Abstract
Record-breaking heavy and persistent precipitation occurred over the Yangtze River Valley (YRV) in June-July (JJ) 2020. An observational data analysis has indicated that the strong and persistent rainfall arose from the confluence of southerly wind anomalies to the south associated with an extremely strong anomalous anticyclone over the western North Pacific (WNPAC) and northeasterly anomalies to the north associated with a high-pressure anomaly over Northeast Asia. A further observational and modeling study has shown that the extremely strong WNPAC was caused by both La Niña-like SST anomaly (SSTA) forcing in the equatorial Pacific and warm SSTA forcing in the tropical Indian Ocean (IO). Different from conventional central Pacific (CP) El Niños that decay slowly, a CP El Niño in early 2020 decayed quickly and became a La Niña by early summer. This quick transition had a critical impact on the WNPAC. Meanwhile, an unusually large area of SST warming occurred in the tropical IO because a moderate interannual SSTA over the IO associated with the CP El Niño was superposed by an interdecadal/long-term trend component. Numerical sensitivity experiments have demonstrated that both the heating anomaly in the IO and the heating anomaly in the tropical Pacific contributed to the formation and maintenance of the WNPAC. The persistent high-pressure anomaly in Northeast Asia was part of a stationary Rossby wave train in the midlatitudes, driven by combined heating anomalies over India, the tropical eastern Pacific, and the tropical Atlantic.
摘要
2020年6-7月, 长江流域出现了创纪录的持续性特大暴雨. 观测资料表明, 与西北太平洋异常反气旋 (WNPAC) 相关的南风异常和与东北亚异常高压相联系的东北风异常交汇, 从而导致该持续性暴雨的发生. 进一步的观测和模式研究表明, 超强的 WNPAC 由赤道太平洋的 La Niña 型海温异常和热带印度洋的暖海温异常共同强迫产生. 与传统的中太平洋型 (CP) El Niño 的缓慢衰减不同, 2020 年初 CP El Niño 快速衰减, 到初夏演变为 La Niña. ENSO 的快速位相转换对 WNPAC 的形成发挥着关键的作用. 同时, 与 CP El Niño 相关的印度洋年际尺度海温异常叠加了年代际分量, 导致热带印度洋海温出现极端增暖. 数值试验表明, 热带印度洋和太平洋的热源对 WNPAC 的形成和维持均有贡献. 持续的东北亚高压异常则是中纬度静止 Rossby 波列的一部分, 由印度、 热带东太平洋和热带大西洋的热源共同强迫产生.
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Acknowledgements
This work was jointly supported by China National Key R&D Program 2018YFA0605604, NSFC Grant No. 42088101, NOAA NA18OAR4310298, and NSF AGS-2006553. This is SOEST contribution number 11354, IPRC contribution number 1524, and ESMC number 350.
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Article Highlights
• The Yangtze River Valley experienced record-breaking strong and persistent rainfall in June–July 2020 due to the confrontation of a strong anomalous anticyclone over the western North Pacific to the south and cold/dry advection induced by anomalous northeasterly to the north.
• The extremely strong anomalous anticyclone over the western North Pacific resulted from a combined effect of a quick El Niño to La Niña phase transition and strong Indian Ocean warming.
• The unusual Indian Ocean warming was a result of superposition of an interannual and an interdecadal/long-term trend component.
• The persistent northeasterly anomaly in Northeast Asia was part of a zonally oriented Rossby wave train, forced by heating anomalies over India, the eastern Pacific, and the Atlantic.
This paper is a contribution to the special issue on Summer 2020: Record Rainfall in Asia — Mechanisms, Predictability and Impacts.
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Pan, X., Li, T., Sun, Y. et al. Cause of Extreme Heavy and Persistent Rainfall over Yangtze River in Summer 2020. Adv. Atmos. Sci. 38, 1994–2009 (2021). https://doi.org/10.1007/s00376-021-0433-3
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DOI: https://doi.org/10.1007/s00376-021-0433-3
Key words
- Yangtze River floods
- anomalous anticyclone over the western North Pacific
- CP and EP El Niño
- Indian Ocean warming
- La Niña
- Rossby wave train
关键词
- 长江洪涝
- 西北太平洋异常反气旋
- 中太平洋型和东太平洋型厄尔尼诺
- 印度洋增暖
- 拉尼娜
- 罗斯贝波列