Abstract
The unprecedented Zhengzhou heavy rainfall in July 2021 occurred under the background of a northward shift of the western Pacific subtropical high (WPSH). Although the occurrence of this extreme event could not be captured by seasonal predictions, a skillful prediction of the WPSH variation might have warned us of the increased probability of extreme weather events in Central and Northern China. However, the mechanism for the WPSH variation in July 2021 and its seasonal predictability are still unknown. Here, the observed northward shift of the WPSH in July 2021 is shown to correspond to a meridional dipole pattern of the 850-hPa geopotential height to the east of China, the amplitude of which became the strongest since 1979. The meridional dipole pattern is two nodes of the Pacific—Japan pattern. To investigate the predictability of the WPSH variation, a 21-member ensemble of seasonal predictions initiated from the end of June 2021 was conducted. The predictable and unpredictable components of the meridional dipole pattern were identified from the ensemble simulations. Its predictable component is driven by positive precipitation anomalies over the tropical western Pacific. The positive precipitation anomalies are caused by positive horizonal advection of the mean moist enthalpy by southwesterly anomalies to the northwestern flank of anticyclonic anomalies excited by the existing La Niña, which is skillfully predicted by the model. The leading mode of the unpredictable component is associated with the atmospheric internal intraseasonal oscillations, which are not initialized in the simulations. The relative contributions of the predictable and unpredictable components to the observed northward shift of the WPSH at 850 hPa are 28.0% and 72.0%, respectively.
摘 要
2021 年 7 月, 在西太副高异常北移的大尺度背景环流下, 郑州发生了一次前所未有的强降水事件. 虽然当前的季节预测系统尚无法预测 “7.21” 郑州极端暴雨事件, 但对其背景环流的准确预测能够为我们应对未来此类极端天气事件的发生及其防灾减灾提供决策依据. 观测分析表明, 2021 年 7 月西太副高异常北移呈现为我国东部沿海地区 850 hPa 经向偶极型位势高度异常, 与东亚夏季 PJ (Pacific-Japan) 遥相关的低层节点相对应, 其振幅为 1979 年以来最强. 为了研究其季节尺度上的可预测性, 基于中国科学院大气物理研究所季节至年代际气候预测系统 (IAP-DecPreS), 针对 2021 年 7 月西太副高的异常北移开展了集合预测试验. 研究表明, 7 月西太副高异常北移既包含可预测信号分量, 也包含不可预测噪音分量. 可预测信号由月平均尺度上热带西太平洋正降水异常驱动, 其来源是 La Niña 冷海温异常激发的副热带北太平洋异常反气旋西侧的异常正湿焓平流. 不可预测噪音则主要源自大气内部的次季节变率. 两者对 2021 年 7 月西太副高异常北移的相对贡献分别为 28.0% 和 72.0%.
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Acknowledgements
This work is supported by the National Natural Science Foundation of China under Grant No. 41988101, the Chinese Academy of Sciences under Grant XDA20060102, the China Postdoctoral Science Foundation under Grant No. 2022T150638 and K. C. Wong Education Foundation.
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Article Highlights
• Northward shift of low-level component of the western Pacific subtropical high (WPSH) in July 2021 broke the record held since 1979.
• Predictable and unpredictable components of the WPSH variation in 2021 were identified based on the seasonal prediction from IAP-DecPreS.
• The predictable and unpredictable components are driven by the existing La Niña and atmospheric internal variability, respectively.
This paper is a contribution to the special collection on the July 2021 Zhengzhou, Henan Extreme Rainfall Event.
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Hu, S., Zhou, T., Wu, B. et al. Seasonal Prediction of the Record-Breaking Northward Shift of the Western Pacific Subtropical High in July 2021. Adv. Atmos. Sci. 40, 410–427 (2023). https://doi.org/10.1007/s00376-022-2151-x
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DOI: https://doi.org/10.1007/s00376-022-2151-x
Key words
- western pacific subtropical high
- seasonal prediction
- seasonal predictability
- La Niña
- Pacific—Japan pattern