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The Concurrent Record-breaking Rainfall over Northwest India and North China in September 2021


doi: 10.1007/s00376-022-2187-y

  • Extremely heavy rainfall occurred over both Northwest India and North China in September 2021. The precipitation anomalies were 4.1 and 6.2 times interannual standard deviation over the two regions, respectively, and broke the record since the observational data were available, i.e., 1901 for India and 1951 for China. In this month, the Asian upper-tropospheric westerly jet was greatly displaced poleward over West Asia, and correspondingly, an anomalous cyclone appeared over India. The anomalous cyclone transported abundant water vapor into Northwest India, leading to the heavy rainfall there. In addition, the Silk Road pattern, a teleconnection pattern of upper-level meridional wind over the Eurasian continent and fueled by the heavy rainfall in Northwest India, contributed to the heavy rainfall in North China. Our study emphasizes the roles of atmospheric teleconnection patterns in concurrent rainfall extremes in the two regions far away from each other, and the occurrence of rainfall extremes during the post- or pre-monsoon period in the northern margins of monsoon regions.
    摘要: 2021年9月,在印度西北部和中国华北地区出现极端强降水。印度西北部的降水异常达到4.1倍的年际标准差,打破自1901年有观测以来的历史记录;同时,华北降水异常达到6.2倍的年际标准差,破1951年以来记录。在该月,西亚地区上对流层西风急流位置显著偏北,相应地,印度地区出现气旋式环流异常。异常的气旋式环流输送大量水汽至印度西北部,导致该地区出现极端降水。此外,欧亚陆地上空的丝绸之路遥相关型,在印度西北部降水的加强下,对华北极端降水起到重要贡献。研究结果强调大气遥相关对两地同时出现极端降水的作用,揭示季风北边缘区在季风盛行前、后时期发生极端降水的成因。
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  • Figure 1.  (a) Percentage of precipitation anomalies (%) in September 2021 relative to climatology. Only the anomalies at the regions where climatological September precipitation is higher than 20 mm, except northwest China with sparse stations, are shown. The white hatching indicates Northwest India and North China. (b) Standardized precipitation averaged over Northwest India in September from 1901 to 2021. The numbers in top right corner show the average precipitation and times of climatological STD in September 2021. (c) Same as (b), but for standardized precipitation averaged over North China.

    Figure 2.  Precipitation (mm) (a) in September 2021, (b) averaged in September during 1979–2020, (c) averaged in July and August during 1979–2020. (d) Difference between the precipitation averaged in July and August and precipitation in September during 1979–2020 (mm). Only the contours in the regions where climatological September precipitation is higher than 20 mm, except northwest China with sparse stations, are shown. The hatching indicates Northwest India and North China as in Fig. 1.

    Figure 3.  (a) Anomalies of 200-hPa zonal wind (m s−1) in September 2021. Dots indicate the anomalies exceeding ±2 times STD. Green and black dash lines show the AWJ axis in September 2021 and the climatological AWJ axis, respectively. The black boxes indicate the regions (25°–35°N, 50°–90°E) and (40°–50°N, 50°–90°E) used to define WAWJ index. (b) Standardized WAWJ index in September from 1979 to 2021. The numbers in right corner show the anomalous times of STD in September 2021 and its rank since 1979.

    Figure 4.  (a) Difference of 200-hPa zonal wind (m s−1) between the average of July and August and it in September during 1979–2020. (b) Green, black, and red dash lines show the AWJ axis in September 2021, the climatological AWJ axis in September, and the climatological AWJ axis averaged in July and August, respectively.

    Figure 5.  (a) Anomalies of 850-hPa wind (m s−1) in September 2021. Black and grey vectors show the wind anomalies exceeding ±2 and ±1 times climatological STD, respectively. The black boxes indicate the regions (25°–35°N, 105°–125°E) and (10°–20°N, 105°–125°E) used to define the anticyclone index in section 4.2. (b) Anomalies of water vapor flux (m s−1 g kg−1) at 850 hPa in September 2021. The contours show the value and the vectors show the anomalies exceeding ±2 (black) and ±1 (grey) times STD. The black hatching indicates North China which is same as in Fig. 1. The grey shading shows the 2000 m topography.

    Figure 6.  Anomalies of vertically integrated water vapor flux from the surface to 100 hPa (kg m−1 s−1) in September 2021. Only the values greater than 40 kg m−1 s−1 are shown. The black hatching indicates North China which is same as in Fig. 1. The grey shading shows the 2000 m topography.

    Figure 7.  850-hPa horizontal wind (m s−1) regressed onto the WAWJ index during 1979–2020. The black vectors show the wind anomalies significant at the 0.05 level, according to Student’s test. The grey shading shows the 2000 m topography.

    Figure 8.  (a) Surface temperature anomalies in September 2021 (°C). Dots indicate the anomalies exceeding ±2 times STD. Black boxes show the region (25°–40°N, 50°–90°E) used for define temperature index. (b) Time series of standardized September temperature anomalies averaged over the region shown in (a) from 1979 to 2021. The numbers in right corner show the anomalous times of STD in September 2021 and the rank of it since 1979.

    Figure 9.  (a) Anomalies of 200-hPa meridional wind (m s−1) in September 2021. Dots indicate the anomalies exceeding ±2 times STD. Green dots indicate the points (40°N, 50°E), (40°N, 80°E), (40°N, 110°E), and (40°N,140°E) used to define SRP index. (b) Standardized SRP index in September from 1979 to 2021. The numbers in right corner show the anomalous times of STD in September 2021 and its rank since 1979.

    Figure 10.  850-hPa horizontal wind (m s−1) regressed onto the SRP index (SRPI) during 1979–2020. The black vectors show the wind anomalies significant at the 0.05 level, according to Student’s test. The grey shading shows the 2000 m topography. The red line (from 33°N, 105°E to 45°N, 125°E) illustrates the rainfall enhancement over North China in September 2021.

    Figure 11.  850-hPa horizontal wind (m s−1) regressed onto the index of anomalous anticyclone over west Pacific (WPAC) during 1979–2020. Here, the index is defined as the difference between the 850-hPa zonal wind anomalies averaged over (25°–35°N, 105°–125°E) and (10°–20°N, 105°–125°E), where the winds were extremely anomalous in September 2021 as shown in Figure 5a. The black vectors show the wind anomalies significant at the 0.05 level, according to Student’s test. The grey shading shows the 2000 m topography. The red line [from (33°N, 105°E) to (45°N, 125°E)] illustrates the rainfall enhancement over North China in September 2021.

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Manuscript received: 04 July 2022
Manuscript revised: 19 September 2022
Manuscript accepted: 27 September 2022
通讯作者: 陈斌, bchen63@163.com
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The Concurrent Record-breaking Rainfall over Northwest India and North China in September 2021

    Corresponding author: Ying NA, naying@bj.cma.gov.cn
  • 1. Beijing Municipal Climate Center, Beijing 100089, China
  • 2. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 3. College of Earth and Planetary Sciences, University of the Chinese Academy of Sciences, Beijing 100049, China

Abstract: Extremely heavy rainfall occurred over both Northwest India and North China in September 2021. The precipitation anomalies were 4.1 and 6.2 times interannual standard deviation over the two regions, respectively, and broke the record since the observational data were available, i.e., 1901 for India and 1951 for China. In this month, the Asian upper-tropospheric westerly jet was greatly displaced poleward over West Asia, and correspondingly, an anomalous cyclone appeared over India. The anomalous cyclone transported abundant water vapor into Northwest India, leading to the heavy rainfall there. In addition, the Silk Road pattern, a teleconnection pattern of upper-level meridional wind over the Eurasian continent and fueled by the heavy rainfall in Northwest India, contributed to the heavy rainfall in North China. Our study emphasizes the roles of atmospheric teleconnection patterns in concurrent rainfall extremes in the two regions far away from each other, and the occurrence of rainfall extremes during the post- or pre-monsoon period in the northern margins of monsoon regions.

摘要: 2021年9月,在印度西北部和中国华北地区出现极端强降水。印度西北部的降水异常达到4.1倍的年际标准差,打破自1901年有观测以来的历史记录;同时,华北降水异常达到6.2倍的年际标准差,破1951年以来记录。在该月,西亚地区上对流层西风急流位置显著偏北,相应地,印度地区出现气旋式环流异常。异常的气旋式环流输送大量水汽至印度西北部,导致该地区出现极端降水。此外,欧亚陆地上空的丝绸之路遥相关型,在印度西北部降水的加强下,对华北极端降水起到重要贡献。研究结果强调大气遥相关对两地同时出现极端降水的作用,揭示季风北边缘区在季风盛行前、后时期发生极端降水的成因。

    • In September 2021, extremely heavy rainfall occurred in both Northwest India and North China, triggering dire economic and societal consequences in these widely separated regions. According to India Meteorological Department, in 2021, 8 states and territories in Northwest India recorded excessive precipitation as compared to their respective September averages (https://weather.com/en-IN/india/monsoon/news/2021-10-01-india-ends-2021-monsoon-season-on-normal-note). Concurrent with the heavy rainfall in Northwest India, precipitation over seven provinces in North China hit a record high since 1961 (Liu and Gao, 2021; Zhou et al., 2022a). Wei River, the tributary of Yellow River, with the peak flood in July and August normally, experienced the most severe autumn flood since 1935 (Li et al., 2022). Floods in four provinces affected more than 6 million people, led to 41 people killed or missing, destroyed ~500,000 hectares of farmland, and produced economic losses totaling 15.34 billion yuan, according to the Chinese government (https://www.mem.gov.cn/xw/yjglbgzdt/202201/t20220123_407199.shtml).

      The heavy rainfall in September 2021 is unexpected in the climatological sense, because the rainy season is prior to September over both Northwest India and North China. Influenced by the Asian summer monsoon, the rainy season is generally July and August over Northwest India and North China, corresponding to the march of Indian and East Asian summer monsoon, respectively, and subsequently in September, precipitation reduces remarkably (e.g., Krishnamurthy and Shukla, 2000; Wang and Lin, 2002).

      The seasonal march of rainy season over both regions is closely connected to that of the upper-tropospheric Asian westerly jet (AWJ) (e.g., Liang and Wang, 1998; Ding and Chan, 2005; Chiang et al., 2017; Chowdary et al., 2019; Choudhury et al., 2021; Li et al., 2021). The AWJ is characterized by subseasonal meridional migration due to the seasonal change in warming and resultant meridional temperature gradient (Yeh et al., 1958; Kuang et al., 2007). When the AWJ reaches the northernmost position about 40°N in midsummer, precipitation over Northwest India and North China also reaches the peak (e.g., Krishnamurthy and Shukla, 2000; Ding and Chan, 2005). In addition, the jet-rainfall relationship also exists on the interannual timescale: During the summer when AWJ is displaced poleward (equatorward), the rain bands tend to be displaced poleward (equatorward), i.e., more (less) rainfall in Northwest India and North China (Liang and Wang, 1998; Lu, 2004; Wei et al., 2015; Du et al., 2016; Hong et al., 2021; Chowdary et al., 2022).

      The summer precipitation variations over Northwest India and North China tend to be in phase with one another (e.g., Kripalani and Singh, 1993; Wu, 2017; Wang et al., 2021). This in-phase relationship in summer rainfall can be well explained by the Rossby waves along the AWJ, known as the Silk Road Pattern (SRP) (Lu et al., 2002; Wu and Wang, 2002; Saeed et al., 2011; Hong and Lu, 2016; Wu, 2017; Yadav, 2017). The Indian rainfall anomalies, affected by the SRP, can in turn trigger the downstream components of SRP, which propagate eastwards along the AWJ (Kripalani et al., 1997; Ding and Wang, 2005; Saeed et al., 2011; Greatbatch et al., 2013; Wei et al., 2014; Lin et al., 2017). The SRP can also contribute to the in-phase variations in precipitation between South Asia and East Asia (Wu, 2017; Liu and Huang, 2019). However, it should be mentioned that most of these previous studies focused on summer.

      In the remainder of this paper, data and methods are presented in section 2. That the extreme rainfall in September 2021 broke the record over both Northwest India and North China is shown in section 3. We illustrate the circulation anomalies responsible for the extreme rainfall and discuss the linkage of the extreme events between the two widely separated regions in section 4. A summary is presented in section 5.

    2.   Data and methods
    • The precipitation over India is from the India Meteorological Department. This dataset has a 0.25° resolution for 1901–2021 (Pai et al., 2014). The precipitation over China is derived from gauge-based observations collected by 2,400 stations for 1951–2021 and is provided by the National Meteorological Information Center of China (http://data.cma.cn/data/index/f0fb4b55508804ca.html). The precipitation anomaly over China in Fig. 1a was calculated at each station respectively and then interpolated to a 0.25°×0.25° grid using a Cressman interpolation scheme. Wind, specific humidity, 2 m air temperature, and vertical integral of water vapor flux are from ERA5 atmospheric reanalysis with a 0.25° resolution for 1979–2021 (Hersbach et al., 2018). The climatological average and interannual standard deviation (STD) are derived for each variable from 1979 to 2020.

      Figure 1.  (a) Percentage of precipitation anomalies (%) in September 2021 relative to climatology. Only the anomalies at the regions where climatological September precipitation is higher than 20 mm, except northwest China with sparse stations, are shown. The white hatching indicates Northwest India and North China. (b) Standardized precipitation averaged over Northwest India in September from 1901 to 2021. The numbers in top right corner show the average precipitation and times of climatological STD in September 2021. (c) Same as (b), but for standardized precipitation averaged over North China.

    3.   Extreme rainfall in September 2021
    • Figure 1a shows the precipitation anomalies for September 2021 in percentage. The percentages highlight the anomalies compared with local climatological-mean precipitation, which exhibits a wide scope, ranging from tiny amounts over the deserts of northwest China to more than 400 mm over the western coastal region of the Indian subcontinent (Fig. 2b). Generally, rainfall was above normal in the northern parts of both India and China, and below normal in the southern parts. The extremely abnormal precipitation appeared over Northwest India and North China, where the precipitation in September 2021 was more than twice the climatological mean, with some areas even more than four times. The rain band appeared over Northwest India and North China in September 2021 (Fig. 2a), in sharp contrast to normal years when the peak rainy season ends by September in both regions and accordingly, the rainfall decreases remarkably (Fig. 2b). Actually, the distribution of rainfall in September 2021 resembles somewhat that for the climatological mean in July and August (Fig. 2c), which is the peak rainy season over Northwest India and North China, and thus the precipitation anomalies resemble the subseasonal differences between July/August and September (Fig. 2d).

      Figure 2.  Precipitation (mm) (a) in September 2021, (b) averaged in September during 1979–2020, (c) averaged in July and August during 1979–2020. (d) Difference between the precipitation averaged in July and August and precipitation in September during 1979–2020 (mm). Only the contours in the regions where climatological September precipitation is higher than 20 mm, except northwest China with sparse stations, are shown. The hatching indicates Northwest India and North China as in Fig. 1.

      We define Northwest India and North China as the areas where the percentage of precipitation anomalies in September 2021 was higher than 100% within two specified areas (18°–30°N, 68°–80°E) and (33°–45°N, 105°–125°E), respectively, as shown by the white hatching in Fig. 1a. The regional-mean precipitation in September 2021 was 308.1 and 226.2 mm for Northwest India and North China, respectively; these values broke the records in both the regions since the observational data have been available, i.e., 1901 and 1951, respectively (Figs. 1b and 1c). The anomaly over Northwest India was 4.1 times STD; the second largest anomaly dated back more than 100 years ago (3.1 in 1917). In addition, the anomaly of precipitation over North China was 6.2 times STD, and this value was much greater than the second largest in 2011 (2.6).

      The correlation coefficient between September precipitation over Northwest India and North China is 0.24 during 1951–2020, significant at the 0.05 confidence level, suggesting that the in-phase relationship in rainfall between Northwest India and North China exists in early autumn, in addition to summer. It should be mentioned that the region definitions for Northwest India and North China are based on the extreme rainfall that occurred in September 2021, and thus may not be appropriate for accurately depicting the statistical relationship in rainfall between these two regions over a longer period. Therefore, the in-phase relationship would be underestimated by the correlation coefficient (0.24), which is calculated by the regional averages.

      Considering the active role of Indian rainfall, i.e., Indian rainfall affecting North China rainfall through the SRP, in the following section we first discuss the causes of extreme precipitation over Northwest India in September 2021 and then turn to North China.

    4.   Circulation anomalies
    • The 200-hPa zonal wind anomalies in September 2021 are shown in Fig. 3a. Westerly and easterly anomalies appear to the north and south of AWJ axes, respectively. The zonal winds are extremely anomalous over West Asia, and the anomalies exceed twice the climatological STD over most of the grid points in this region. These zonal wind anomalies correspond well to the poleward shift of AWJ. In particular, the jet axis (green bold dashed line) was shifted poleward about 5 latitude degrees in September 2021 over West Asia in comparison with the climatology (black bold dashed line). Accordingly, the latitudes of the AWJ axis in September 2021 were comparable to, or even higher than, those for the climatology of July and August (Fig. 4b), when the AWJ usually reaches its northernmost location throughout the year, and the zonal wind anomalies in September 2021 were stronger than the subseasonal differences between July/August and September (Fig. 4a).

      Figure 3.  (a) Anomalies of 200-hPa zonal wind (m s−1) in September 2021. Dots indicate the anomalies exceeding ±2 times STD. Green and black dash lines show the AWJ axis in September 2021 and the climatological AWJ axis, respectively. The black boxes indicate the regions (25°–35°N, 50°–90°E) and (40°–50°N, 50°–90°E) used to define WAWJ index. (b) Standardized WAWJ index in September from 1979 to 2021. The numbers in right corner show the anomalous times of STD in September 2021 and its rank since 1979.

      Figure 4.  (a) Difference of 200-hPa zonal wind (m s−1) between the average of July and August and it in September during 1979–2020. (b) Green, black, and red dash lines show the AWJ axis in September 2021, the climatological AWJ axis in September, and the climatological AWJ axis averaged in July and August, respectively.

      Considering this pattern of extreme zonal wind anomalies over West Asia, an index was defined as the difference between the zonal wind anomalies averaged over (40°–50°N, 50°–90°E) and (25°–35°N, 50°–90°E), to depict the AWJ variability over West Asia (hereafter abbreviated as WAWJ). A positive index represents the poleward displacement of WAWJ, and vice versa. The time series of standardized WAWJ index in September from 1979 to 2021 are shown in Fig. 3b. The anomaly of WAWJ index in 2021 was 2.7 times the climatological STD and was the largest since 1979.

      The lower-tropospheric winds and resultant water vapor flux are crucial for inducing precipitation, and anomalous winds and water vapor flux at 850 hPa (Fig. 5) explain well the precipitation enhancement in September 2021. There were significant cyclonic anomalies over India and the cyclonic wind anomalies were generally over twice the climatological STD (Fig. 5a). These anomalous winds associated with the Indian cyclone transported abundant water vapor over Northwest India (Fig. 5b), consistent with the vertically integrated water vapor flux (Fig. 6), confirming the crucial role of lower-tropospheric winds in transporting total water vapor. It should be mentioned that the monthly circulation anomalies are the sum of various synoptic disturbances including the Cyclone Gulab. This cyclone, which initiated over the Bay of Bengal on September 24 and passed through India afterwards, greatly contributed to the precipitation enhancement over Northwest India (figures not shown).

      Figure 5.  (a) Anomalies of 850-hPa wind (m s−1) in September 2021. Black and grey vectors show the wind anomalies exceeding ±2 and ±1 times climatological STD, respectively. The black boxes indicate the regions (25°–35°N, 105°–125°E) and (10°–20°N, 105°–125°E) used to define the anticyclone index in section 4.2. (b) Anomalies of water vapor flux (m s−1 g kg−1) at 850 hPa in September 2021. The contours show the value and the vectors show the anomalies exceeding ±2 (black) and ±1 (grey) times STD. The black hatching indicates North China which is same as in Fig. 1. The grey shading shows the 2000 m topography.

      Figure 6.  Anomalies of vertically integrated water vapor flux from the surface to 100 hPa (kg m−1 s−1) in September 2021. Only the values greater than 40 kg m−1 s−1 are shown. The black hatching indicates North China which is same as in Fig. 1. The grey shading shows the 2000 m topography.

      The concurrent WAWJ poleward displacement in the upper troposphere and Indian cyclonic anomalies in the lower troposphere may not be accidental. Figure 7 shows the 850-hPa horizontal wind regressed onto the WAWJ index during 1979–2020. There are significant cyclone anomalies at 850 hPa over India in association with the poleward displacement of WAWJ, suggesting the close relationship between these circulation anomalies in the upper and lower troposphere.

      Figure 7.  850-hPa horizontal wind (m s−1) regressed onto the WAWJ index during 1979–2020. The black vectors show the wind anomalies significant at the 0.05 level, according to Student’s test. The grey shading shows the 2000 m topography.

      A question naturally arises: What is the reason for the poleward displacement of WAWJ in September 2021? Considering that the meridional temperature gradient plays a crucial role in determining the meridional migration of AWJ, we examined the surface air temperature anomalies in September 2021 (Fig. 8). The temperature anomalies were extremely positive over West Asia, exceeding twice the climatological STD. On the other hand, the temperatures were below normal to the north of about 40°N in the Eurasian continent, and the temperature anomalies were generally weak in the Indian Ocean. As a result, the meridional temperature gradient was strengthened over the mid-high latitudes and weakened over the low latitudes in West Asia, corresponding to the poleward displaced WAWJ. The anomaly of temperature averaged over West Asia (25°–40°N, 50°–90°E) in September 2021 was the largest since 1979 (Fig. 8b), and the correlation coefficient between the temperature averaged over this region and the WAWJ index is 0.41 during 1979–2020, significant at the 0.01 confidence level. Thus, the extreme temperature anomalies over West Asia in September 2021 may have contributed to the poleward displacement of WAWJ.

      Figure 8.  (a) Surface temperature anomalies in September 2021 (°C). Dots indicate the anomalies exceeding ±2 times STD. Black boxes show the region (25°–40°N, 50°–90°E) used for define temperature index. (b) Time series of standardized September temperature anomalies averaged over the region shown in (a) from 1979 to 2021. The numbers in right corner show the anomalous times of STD in September 2021 and the rank of it since 1979.

    • Over East Asia, in September 2021 there were also westerly anomalies to the north of the jet axis and easterly anomalies to the south, corresponding to the poleward displacement of jet axis (Fig. 3a), although these anomalies were not so extreme as those over West Asia. The latitude of the jet axis in September 2021 was also comparable to, or even higher than, the climatological location of July–August jet axis (Fig. 4b). As mentioned in the introduction, the poleward shifted jet would favor the heavier rainfall in North China.

      The lower-tropospheric circulation anomalies favored water vapor transport into North China in September 2021 (Fig. 5). Highly anomalous southwesterlies in association with the anticyclonic anomaly over South China and the western North Pacific transported more water vapor into the western part of North China, and the southerly to southeasterly anomalies over the Yellow Sea and coastal regions of China enhanced water vapor transport into the eastern part of North China. The combination of the anomalous southwesterlies and southeasterlies induced the extreme precipitation over North China. It should be mentioned that the southerly/southeasterly anomalies, albeit much weaker than the southwesterly anomalies, were crucial for water vapor convergence over North China (Figs. 5b and 6), due to the sharp south-to-north decrease of specific humidity over eastern China in September.

      The southerly to southeasterly anomalies at 850 hPa over the coastal regions of China and the Yellow Sea correspond to southerly anomalies in the upper troposphere. In September 2021 there were southerly anomalies at 200 hPa over eastern China, and there were also strong southerly and northerly anomalies over the midlatitude Eurasian continent and Japan (Fig. 9a). These meridional wind anomalies resembled well the SRP, and the SRP in September 2021 was extremely anomalous and broke the record (Fig. 9b). Here, the SRP index is defined as the algebraic sum of 200-hPa meridional wind anomalies at the four centers along 40°N as marked in Fig. 9a, i.e., V200(50°E) -V200(80°E) +V200(110°E) -V200(140°E).

      Figure 9.  (a) Anomalies of 200-hPa meridional wind (m s−1) in September 2021. Dots indicate the anomalies exceeding ±2 times STD. Green dots indicate the points (40°N, 50°E), (40°N, 80°E), (40°N, 110°E), and (40°N,140°E) used to define SRP index. (b) Standardized SRP index in September from 1979 to 2021. The numbers in right corner show the anomalous times of STD in September 2021 and its rank since 1979.

      The concurrence between the SRP and lower-tropospheric southerly/southeasterly anomalies over the coastal regions of China and Yellow Sea is not limited to the specific case of September 2021, but also exists in normal years. Figure 10 shows the 850-hPa horizontal wind (m s−1) regressed onto the SRP index during 1979–2020. Stronger SRP corresponds to significant southeasterly anomalies over the Yellow Sea and the northern part of East China, which leads to the wind convergence over North China. Therefore, the extremely anomalous SRP in September 2021 was associated with anomalous lower-tropospheric southeasterlies and wind convergence over North China.

      Figure 10.  850-hPa horizontal wind (m s−1) regressed onto the SRP index (SRPI) during 1979–2020. The black vectors show the wind anomalies significant at the 0.05 level, according to Student’s test. The grey shading shows the 2000 m topography. The red line (from 33°N, 105°E to 45°N, 125°E) illustrates the rainfall enhancement over North China in September 2021.

      The SRP is crucial to the linkage between circulation and precipitation anomalies over West and East Asia. Hong et al. (2021) stated that the poleward displacement of WAWJ is associated with the enhanced SRP, and the SRP can be significantly fueled by the precipitation over Northwest India through latent heat release (Ding and Wang, 2005). Our results show the SRP can lead to the enhanced precipitation over North China and suggest that the anomalous SRP was responsible for the concurrence of extremely heavy rainfall over the two widely separated regions in September 2021.

      On the other hand, the anomalous anticyclone over South China and the western North Pacific in September 2021 might be an independent factor. As shown in Fig. 5, the southwesterly anomalies associated with the anticyclonic anomaly transported more water vapor northward from ocean in this month. During 1979–2020, it shows weak or even contrasting relationships to the WAWJ displacement (Fig. 7), SRP (Fig. 10), and lower-tropospheric cyclonic anomaly over India (Fig. 11). Therefore, we can conclude that both this anomalous anticyclone and the SRP contributed to the extremely heavy rainfall in North China in September 2021.

      Figure 11.  850-hPa horizontal wind (m s−1) regressed onto the index of anomalous anticyclone over west Pacific (WPAC) during 1979–2020. Here, the index is defined as the difference between the 850-hPa zonal wind anomalies averaged over (25°–35°N, 105°–125°E) and (10°–20°N, 105°–125°E), where the winds were extremely anomalous in September 2021 as shown in Figure 5a. The black vectors show the wind anomalies significant at the 0.05 level, according to Student’s test. The grey shading shows the 2000 m topography. The red line [from (33°N, 105°E) to (45°N, 125°E)] illustrates the rainfall enhancement over North China in September 2021.

      Recently, Liu et al. (2022) investigated the effects of tropical SST anomalies on the extreme rainfall event over North China in September 2021. They suggested that the SST anomalies in the tropical Indian Ocean, Pacific and Atlantic strengthened the convection over the Maritime Continent and northern Indian Peninsula, which contributed to the anticyclonic anomaly over the western North Pacific and resulted in extreme rainfall over North China. The current study, however, emphasizes the role of internal atmospheric variability, i.e., the midlatitude atmospheric circulations, including the SRP and related lower-tropospheric southerlies or southeasterlies.

    5.   Summary
    • Record-breaking precipitation occurred simultaneously over Northwest India and North China in September 2021. The precipitation extreme over Northwest India was supported by a pronounced poleward displacement of the WAWJ and correspondingly enhanced water vapor transport by the anomalous Indian cyclone. The WAWJ displacement to the northernmost position might have been forced by the extremely warm temperature over West Asia and resultant intensification of the meridional temperature gradient. Furthermore, the SRP was highly anomalous in September 2021, possibly induced by the Indian rainfall anomalies. The anomalous SRP favored the extreme rainfall in North China through the poleward displaced upper-tropospheric westerly jet over East Asia and the lower-tropospheric southeasterly anomalies, in combination with the water vapor transported by the anomalous anticyclone over the western North Pacific.

      The present study indicates that the precipitation and circulation in September 2021 resembled those in the peak rainy season, i.e., July and August. This may be explained by the extremely warm temperatures over the Eurasian continent, which could induce the poleward displacement of the upper-tropospheric westerly jet (e.g., Lu et al., 2007; Seidel et al., 2008; Pena-Ortiz et al., 2013; Simpson et al., 2014). Such a poleward displacement would be more remarkable during the seasonal transitions, when the surface temperatures experience rapid changes and thus the jet location is more sensitive to the temperature changes (Voigt and Shaw, 2016; Chen et al., 2020). Therefore, climate extremes during the seasonal transitions should be emphasized, in addition to the extremes during the peak warm or cold seasons that are currently under widespread investigation (Endo et al., 2021; Yang et al., 2021; Zhou et al., 2022b). On the other hand, our results suggest that climate extremes in one region can be related to, or lead to, those in other regions, through the atmospheric teleconnection patterns. It is widely believed that climate extremes would occur more frequently and their intensities would be enhanced under global warming. Therefore, the more frequent and stronger climate extremes would trigger the atmospheric teleconnections more frequently, which in turn could induce the climate extremes in widely separated regions worldwide (Lau and Kim, 2012; Orsolini et al., 2015; Boers et al., 2019).

      Acknowledgements. The authors greatly appreciate the comments and suggestions from the two anonymous reviewers. This study was supported by the National Natural Science Foundation of China (Grant No. 42105064), the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (Grant No. 2019QZKK0102) and China Meteorological Administration program (Grant No. CXFZ2021J030).

      Data Availability Statements. The gauge-based precipitation dataset over China can be accessed at http://data.cma.cn/data/index/f0fb4b55508804ca.html. The precipitation dataset over India can be accessed at https://www.imdpune.gov.in. ERA5 atmospheric reanalysis can be accessed at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels?tab=overview.

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