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

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.

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.

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Acknowledgements

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.

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Correspondence to Yu Liu.

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Article Highlights

• The extremely strong Northwest Pacific anticyclone anomaly (NWPAC) is responsible for extreme rainfall in summer 2020.

• The decadal warming trend of the tropical Indian Ocean contributes to the extreme NWPAC that appears after a weak El Niño.

• Considering the 2019/20 event as a fast-decaying El Niño provides a better explanation of the extreme NWPAC and rainfall anomalies.

This paper is a contribution to the special issue on Summer 2020: Record Rainfall in Asia—Mechanisms, Predictability and Impacts.

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Fang, C., Liu, Y., Cai, Q. et al. Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Niño?. Adv. Atmos. Sci. 38, 2067–2081 (2021). https://doi.org/10.1007/s00376-021-1009-y

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  • DOI: https://doi.org/10.1007/s00376-021-1009-y

Key words

  • extreme rainfall
  • Northwest Pacific anticyclone anomaly (NWPAC)
  • Indo-western Pacific Ocean capacitor (IPOC)
  • Tropical Indian Ocean warming trend
  • fast-decaying El Niño

关键词

  • 极端降水
  • 西北太平洋反气旋异常
  • 印度–西太平洋电容器
  • 热带印度洋变暖趋势
  • 快速衰退型 El Niño