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# Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Niño?

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This study was jointly supported by grants from the Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) (Grant No. XDB40000000), the CAS (Grant No. QYZDJ–SSW–DQC021), the National Natural Science Foundation of China (Grant No. 41630531), and the State Key Laboratory of Loess and Quaternary Geology. We thank the supercomputer center of the Pilot Qingdao National Laboratory for Marine Science and Technology and Beijing Super Cloud Computing Center, who offered computing services. We also thank Dr. X.Z. LI, H. LIU, and L. LIU from the Institute of Earth Environment, CAS, who offered suggestions for our numerical experiments

• In summer 2020, extreme rainfall occurred throughout the Yangtze River basin, Huaihe River basin, and southern Yellow River basin, which are defined here as the central China (CC) region. However, only a weak central Pacific (CP) El Niño happened during winter 2019/20, so the correlations between the El Niño–Southern Oscillation (ENSO) indices and ENSO-induced circulation anomalies were insufficient to explain this extreme precipitation event. In this study, reanalysis data and numerical experiments are employed to identify and verify the primary ENSO-related factors that cause this extreme rainfall event. During summer 2020, unusually strong anomalous southwesterlies on the northwest side of an extremely strong Northwest Pacific anticyclone anomaly (NWPAC) contributed excess moisture and convective instability to the CC region, and thus, triggered extreme precipitation in this area. The tropical Indian Ocean (TIO) has warmed in recent decades, and consequently, intensified TIO basinwide warming appears after a weak El Niño, which excites an extremely strong NWPAC via the pathway of the Indo-western Pacific Ocean capacitor (IPOC) effect. Additionally, the ENSO event of 2019/20 should be treated as a fast-decaying CP El Niño rather than a general CP El Niño, so that the circulation and precipitation anomalies in summer 2020 can be better understood. Last, the increasing trend of tropospheric temperature and moisture content in the CC region after 2000 is also conducive to producing heavy precipitation.
摘要: 2020夏季在整个长江流域、淮河流域以及黄河流域南部[本文定义为中国中部（CC）地区]均发生了极端强降水事件。但2019/20年冬赤道太平洋仅发生了一次弱的中太平洋型（CP）El Niño，因而根据厄尔尼诺–南方涛动（ENSO）指数与ENSO激发的环流异常间的相关关系难以解释此次极端降水事件。本研究利用再分析资料和数值模拟试验找出并验证了造成此次极端降水事件与ENSO相关的主要因素。水汽收支和湿静能分析表明：2020年夏季异常强烈的西北太平洋反气旋异常（NWPAC）西北侧的西南气流异常向CC地区输送大量水汽并提供对流不稳定条件，从而引发了该地区的极端降水。近几十年来，热带印度洋（TIO）变暖，使得弱El Niño后也能出现强的TIO海盆增暖，进而通过印度–西太平洋电容器效应激发出极端强烈的NWPAC。此外，2019/20年的ENSO事件应被视为一次快速衰减的CP El Niño，而不是一次一般的CP El Niño，以便更好地理解2020年夏季的环流和降水异常。最后，2000年后CC区域对流层大气温度和水汽含量的上升趋势也有利于更多降水产生。
• Figure 1.  (a) Precipitation anomaly percentages (shaded contours, units: %) in summer (JJA) 2020 based on observed data by meteorological stations. The gray (black) dots indicate the stations where the values exceed the 95th percentile or are below the 5th percentile (are the highest or lowest) during 1979–2020. The green box indicates the CC region; the purple lines indicate the Yellow River, Huaihe River, and Yangtse River respectively from the north to the south (the same in the following). (b) Pacific SST anomalies (units: °C) in winter (D(0)JF(1)) 2019/20, where the hatched areas indicate the anomalies exceed the 95th percentile or are below the 5th percentile during 1979–2020. The red and yellow boxes indicate the regions for defining the Niño-3 index and El Niño Modoki index (EMI), respectively. The SST data are from ERSSTv5. (c) The time series of the area-weighted average JJA precipitation anomaly percentages in the central China (CC) region (bars, units: %), D(0)JF(1) Niño-3 index (red line), and D(0)JF(1) EMI (yellow line) during 1979–2020.

Figure 2.  Monthly precipitation anomaly percentages (shaded contours, units: %) during JJA (a–c) 1980, (d–f) 1998, (g–i) 2016, and (j–l) 2020, respectively, where (a, d, g, j), (b, e, h, k), (c, f, i, l) are for June, July, and August, respectively. The gray (black) dots indicate the stations where the values exceed the 95th percentile or are below the 5th percentile (are the highest or lowest) during 1979–2020. The green boxes indicate the CC region. The data are derived from station observation.

Figure 3.  JJA-mean wind (vectors, units: m s−1) and geopotential height (shaded contours, units: gpm) anomalies in 2020 at (a) 850 hPa and (b) 500 hPa, respectively; (c) JJA-mean SST (shaded contours, units: °C) anomalies in 2020; (d) JJA-mean wind at 200 hPa (vectors, units: m s−1) and TT (shaded contours, units: gpm) anomalies in 2020; (e) the vectors are composites of JJA-mean meridional wind (units: m s−1) and vertical velocity (units: 10−2 Pa s−1) anomalies in 2020 at the meridional-vertical section averaged within 101°–123°E, and the shaded contours are only for vertical velocity anomalies; (f) is similar to (e) but for composites of zonal wind and vertical velocity anomalies at the zonal-vertical section averaged within 27°–36°N. The vectors in black (gray) denote that the anomalies either (neither) exceed the 95th percentile or (nor) are below the 5th percentile during 1979–2020; the hatched areas indicate where the anomalies exceed the 95th percentile or are below the 5th percentile during 1979–2020 for the shaded contours. The black box in (a), (c), and (d) indicate the NWP, TIO, and MC regions, respectively. The green boxes indicate the CC region. The SST data are from ERSSTv5, and the other variables are from ERA5.

Figure 4.  (a) JJA-mean vertical integrated water vapor flux (WVF) (vectors, units: kg m−1 s−1) and moisture content (that is, specific humidity) (shaded contours, units: kg m−2) anomalies in 2020 and climatological JJA-mean summer moisture content (blue contours, units: kg m−2) during 1979–2020; (b) JJA-mean divergence anomalies of integrated WVF (shaded contours, units: 10−5 kg m−2 s−1) in 2020. The meaning of the black and gray vectors and hatched areas are the same as in Fig. 3. The green boxes indicate the CC region. The wind and specific humidity data are from ERA5.

Figure 5.  Scatterplots between the FD EMI and the JJA-mean values of each term in the moisture equation, Eq. (1), for each year during 1979–2020, where (a–g) correspond to each term from left to right in Eq. (1) in sequence. All the values are averaged within the CC region. The thick gray lines are linear fittings of each scatter; the correlation coefficients (r) and corresponding significance levels (p) are marked at the top-left corner of each panel. The vertical and horizontal dashed lines indicate the critical values of FD CP El Niño/La Niña definitions and ±1 standard deviation values of each term, respectively. The scatters of 1979–99 (2000–19) are marked in blue (pink); the years 1980, 1998, 2016, and 2020 are marked with squares, triangles, diamonds, and pentagrams, respectively. The data to calculate the terms in Eq. (1) are from ERA5 except for the precipitation data, which is obtained from station observation.

Figure 6.  Vertical profiles of vertical velocity anomalies (ω', blue line, units: 10−2 Pa s−1) in summer 2020 and climatological JJA-mean MSE ($\overline{{\phi }_{m}}$, pink line, units: J kg−1) during 1979–2020 averaged within the CC region.

Figure 7.  Similar to Fig. 5 but for the FD EMI with each term in the MSE equation, Eq. (2).

Figure 8.  Scatterplots for the (a) D(0)JF(1) Niño-3 index, (b) D(0)JF(1) EMI, and (c) FD EMI with JJA-mean TIO SST anomalies, respectively. Scatterplots between JJA-mean (d) TIO SST and MC TT, (e) MC TT and NWPACI, (f) NWPACI and CC precipitation anomalies, respectively. The dashed lines indicate the critical values defining El Niño/La Niña for the Niño indices and ±1 standard deviation for other factors, respectively.

Figure 9.  (a) Time series of the JJA-mean TIO SST anomalies (blue line, units: °C), MC TT anomalies (pink line, units: gpm), and NWPACI (yellow line, units: 10−6 s−1) during 1979–2020. The thick lines are linear fittings, respectively. The green points and red crosses indicate the years that positive NWPACI matches (1983, 1987, 1988, 1991, 1998, 2003, 2007, 2010, 2020) and mismatches (1980, 1993, 1995, 1996, 2013, 2015, 2016) positive TIO SST and MC TT anomalies, respectively. (b) Time series of JJA-mean vertically-integrated moisture content (blue line, units: kg m−2) and TT (pink line, units: gpm) anomalies in the CC region; the thick lines are quadratic fittings.

Figure 10.  (a) Bars of the boundary condition JJA-mean TIO SST (blue bars, units: °C) and simulated JJA-mean MC TT (pink bars, units: gpm) and NWPACI (yellow bars, units: 10−6 s−1) for each sensitivity experiment minus CLIM; one special case is for ELCP_FD_ALL_TIOW minus CLIM_TIOW (the last group of bars). (b) JJA-mean wind (vectors, units: m s−1) and precipitation (shaded contours, units: mm d−1) at 850 hPa, (c) wind (vectors, units: m s−1) at 200 hPa and TT (shaded contours, units: gpm) for the results of 20_ALL minus CLIM. (d, e) and (f, g) are similar to (b, c) but for ELCP_FD_ALL minus ELCP_ALL and ELCP_FD_ALL_TIOW minus CLIM_TIOW, respectively. The vectors in black (gray) denote wind differences above (below) the 95% confidence level based on Student’s t-test; the hatched areas indicate the differences above the 95% confidence level based on Student’s t-test for the shaded contours. The black boxes in (b, d, f) and (c, e, g) indicate the NWP and MC regions, respectively. The green boxes indicate the CC region.

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## Manuscript History

Manuscript revised: 14 March 2021
Manuscript accepted: 15 April 2021
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Niño?

###### Corresponding author: Yu LIU, yuliu2018@ieecas.cn;
• 1. The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, 710061, China
• 2. CAS Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi’an, 710061, China
• 3. Laboratory for Ocean Dynamics and Climate, Pilot Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266000, China

Abstract: In summer 2020, extreme rainfall occurred throughout the Yangtze River basin, Huaihe River basin, and southern Yellow River basin, which are defined here as the central China (CC) region. However, only a weak central Pacific (CP) El Niño happened during winter 2019/20, so the correlations between the El Niño–Southern Oscillation (ENSO) indices and ENSO-induced circulation anomalies were insufficient to explain this extreme precipitation event. In this study, reanalysis data and numerical experiments are employed to identify and verify the primary ENSO-related factors that cause this extreme rainfall event. During summer 2020, unusually strong anomalous southwesterlies on the northwest side of an extremely strong Northwest Pacific anticyclone anomaly (NWPAC) contributed excess moisture and convective instability to the CC region, and thus, triggered extreme precipitation in this area. The tropical Indian Ocean (TIO) has warmed in recent decades, and consequently, intensified TIO basinwide warming appears after a weak El Niño, which excites an extremely strong NWPAC via the pathway of the Indo-western Pacific Ocean capacitor (IPOC) effect. Additionally, the ENSO event of 2019/20 should be treated as a fast-decaying CP El Niño rather than a general CP El Niño, so that the circulation and precipitation anomalies in summer 2020 can be better understood. Last, the increasing trend of tropospheric temperature and moisture content in the CC region after 2000 is also conducive to producing heavy precipitation.

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