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The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020

Abstract

During June and July of 2020, the Yangtze River basin suffered from extreme mei-yu rainfall and catastrophic flooding. This study explores the seasonal predictability and associated dynamical causes for this extreme Yangtze River rainfall event, based on forecasts from the Met Office GloSea5 operational forecast system. The forecasts successfully predicted above-average rainfall over the Yangtze River basin, which arose from the successful reproduction of the anomalous western North Pacific subtropical high (WNPSH). Our results indicate that both the Indian Ocean warm sea surface temperature (SST) and local WNP SST gradient were responsible for the westward extension of the WNPSH, and the forecasts captured these tropical signals well. We explore extratropical drivers but find a large model spread among the forecast members regarding the meridional displacements of the East Asian mid-latitude westerly jet (EAJ). The forecast members with an evident southward displacement of the EAJ favored more extreme Yangtze River rainfall. However, the forecast Yangtze River rainfall anomaly was weaker compared to that was observed and no member showed such strong rainfall. In observations, the EAJ displayed an evident acceleration in summer 2020, which could lead to a significant wind convergence in the lower troposphere around the Yangtze River basin, and favor more mei-yu rainfall. The model forecast failed to satisfactorily reproduce these processes. This difference implies that the observed enhancement of the EAJ intensity gave a large boost to the Yangtze River rainfall, hindering a better forecast of the intensity of the event and disaster mitigation.

摘要

2020 年夏季 (6、 7 月) 长江流域遭遇了特大梅雨降水和洪涝灾害. 本研究利用英国气象局 GloSea5 业务预测系统, 分析了此次极端降水事件的季节可预测性和相关的动力因子. 研究表明, 模式成功再现了长江流域的降水正异常, 并很好地预测出了西北太平洋副热带高压 (西太副高) 的异常特征. 预测信号主要来自于印度洋和西北太平洋局地的海温异常, 它们是导致西太副高偏西的主要原因. 该研究进一步分析了热带外的驱动因子, 发现模式积分成员之间的差异主要表现为东亚副热带西风急流的经向偏移. 模式中急流偏南的积分, 预测的长江流域降水偏多. 但模式所有积分成员预测的长江流域降水均弱于观测, 对应模式集合预测的降水强度也明显偏弱. 观测结果显示, 2020 年夏季东亚西风急流强度明显增强, 它可以导致长江流域地区对流层低层风场辐合, 是导致梅雨降水偏多的重要因子. 而模式无法再现这些过程, 因此不能有效地预测出梅雨降水的强度特征和更好地防灾减灾.

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Acknowledgments

This work was supported by the National Key Research and Development Program of China (Grant No. 2018YFC1506005) and the National Natural Science Foundation of China (Grant Nos. 41721004 and 41775083). This work and its contributors were also supported by the UK-China Research and Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund.

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Correspondence to Chaofan Li.

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Article Highlights

• The above-average Yangtze River rainfall in summer 2020 is successfully forecasted, but with a weaker intensity.

• Predictable sources arise mainly from the anomalous western North Pacific subtropical high modulated by local and Indian Ocean SST.

• Intensity changes of the East Asian westerly jet are not well simulated, hindering a better forecast of the intensity of the event.

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

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Li, C., Lu, R., Dunstone, N. et al. The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020. Adv. Atmos. Sci. 38, 2055–2066 (2021). https://doi.org/10.1007/s00376-021-1092-0

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  • DOI: https://doi.org/10.1007/s00376-021-1092-0

Key words

  • seasonal forecast
  • Yangtze River rainfall
  • western North Pacific
  • subtropical high
  • westerly jet

关键词

  • 季节预测
  • 长江流域降水
  • 西北太平洋
  • 副热带高压
  • 西风急流