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虞越越, 李亚飞, 任荣彩, 等. 2022. 2020/2021年冬季大范围低温寒潮过程中一种典型的平流层—对流层耦合演变模态[J]. 大气科学, 46(6): 1484−1504. doi: 10.3878/j.issn.1006-9895.2206.21250
引用本文: 虞越越, 李亚飞, 任荣彩, 等. 2022. 2020/2021年冬季大范围低温寒潮过程中一种典型的平流层—对流层耦合演变模态[J]. 大气科学, 46(6): 1484−1504. doi: 10.3878/j.issn.1006-9895.2206.21250
YU Yueyue, LI Yafei, REN Rongcai, et al. 2022. A Typical Evolution Mode of Stratosphere–Troposphere Coupling during the Cold Air Outbreak Events in 2020/2021 Winter [J]. Chinese Journal of Atmospheric Sciences (in Chinese), 46(6): 1484−1504. doi: 10.3878/j.issn.1006-9895.2206.21250
Citation: YU Yueyue, LI Yafei, REN Rongcai, et al. 2022. A Typical Evolution Mode of Stratosphere–Troposphere Coupling during the Cold Air Outbreak Events in 2020/2021 Winter [J]. Chinese Journal of Atmospheric Sciences (in Chinese), 46(6): 1484−1504. doi: 10.3878/j.issn.1006-9895.2206.21250

2020/2021年冬季大范围低温寒潮过程中一种典型的平流层—对流层耦合演变模态

A Typical Evolution Mode of Stratosphere–Troposphere Coupling during the Cold Air Outbreak Events in 2020/2021 Winter

  • 摘要: 平流层爆发性增温(SSW)超前于对流层环流异常,是延长冬季寒潮低温预报时效的重要途径之一。然而强SSW事件前后地面温度响应的区域和时间存在不确定性,其中涉及的平流层—对流层耦合过程和机理也不十分清楚。本文采用1979~2021年ERA5再分析数据集,研究了2020/2021年冬季“偏心型”强SSW事件前后中高纬度地区地面温度异常的演变特征,并分析了其与等熵大气经向质量环流平流层—对流层分支的耦合演变模态的动力联系。结果表明,伴随此次强SSW事件,亚洲和北美中纬度地区的寒潮低温事件分别在绕极西风反转为东风之前和再次恢复为西风之后发生。SSW前后大气经向质量环流的平流层向极地暖支与对流层高层向极暖支、低层向赤道冷支之间呈现出三个阶段的耦合演变模态: 同位相“加强—加强”、反位相“加强—减弱”以及反位相“减弱—加强”。加强的质量环流对流层向赤道冷支是SSW前后寒潮低温事件的主要原因,而加强的向极地平流层暖支是SSW发生及其伴随的北极涛动负位相持续加强的主要原因。大气经向质量环流不同的垂直耦合模态取决于行星波槽脊在对流层顶和对流层中低层两个关键等熵面上的西倾角异常。西倾角异常表征大气波动的斜压性,主要通过影响关键等熵面以上向极地的净质量输送和其下向赤道的净质量输送进行调控。尤其在SSW发生后的极涡恢复期,对流层顶处异常偏弱的斜压性会加强对流层向极地暖支,进而加强向赤道冷支,有利于寒潮低温的发生。本次SSW事件前后大气经向质量环流三支的耦合演变模态,与历年平流层北半球环状模(NAM)负事件中极区平流层温度异常信号下传滞后的平流层—对流层耦合演变类型相一致,其在波动尺度方面也存在共同特征,即SSW事件或NAM负事件前期对流层一波加强且上传,后期对流层二波加强但较难上传。

     

    Abstract: Stratospheric sudden warming (SSW) events are a critical predictability source of the extended-range prediction of cold air outbreaks (CAO) because of their temporal lead information relative to the tropospheric circulation changes. Nevertheless, there is large uncertainty in the region and time of the surface air temperature in response to the SSW events, and the stratosphere–troposphere coupling process and the mechanism involved still remain unclear. Using the ERA5 reanalysis data from 1979 to 2021, this study focuses on the “displacement” type major SSW event of 2020/2021 winter and investigates the characteristics of the extratropical surface air temperature anomalies and the evolution of the coupling mode of the poleward warm air stratospheric branch (WB_ST), poleward warm air tropospheric branch (WB_TR), and equatorward cold branch (CB) of isentropic meridional mass circulation (IMMC), as well as the related wave dynamics. Results show that associated with this SSW event, the CAO events across Eurasia occurred before the subpolar westerly became easterly, while the CAO event over North America occurred after the subpolar westerly recovered. This mainly resulted from the three-stage coupling modes among the three IMMC branches during the stratospheric polar vortex oscillation, i.e., in-phase strengthening of WB_ST and WB_TR/CB, out-of-phase with a strengthened WB_ST but a weakened WB_TR/CB, and out-of-phase with a weakened WB_ST but a strengthened WB_TR/CB. The stronger CB resulted in the CAOs associated with SSW, while the stronger WB_ST predominantly contributed to the occurrence of SSW and the continuous strengthening of the negative phase of the Arctic Oscillation. The coupling mode of the three IMC branches depended on the westward tilt of waves at two critical levels, namely the tropopause level and the middle and lower tropospheric level. Anomalously strong westward tilt caused the net poleward mass transport above this level and the net equatorward transport below it and vice versa. Especially during the polar vortex recovery, the anomalously weak baroclinicity near the tropopause strengthens the WB_TR and, subsequently, the CB due to the mass continuity, which is conducive to the occurrence of CAOs. The coupling evolution mode of IMMC around the SSW in 2020/2021 winter was found to be highly consistent with that during the negative stratospheric Northern Annular Mode (NAM) events that were characterized by a lagged downward propagation of the polar stratospheric temperature anomaly to the lower troposphere. The two share common features in terms of the wave scale, i.e., the wavenumber-1 waves tend to be stronger and propagate upward in the early stage of the SSW event or the negative NAM event, and the wavenumber-2 waves are strengthened but restrained within the troposphere after the SSW.

     

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