the strongest mean rainfall intensity, and the maximum number of stations/days with rainstorm. (2) The extremely long duration of the 2020 Meiyu season lies in the farily early onset and late retreat of Meiyu in this particular year. The early onset of Meiyu is due to the earlier-than-normal first northward shift and migration of the key influential systems including the northwestern Pacific subtropical high (NWPSH) and the South Asian high (SAH) along with the East Asian summer monsoon, induced by weak cold air activities from late May to early mid-June. However, the extremely late retreat of Meiyu is because of later-than-normal second northward shift of the associated large-scale circulation systems accompanied with strong cold air activities, and extremely weak and southward located ITCZ over Northwest Pacific in July. (3) The extremely more than normal Meiyu rainfall is represented by its long duration and strong rainfall intensity. The latter is likely attributed to extreme anomalies of water vapor convergence and vertical ascending motion over the YHRB, resulting from the compound effects of the westward extended and enlarged NWPSH, the eastward extended and expanded SAH, and the strong water vapor transport associated with the low-level southerly wind. The extremely warm SST in the tropical Indian Ocean seems to be the key factor to induce the above-mentioned anomalous large-scale circulations. The results from this study serve to improve understanding of formation mechanisms of the extreme Meiyu in China and may help forecasters to extract useful large-scale circulation features from numerical model products to improve medium-extended-range operational forecasts.

2020年江淮流域梅雨极端异常特征及大尺度环流成因

本文分析了2020年江淮流域梅雨极端异常特征及其大尺度环流成因。2020年江淮流域梅雨季具有入梅早、出梅晚,长度长、梅雨量多、降雨强度强等极端异常特征。梅雨季极端异常偏长, 体现在入梅极端异常偏早和出梅极端异常偏晚。梅雨量极端异常偏多,是受梅雨季极端异常偏长和降雨强度极端异常偏强的共同影响。梅雨季降雨强度极端异常偏强,是在副高西部脊极端异常偏西和范围极端异常偏大、南亚高压东部脊极端异常偏东和范围极端异常偏大、低层偏南气流水汽输送极端异常偏强的共同影响下形成的;而热带印度洋海温极端异常偏暖是诱发上述环流极端异常特征的关键因子。本文所得结论有助于提高对极端梅雨形成机制的认识,有助于预报员从数值模式预报中提取大尺度环流特征来制作中期和延伸期梅雨预报。
1.   Introduction
  • Meiyu in China refers to the persistent rainy weather happening mainly over the Yangtze–Huai River basin (YHRB) in early summer, with occurrence of frequent and concentrated regional rainstorms. Under the background of climate warming, the number of days and amount of heavy rainfall in most parts of China show increasing trends (Jiang et al., 2014). Meiyu not only affects the droughts and floods in the YHRB, but also associates with precipitation patterns throughout China. Studies have shown that Meiyu, as a product of the seasonal transition of the East Asian atmospheric circulation, is modulated by the interaction among multiple members of the Asian summer monsoon system. Moreover, the thermal conditions in the tropical oceans and other external factors can also exert influences on Meiyu (Niu and Jin, 2009; Yuan et al., 2017).

    Around the time when Meiyu starts, atmospheric circulation changes significantly over the Indian Peninsula, the YHRB and east coast of China, and the central North Pacific (Liu et al., 2011). In the years of Meiyu early onset, the South Asian high (SAH) and East Asian subtropical westerly jet establish earlier and the South China Sea summer monsoon also bursts earlier (Zhao et al., 2018a). When the “ + − + ” wave train appears in the region from the Ural Mountain to the Okhotsk Sea in the mid–high latitudes of Asia and from the low to high latitudes of East Asia, the rainfall in the Meiyu season is more abundant than normal (Zhang and Tao, 1998). The two seasonal northward shifts of the northwestern Pacific subtropical high (NWPSH) are closely related to the Meiyu onset and retreat. The active convection of the intertropical convergence zone (ITCZ) can be regarded as the precursory signal for the strengthening and northward shift of the NWPSH, and can impact the northward shift and westward extension of the NWPSH (Xu et al., 2001). Weaker monsoon troughs along with less frequency of tropical cyclones are found to be responsible for stronger intensity of the NWPSH and Meiyu rainfall (Zhu et al., 2017). The distinctive tropospheric warming and stratospheric cooling in the midlatitudes can lead to elevated tropopause in the subtropics, widening of the subtropics over East Asia, and the northward shift of the Meiyu belt (Si et al., 2009).

    Since the beginning of the 21st century, Meiyu in China has been featured with late onset and early retreat, along with short duration and weak rainfall intensity (Jiang and Gao, 2013). In particular, the Meiyu in 2016 and 2020 was characterized by historically rare early onset, long duration, and strong rainfall intensity (Zhao and Niu, 2019; Zhang et al., 2020; Li et al., 2021), which brought about serious flooding disasters to the YHRB. The study on Meiyu has once again become a hot topic. Wang et al. (2020) and Liu et al. (2021) analyzed the influence of the atmospheric circulation anomalies from June to July in summer 2020 and the external forcing factors such as sea surface temperature (SST) on Meiyu. Ding et al. (2021) pointed out that the record-breaking Meiyu over the YHRB in 2020 is closely related to the typical quasi-biweekly oscillation of the East Asian summer monsoon (EASM) circulation.

    In the present study, we focus on sorting out the extremely anomalous features of Meiyu over the YHRB in 2020, and explore the causes of the extreme Meiyu in association with the anomalous evolution of the large-scale circulations, which is suspected to lead to the extreme anomalies in Meiyu onset/retreat over the YHRB in 2020. Especially, we discuss from the perspective of the seasonal transition of the atmospheric circulation in East Asia, and the extremely anomalous characteristics of the key influential systems in the upper, middle, and lower troposphere. The purpose of this study is to obtain a further understanding on the formation mechanism of the extreme Meiyu in 2020 in China, providing scientific basis for better Meiyu forecast in the future.

2.   Data and methods
  • Data used in this paper are as follows: the Meiyu onset and retreat dates for three sub-regions (i.e., the regions to the south, along, and north of the Yangtze River) of the YHRB (28°–34°N, 110°–123°E), released by the National Climate Center of China Meteorological Administration (CMA) (National Climate Center, 2018; Wang and Zheng, 2018; Chen et al., 2019; Ding et al., 2020; Dai et al., 2021); the daily precipitation data from National Meteorological Information Center of CMA; the NCEP daily global atmospheric reanalysis data on a horizontal resolution of 2.5° × 2.5° and 17 layers from 1000 to 10 hPa (Kanamitsu et al., 2002); and the daily data of optimal interpolation SST (Reynolds et al., 2007) and interpolated outgoing longwave radiation (OLR) (Liebmann and Smith, 1996) from NOAA with horizon-tal resolution of 0.25° × 0.25° and 2.5° × 2.5°, respectively.

    The data period in this study is defined as from 1981 to 2020, and the climatological mean is the average from 1981 to 2010. However, the SST data are available from 1982 to 2020; due to limitation in the starting time, the climatology mean of the SST data is the average from 1982 to 2010.

  • According to GB/T 33671-2017 Meiyu Monitoring Indices (General Administration of Quality Supervision, Inspection, and Quarantine of the People’s Republic of China, 2017), the onset (retreat) date of the Meiyu season over the YHRB is the earliest (latest) one in the three sub-regions. The Meiyu rainfall (mm) is the accumulation of the daily area averaged precipitation over the YHRB (277 meteorological stations in total) during the Meiyu season, and the mean rainfall intensity (mm day−1) is the mean of the daily area averaged precipitation over the YHRB during the Meiyu season. In addition, identification of the regional rainstorm process is based on weather events with a combination of subjective and objective methods (Niu et al., 2018).

  • The area index, westward ridge point (WRP), and ridge line position (RLP) of the NWPSH at 500 hPa, and the area index, eastward ridge point (ERP), and RLP of the SAH at 200 hPa are calculated by using the definitions of Niu and Zhai (2013). The western RLP of the NWPSH refers to the average of the RLP of the NWPSH over 110°–130°E, and the eastern RLP of the SAH refers to the average of the RLP of the SAH over 110°–125°E.

    With reference to previous studies (Gao and Xue, 2006; Li and Li, 2014) in combination with the climatology distribution characteristics of the meridional wind reflected in the atmospheric reanalysis data, the Philippines and New Guinea cross-equatorial flow (CEF) indices are defined as the area averaged meridional wind over the region 2.5°S–2.5°N, 125°–135°E and the region 2.5°S–2.5°N, 142.5°–152.5°E at 925 hPa, respectively.

    The Indian Ocean basin-wide mode (IOBW) index is the area averaged SST anomaly in the tropical Indian Ocean (IO) (20°S–20°N, 40°–110°E; Yuan et al., 2017).

    The ITCZ intensity index over the Northwest Pacific is the number of grid points with OLR ≤ 220 W m−2 over the region 5°–20°N, 110°–140°E. The larger the index value is, the stronger the intensity of the ITCZ and the more active the tropical convection would be in the Northwest Pacific.

    The north boundary of the EASM is determined based on both the distribution and the thermal properties of air flows, in which southwesterly flows with potential pseudo-equivalent temperature (θse) exceeding a certain threshold value are examined, and two thresholds of 335 and 340 K are adopted according to Li et al. (2013). In the context, the daily evolution of θse, wind vector, meridional wind, and precipitation averaged over 110°–123°E during the Meiyu season and its adjacent periods in the past 40 years are analyzed and compared. The results show that in most cases the movement to the north and south of the θse isolines of 335 and 340 K are basically consistent, with a very small interval. However, there is sometimes a large interval between the two θse isolines in the late Meiyu season and before and after the Meiyu retreat date. In such a case, the θse isoline of 340 K is closer to the strong gradient zone of southerly wind associated with the southwesterly flows and the northern edge of the Meiyu belt than that of 335 K. For this reason, the north boundary of the EASM is determined by the latitude location of the north edge of the southwesterly flow with θse exceeding 340 K.

    Among the Meiyu monitoring indices, the north and south boundaries of the western RLP of the NWPSH are defined to correspond to the Meiyu onset (≥ 18°N) and retreat (≥ 27°N) over the YHRB. However, the north and south boundaries of the eastern RLP of the SAH are not clearly related to the Meiyu onset and retreat, and thus they need to be further standardized. Firstly, the daily western RLP of the NWPSH and the daily eastern RLP of the SAH at 200 hPa in the Meiyu season over the YHRB from 1981 to 2020 are sorted from south to north, respectively, to obtain their respective RLPs at different quantiles in the season (Fig. 1a). Secondly, the number of days when the RLP falls in the latitude range corresponding to each percentile interval among the nine percentile intervals is calculated during the maximum possible duration period of Meiyu over the YHRB. The 9 percentile intervals are cut from the two tail ends (5% and 95%) to the middle at an interval of every 5 percentages, and the maximum possible duration period of Meiyu is from the earliest onset date of 25 May (1995 and 2016) to the latest retreat date of 7 August (1993) during 1981 to 2020. Finally, the correlation coefficients between the number of days of the RLP falling in the latitude range corresponding to each percentile interval and the duration days of the Meiyu season over the YHRB during 1981 to 2020 are calculated.

    Figure 1.  (a) The western RLP of the NWPSH at 500 hPa and the eastern RLP of the SAH at 200 hPa at the different percentile in the Meiyu season and (b) correlation coefficient (r) between the number of days of the RLP falling in the latitude range corresponding to each percentile interval during maximum possible duration period of Meiyu and the duration days of the Meiyu season over the YHRB from 1981 to 2020.

    The results show that the correlation is the highest between the number of days when the western RLP of the NWPSH falls in the latitude range corresponding to 15%–85% and the duration days of the Meiyu season, with a correlation coefficient of 0.48 (passing the t-test at the significance level of α = 0.05; Fig. 1b). This indicates that the more the number of days that the western RLP of the NWPSH falls in the latitude range corresponding to 15%–85%, the more favorable the occurrence and maintenance of Meiyu and the longer the duration of the Meiyu season would be. The western RLP of the NWPSH at 15% and 85% quartile is 18.4 and 26.6°N, respectively, closest to the south and north boundaries of the western RLP of the NWPSH in the obtained Meiyu monitoring indices.

    By analogy, the number of days that the eastern RLP of the SAH at 200 hPa falls in the latitude range corresponding to 10%–90% has the highest correlation to the duration days of the Meiyu season, with a correlation coefficient of 0.54, indicating that the more the number of days that the eastern RLP of the SAH at 200 hPa is located in the latitude range corresponding to 10%–90%, the more favorable the occurrence and maintenance of Meiyu and the longer the duration of the Meiyu season would be. The eastern RLP of the SAH at 200 hPa corresponding to 10% and 90% quartile is 21.2° and 30.2°N, respectively. Therefore, when the north and south boundaries of the eastern RLP of the SAH at 200 hPa is farther north of 21.2° and 30.2°N respectively, it is more conducive to early onset and late retreat of Meiyu over the YHRB.

3.   Extremely anomalous features of Meiyu in 2020
  • According to the monitoring information, the onset date of the Meiyu season over the YHRB in 2020 is 29 May with a standardized anomaly of −1.18σ, and is the fourth earliest Meiyu season since 1981, but later than that in 2016, 1995, and 2015. The Meiyu retreat date is 2 August with a standardized anomaly of 1.44σ, the third latest since 1981, but only earlier than that in 1993 and 1998. The number of duration days of 2020 Meiyu is 65 days with a standardized anomaly of 2.03σ, the longest duration since 1981 (Fig. 2a). The 2020 Meiyu rainfall reaches 780.9 mm with a standardized anomaly as high as 3.08σ, the maximum since 1981. The mean rainfall intensity of 2020 Meiyu is 12.1 mm day−1 with a standardized anomaly of 1.84σ, which is the strongest since 1981 (Fig. 2b).

    Figure 2.  (a, b) Normalized and (c, d) original time series of the extreme properties of heavy rain in the Meiyu season over the YHRB from 1981 to 2020: (a) Meiyu onset date, retreat date, and duration days, (b) Meiyu rainfall, mean rainfall intensity, and duration days, (c) sum of the daily number of stations with rainstorm (blue) or heavy rainstorm (red) during the Meiyu season, and (d) the number of stations with daily precipitation ranking the top five. Note that r in (a) and (b) denotes correlation coefficient.

    It is evident in Fig. 3a that heavy rainfall processes occurred frequently in the Meiyu season of 2020, with 15 processes identified. Although the heavy rainfall belt oscillated from north to south, it is still relatively stable within the latitude range of the YHRB. The accumulated precipitation over the YHRB exceeded 500 mm in gene-ral, and reached 900–1700 mm (1–2 times more than normal) in the center area, in eastern Hubei Province, southern Anhui Province, northeastern Jiangxi Province, and northwestern Zhejiang Province (Figs. 3b, c). Sum of daily number of stations with rainstorm (heavy rainstorm) in the 2020 Meiyu season is as high as 1275 (285), ranking the maximum since 1981. The number of stations that experienced the top 5 heaviest daily precipitation ranks the second in the Meiyu season since 1981, next to 2016 (Figs. 2c, d).

    Figure 3.  (a) Time–latitude evolution of daily precipitation averaged over 110°–123°E from May to August, and distributions of (b) accumulated precipitation and (c) its percentage anomalies during the Meiyu season over the YHRB in 2020.

    To sum up, the Meiyu season over the YHRB in 2020 exhibits extremely anomalous features in many aspects such as the Meiyu onset (retreat) date, duration, rainfall amount, and rainfall intensity. The 2020 Meiyu is characterized by extremely early onset, late retreat, long duration, abundant Meiyu rainfall, and strong rainfall intensity. This extremely long Meiyu season over the YHRB in 2020 is due to the extremely early onset and late retreat of Meiyu. Statistical results from 1981 to 2020 show that the duration of the Meiyu season is highly positively correlated with the onset date but is highly negatively correlated with the retreat date, with correlation coefficient of −0.62 and 0.67, respectively. The Meiyu rainfall amount is related to both the Meiyu duration and the rainfall intensity. The correlation coefficient between the Meiyu rainfall and the duration (mean rainfall intensity) in the Meiyu season from 1981 to 2020 is as high as 0.83 (0.73). This indicates that the longer the duration of the Meiyu season and the stronger the rainfall intensity, the greater the Meiyu rainfall. Next, we focus on analysis and discussion of the causes of the extremely anomalous features of the onset date, retreat date, and rainfall intensity of the Meiyu season over the YHRB in 2020.

4.   Large-scale circulations associated with the extremely anomalous Meiyu in 2020
  • The early onset of Meiyu is related to the early seasonal adjustment and migration of the East Asian atmospheric circulation from winter to summer (Zhao et al., 2018b). From Fig. 4, it is seen that the western RLP of the NWPSH is obviously northward than normal from late May to early mid-June, of which the 5-day sliding average is stably located to the north of 18°N since 30 May. This indicates that the western ridge of the NWPSH has finished its first seasonal northward shift, which is 7 days earlier than normal (6 June). The eastern RLP of the SAH at 200 hPa also migrated northward to the north of 21.2°N from 31 May and stably located in the latitude range conducive to the Meiyu onset, which is 11 days earlier than normal (11 June). Under the guidance of the mid–upper level circulation system, the north boundary of the EASM at the low level (the north edge of southwesterly winds with θse more than 340 K at 850 hPa) migrated northward into the YHRB at an earlier time than normal. Afterwards, the EASM continuously poured in and transported abundant water vapor and energy to the YHRB, and frequently converged with cold air in the region, resulting in frequent heavy rainfall processes over the YHRB (Figs. 5a, b) and the Meiyu was initiated on 30 May. Thus, the extremely early Meiyu onset is mainly due to the early first seasonal northward shift and migration of the key influence systems, such as the NWPSH, SAH, and EASM, which are responsible for the earlier transition of the East Asian atmospheric circulation pattern from winter to early summer.

    Figure 4.  Evolution of the daily (a) western RLP of the NWPSH at 500 hPa and (b) eastern RLP of the SAH at 200 hPa from May to August in 2020. Red lines denote the original time series, black lines represent the 5-day sliding average, and green lines represent the climatological mean.

    Figure 5.  Time–latitude cross-sections averaged over 110°–123°E of the daily (a) wind vector (arrow), meridional wind (brown line and shading), and θse isoline of 340 K (bold black line) at 850 hPa; (b) water vapor flux vector (arrow), and its meridional component (brown line) and divergence (shading) at 850 hPa; and (c) geopotential height at 500 hPa (bold black line denotes the 588-dagpm isoline of the NWPSH while black dot–dashed line shows the ridge line of the NWPSH) and temperature anomalies at 850 hPa (shading) from May to August in 2020. In (a)–(c), green dashed lines denote the latitude range of the YHRB.

    Previous studies have shown that the northward shift and southward retreat of the NWPSH, SAH, EASM, and other key influence systems are affected by the cold air carried by the westerly trough moving southward from the mid–high latitudes, and by the intensity and location of the ITCZ (Zhang and Tao, 1999; Tao and Wei, 2006). From late May to early mid-June in 2020, the Arctic region was dominated by polar vortex, the cold air from the polar region to the south was significantly weakened, and the front zone at mid–high latitudes contracted northward. Although the low troughs appeared frequently within the latitude range of the YHRB, temperature anomalies at 850 hPa were positive, suggesting that the cold air activities over the YHRB are frequent but weaker than normal (Fig. 5c). Weaker-than-normal cold air activities are favorable for seasonal northward shift and the norther-than-normal location of the NWPSH. According to the climatological mean, the date when the northern boundary of OLR less than 220 W m−2 averaged over 110°–140°E in the Northwest Pacific extends to the north of 10°N is basically consistent with the Meiyu onset date (8 June) over the YHRB (the gray line in Fig. 6), which is very close to normal situation (Fig. 6a). It can be seen from the above analysis that the early first seasonal northward shift and migration of the key influence systems including the NWPSH, SAH, and EASM in 2020 are mainly affected by the weak cold air activities from late May to early mid-June.

    Figure 6.  Time–latitude cross-sections averaged over 110°–140°E of daily geopotential height at 500 hPa above 586 dagpm (bold dark-red solid line shows the 588-dagpm isoline of NWPSH, dark-red dot–dashed line shows the ridge line of NWPSH, and black lines show the climatic NWPSH and its ridge line) and OLR below 220 W m−2 (blue shading; gray line denotes the climatic isoline of 220 W m−2) from May to August: (a) 2020, (b) composite for the Meiyu season with anomalously late retreat date and long duration, and (c) composite for the Meiyu season with anomalously early retreat date and short duration.

  • The second northward shift of the NWPSH is closely related to the Meiyu retreat, implying establishment of the midsummer atmospheric circulation pattern over East Asia. After the Meiyu onset in 2020, although the western RLP of the NWPSH at 500 hPa displayed a tendency of northward migration slowly, it was oscillating from north to south in the latitude range of 18°–27°N, conducive to the maintenance of Meiyu. Until 29 July, the western RLP of the NWPSH settled north of 27°N and finished the second seasonal northward shift, 8 days later than normal (21 July). Similarly, the eastern RLP of the SAH at 200 hPa was relatively stable in the latitude range of 21.2°–30.2°N, conducive to the maintenance of Meiyu after the Meiyu onset. It was not until 1 August that it moved to the north of the northern boundary (30.2°N), also later than normal (Fig. 4). Regulated by the mid–upper level circulation system, the north boundary of the low-level EASM was relatively stable in the latitude range of the YHRB after the Meiyu onset, and strong meridional gradient of the southerly flows over the south side of the north EASM boundary also maintained in the latitude range of the YHRB, leading to continuous moisture convergence for frequent heavy rain and long duration of the Meiyu season. It was not until late July that the north edge of the low-level EASM crossed the north flank of the YHRB, and then the Meiyu retreated on 2 August (Figs. 5a, b). Obviously, the extremely late Meiyu retreat is mainly caused by the delayed second northward shift and migration of the NWPSH, SAH, and EASM, which contributes to the delayed setup of the midsummer atmospheric circulation pattern over East Asia.

    Why was the second seasonal northward shift of the NWPSH delayed? Why was the establishment of the midsummer atmospheric circulation pattern over East Asia postponed? Compared with June, the Arctic region in July 2020 turned to be dominated by the polar high, and the region near the Okhotsk Sea was affected frequently by a strong blocking high, forcing the cold air to invade the lower latitudes. Though low troughs still appeared frequently, temperature anomalies at 850 hPa turned negative in the latitude range of the YHRB (Fig. 5c). This indicates that the cold air activities over the YHRB were more frequent and stronger than normal. The stronger-than-normal cold air is bound to suppress the seasonal northward shift of the NWPSH.

    Climatologically, the northern boundary of OLR less than 220 W m−2 averaged over 110°–140°E in Northwest Pacific is south of 17°N during the Meiyu season, and steadily crosses 17°N until about the retreat date of Meiyu. The date of crossing 17°N of the above northern boundary of OLR less than 220 W m−2 was 7 days later than normal in 2020. In addition, the ITCZ intensity over Northwest Pacific during most time of July (Fig. 7a) was weaker than normal. The ITCZ intensity index over Northwest Pacific was the third lowest in July since 1981 (Fig. 8a), with a normalized anomaly as low as −1.45σ. The ITCZ intensity index of Northwest Pacific in July is positively correlated with the western RLP of the NW-PSH, with a correlation coefficient of 0.59 (significant at 0.05). This means that the weaker the ITCZ, the more southward the western ridge of the NWPSH would be.

    Figure 7.  Normalized time series of the daily (a) ITCZ intensity index over Northwest Pacific and (b) Philippines CEF index (red line) and New Guinea CEF index (green line) from May to August in 2020.

    Figure 8.  Normalized time series of (a) ITCZ intensity index over Northwest Pacific, (b) Philippines CEF index, and (c) New Guinea CEF index, in July from 1981 to 2010.

    Furthermore, the years for the Meiyu season with anomalously late retreat (σ more than 1.0) and long duration (σ more than 1.0) over the YHRB were selected, including 1993, 1998, 1999, and 2020. For comparison, the years for the Meiyu season with anomalously early retreat (σ less than −1.0) and short duration (σ less than −1.0) were also extracted, which are 2001, 2002, and 2013. The composite results show clearly that the anomalous characteristics of the ITCZ in Northwest Pacific and the NWPSH for the composite of the Meiyu season with anomalously late retreat and long duration are very similar to those in 2020. That is to prove, the ITCZ convection over Northwest Pacific is obviously weaker than normal, the date of the northern boundary of OLR less than 220 W m−2 averaged over 110°–140°E extending to north of 17°N is later (in late July), and the western RLP of the NWPSH is obviously more southward (Fig. 6b). The reverse is true for the composite Meiyu season with anomalously early retreat and short duration (Fig. 6c).

    The intensity and location of the ITCZ over North-west Pacific can be affected by the CEFs from the Southern Hemisphere. The weaker the CEFs in the east of 100°E, the weaker the intensity of the ITCZ and the more southward its location would be (Bi et al., 2004; Liu et al., 2009; Sui and Wu, 2017). In July 2020, the Philippines and New Guinea CEFs were continuously weaker than normal (Fig. 7b), and their indices ranked the fourth lowest and the lowest since 1981 with normalized anomalies of −1.85σ and −1.49σ, respectively (Figs. 8b, c). In summary, the late second northward shift of the NWPSH and the other key influence systems in 2020 are also influenced by the extremely weak and further southward ITCZ over Northwest Pacific, as well as the extremely weak Philippines and New Guinea CEFs in July 2020.

  • During the Meiyu season over the YHRB, the range and intensity of the key influential systems such as the NWPSH have a significant impact on the rainfall intensity. The WRP and area index of the NWPSH are closely related to the mean rainfall intensity (with correlation coefficient of −0.49 and 0.46, respectively). Table 1 indicates that the more westward the WRP and the larger the coverage of the NWPSH, the stronger the rainfall intensity tends to be in the Meiyu season. In the Meiyu season of 2020, the WRP of the NWPSH is 108°E, about 20° more westward than normal (Fig. 9a). It is the second westernmost, just eastward to that in 2010 since 1981. The range of the NWPSH is the third largest and only smaller than that in 2010 and 2016 since 1981 (Fig. 10a). It can be inferred that the extremely westward extended western ridge and extremely large coverage of the NWPSH are the key factors leading to the extremely strong rainfall intensity of the Meiyu season in 2020.

    NWPSH SAH at 200 hPaIOBW indexAverage over the YHRB
    WRPArea indexERPArea indexWater vapor flux divergence at 850 hPaOmega at 500 hPa
    −0.49#0.46# 0.31*0.33#0.44#−0.60#−0.67#
    Note: # (*) denotes the value passing the t-test at the significance level of α = 0.05 (0.10).

    Table 1.  Correlation coefficients between the mean rainfall intensity and averaged circulation indices, related physical quantities, and the IOBW index in the Meiyu season from 1981 to 2020

    Figure 9.  Mean large-scale circulations for the Meiyu season of 2020: (a) geopotential height (black solid lines) and its anomalies (shading) at 500 hPa, (b) geopotential height (black solid lines), zonal wind greater than 30 m s−1 (blue lines), and divergence (shading) at 200 hPa, (c) water vapor flux (vector) and its divergence (shading) at 850 hPa, (d) anomalies of water vapor flux (vector) and its divergence (shading) at 850 hPa. In (a) and (b), the bold black solid lines show the range of the subtropical highs at 500 hPa and the SAH at 200 hPa denoted by the 588- and 1252-dagpm isolines respectively; the black dashed lines show the ridge line of the highs; and the green lines show the climatic range and ridge line of the highs. The rectangle in (c) and (d) denotes the region 20°–33°N, 105°–123°E.

    Figure 10.  Standardized time series of the averaged circulation characteristic indices, relative physical quantities, and the IOBW index in the Meiyu season of the YHRB from 1981 to 2020: (a) the WRP and area index of the NWPSH at 500 hPa, (b) the ERP and area index of the SAH at 200 hPa, (c) the IOBW index and area averaged vertical motion (omega) over the YHRB at 500 hPa, and (d) area averaged divergence over the YHRB and meridional component of the water vapor flux in the region 20°–33°N, 105°–123°E at 850 hPa.

    It is found that the extreme anomalies of the NWPSH are linked to the extreme anomalies of the SAH and the warm SST in the tropical IO. Studies have shown that the SAH can affect the development of the NWPSH by dynamic and thermal mechanisms, and they have a tendency to move towards (backwards) each other (Ren et al., 2007). In the Meiyu season, there is a significant negative correlation (with correlation coefficient of −0.59) between the ERP of the SAH at 200 hPa and the WRP of the NWPSH at 500 hPa, and a significant positive correlation (with correlation coefficient of 0.81) between the area indices of the two system (Table 2). Meanwhile, the ERP and area index of the SAH are significantly correlated with the mean rainfall intensity (with correlation coefficients of 0.31 and 0.33, respectively) (Table 1). This suggests that the more eastward the eastern ridge and the larger the coverage of the SAH, the more westward the western ridge and the larger the coverage of the NWPSH, and the stronger the rainfall intensity would be. In the Meiyu season of 2020, the ERP of the SAH denoted by the 1252-dagpm was 136°E, about 27° more eastward than normal (Fig. 9a), ranking the fourth easternmost since 1981, and the coverage of the SAH is the fourth largest (Fig. 10b). On the other hand, the warm SST anomalies over the tropical IO can trigger an anomalous anticyclone circulation over Northwest Pacific through the Matsuno–Gill response (Matsuno, 1966; Gill, 1980), which facilitates the westward extension and strengthening of the NWPSH (Xie et al., 2009; Yuan et al., 2012, 2017), further enhancing the rainfall intensity. The IOBW index in the Meiyu season is significantly negative and positive correlated with the WRP and area index of the NWPSH (correlation coefficients are −0.53 and 0.63, respectively), and is significantly positive correlated with the mean rainfall intensity (correlation coefficient is 0.44). Thus, when the SST over the tropical IO is uniformly warmer than normal, the western ridge of the NWPSH tends to be more westward and its coverage would be larger than normal, and the rainfall intensity would resultantly be more intense. In the Meiyu season of 2020, the SSTs over the tropical IO were extremely warm than normal and the IOBW index ranked the highest since 1981 (Fig. 10c).

    SAH at 200 hPaIOBW index
    ERPArea index
    WRP of NWPSH−0.59#−0.62#−0.53#
    Area index of NWPSH 0.75# 0.81# 0.63#
    Note: # denotes the value passing the t-test at the significance level of α = 0.05.

    Table 2.  Correlation coefficients between averaged circulation characteristic indices and IOBW index in the Meiyu season from 1981 to 2020

    The low-level anomalous anticyclone circulation in the Northwest Pacific triggered by the warm SST in the tropical IO is conducive to the strengthening of the southerly flow to its west side, and the westward anomaly of the western ridge of the NWPSH is favorable for guiding the low-level southerly flow to turn along its western edge into China, providing abundant moisture and energy for the occurrence of heavy rainfall. In the 2020 Meiyu season, the western ridge of the NWPSH exhibited extremely westward anomaly; the water vapor transport by the southerly flow turning along the western edge of the NWPSH also displayed extremely strong anomaly. The shear line formed by the confluence of cold and warm air flows was located near 33°N, and the region south of the shear line (20°–33°N, 105°–123°E) was dominated by southerly flows, in which more abundant water vapor was transported (Figs. 9c, d) and the area averaged meridional water vapor flux was the fifth strongest since 1981 (Fig. 10d). Meanwhile, the circulation over the mid–high latitudes in Asia displayed a pattern of double blocking highs, with the ridges near the Ural Mountains and Okhotsk sea, and the low troughs near Lake Balkhash, Lake Baikal, and the East China Sea, respectively (Figs. 9a, b). The cold air carried by the low troughs frequently invaded the YHRB from the west, northwest, and east. The confluence of the cold air and the extremely strong warm–wet southerly flows produced obviously stronger than normal moisture convergence at 850 hPa over the YHRB (Figs. 9c, d); the area averaged moisture convergence (divergence) over the YHRB is the third strongest (lowest) in the Meiyu season since 1981 (Fig. 10d). The low-level moisture convergence is crucial to the formation of heavy rainfall. Analysis suggests that the area averaged water vapor flux divergence at 850 hPa is highly pertinent to the mean rainfall intensity in the Meiyu season with a correlation coefficient of −0.60. This indicates that stronger low-level moisture convergence is prone to cause stronger rainfall intensity. Thus, another key factor inducing the extremely strong rainfall intensity is the extremely strong moisture convergence resulting from the extremely strong low-level water vapor transport associated with the southerly flow turning along the western edge of the NWPSH and the confluence of cold and warm air flows.

    When the SAH extends eastward, the upper-level divergence located at its northeastern quadrant also moves eastward, providing necessary dynamic conditions for the formation of heavy rainfall over the YHRB. In the Meiyu season of 2020, the eastern ridge of the SAH was extremely anomalously eastward. The YHRB was controlled by strong upper-level divergence over the north side of the SAH ridge line and the south side of the westerly jet belt (Fig. 9b). The intensity of the divergence is stronger than normal (figure omitted). Due to the combined effects of the extremely strong moisture convergence at the low level and the stronger-than-normal divergence at the upper level, extremely strong vertical ascending motion was induced over the YHRB, where the area averaged vertical ascending motion is the second strongest (i.e., the second lowest omega) in the Meiyu season since 1981 (Fig. 10c). The statistical results in Table 1 show that a highly negative correlation exists between the area averaged vertical motion at 500 hPa and the mean rainfall intensity over the YHRB in the Meiyu season with correlation coefficient of −0.67, indicating that the stronger the vertical ascending motion, the stronger the rainfall intensity would be. Therefore, the extremely strong vertical ascending motion is also one of the key factors resulting in the extremely strong rainfall intensity in the Meiyu season of 2020.

    To sum up, the extremely strong Meiyu rainfall intensity in 2020 is a result of the extremely strong water vapor convergence and vertical ascending motion over the YHRB, which occurred due to the combined effects of the extremely westward extended and enlarged NWPSH, extremely eastward extended and expanded SAH, and extremely strong water vapor transport associated with the low-level southerly wind. The extremely warm SST in the tropical IO is supposed to have exerted important influences on the formation of the extremely anomalous large-scale circulations mentioned above.

5.   Conclusions and discussion
  • The extremely anomalous features of Meiyu over the YHRB in 2020 and associated large-scale circulation anomalies are investigated in this paper, based on a comparative analysis of the features of the Meiyu in 2020 and the climatological features of the Meiyu season during 1981–2020. The main conclusions are summarized as follows (Fig. 11).

    Figure 11.  Schematic diagram showing large-scale circulation anomalies responsible for the extremely anomalous features of Meiyu in 2020 over the YHRB.

    (1) The extremely anomalous features of the Meiyu season in 2020 are very prominent. The Meiyu in 2020 is characterized by the fourth earliest of the onset date, the third latest of the retreat date, the longest duration, the maximum amount of Meiyu rainfall, the strongest mean rainfall intensity, and the maximum summation of daily number of stations with rainstorm and heavy rainstorm in the Meiyu season since 1981.

    (2) The extremely long duration of the Meiyu season in 2020 is represented by the extremely early onset and late retreat of Meiyu. The former is mainly induced by the early first northward shift and migration of the key influence systems including the NWPSH, SAH, and EASM; and the corresponding earlier transition of the East Asian atmospheric circulation pattern from winter to early summer under the influence of the weak cold air activities from late May to early mid-June. On the other hand, the extremely late Meiyu retreat is mainly due to the late second northward shift and migration of the NWPSH and the other key influence systems, and the corresponding delayed establishment of the midsummer atmospheric circulation pattern in East Asia, induced by strong cold air activities and the extremely weak and southward located ITCZ over Northwest Pacific in July. The phased anomalous cold air activities are likely related to the downstream dispersion of the wave energy associated with the positive and negative phases of the North Atlantic Oscillation (NAO) (Liu et al., 2020). The extremely weak and further southward ITCZ over the Northwest Pacific are related to the extremely weak Philippines and New Guinea CEFs in July.

    (3) The extremely more than normal Meiyu rainfall is an inevitable result of the extreme long duration and strong rainfall intensity in 2020. The latter is resulted from the extreme strong water vapor convergence and vertical ascending motion over the YHRB, due to the extremely westward extended and enlarged NWPSH, the extremely eastward extended and expanded SAH, and the extremely strong water vapor transport associated with the low-level southerly wind turning along the western edge of the NWPSH. The extremely warm SST in the tropical IO is the key factor to induce the above-mentioned extremely anomalous large-scale circulations.

    Takaya et al. (2020) and Zhou et al. (2021) pointed out that the extreme warm anomaly over the tropical IO in the early summer of 2020 was originated from the super Indian Ocean dipole (IOD) episode in the autumn of 2019. In the present study, we also examined the correlation between the preceding autumn (August–October) IOD index and the early summer (June–July) IOBW index since 1982. The results showed that there was indeed a positive correlation between the preceding autumn IOD index and the early summer IOBW index, but the correlation coefficient (0.31) failed to pass the t-test at the significance level of α = 0.05. Zheng and Wang (2021) have proved that the positive SST anomalies in the western North Atlantic in May can induce positive geopotential height anomalies in the midlatitudes of North Atlantic in June, and then lead to positive geopotential height anomalies in the midlatitudes of East Asia by the atmospheric wave train cross the European continent. The preceding positive SST anomaly in the western North Atlantic is probably an important factor for the extremely large coverage of the SAH in the early summer of 2020. It is also worth further in-depth study on the modulation and influence mechanisms of the SST anomalies and the role of circulation systems in the Southern Hemisphere in the formation of the extremely anomalous Meiyu in 2020.

    The conclusions obtained in this study are expected to support the medium-to-extended-range forecast. This can be achieved through extracting large-scale circulation features from the numerical model products. For example, the ECMWF ensemble mean forecast initiated at 1200 UTC 22 May and 20 July in 2020 revealed that the cold air activities would be weakened and the front zone at the mid–high latitudes would shrink and move northward from 26 May, the RLP of the NWPSH would also shift northward and be stably located to the north of 18°N from 30 May (Fig. 12a), reaching the threshold for Meiyu onset. Moreover, around 30 July, the cold air activities would be weakened over the YHRB compared with the previous period, the front zone at mid–high latitudes would incur further northward contraction, the temperature anomalies at 850 hPa would turn from negative to positive over the latitude range of the YHRB, and the RLP of the NWPSH would obviously shift northward to the north of 27°N (Fig. 12b), reaching the threshold for Meiyu retreat. The above model forecast results are basically consistent with the large-scale circulation evolution characteristics during and around the Meiyu season in 2020. This proves that it is possible to make basically accurate forecasts of the Meiyu onset and retreat dates for the YHRB in the medium-to-extended range through analyzing the large-scale circulation characteristics reflected in the numerical model products based on the conclusions of this study.

    Figure 12.  Time–latitude cross-sections averaged over 110°–123°E of geopotential height at 500 hPa (bold black line shows the 588-dagpm isoline of NWPSH and black dot–dashed line shows the NWPSH ridge line) and temperature anomalies at 850 hPa (shading) based on the ensemble mean forecast of ECMWF initialized at 1200 UTC of (a) 22 May and (b) 20 July with lead time of 15 days and initial fields in the previous 15 days, respectively, in 2020 (purple vertical line indicates initial time; green dashed lines show the latitude range of the YHRB).

    Acknowledgments. The authors are grateful to the editors and anonymous reviewers for their constructive comments that have significantly improved the quality of this paper.

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