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秋季北极海冰影响平流层极涡“动态年循环”的机理

Mechanisms of Arctic Sea Ice in Autumn Influencing the Varying Annual Cycle of the Stratospheric Polar Vortex

  • 摘要: 本文利用ERA5再分析数据集和NOAA海冰密集度(SIC)数据,通过集合经验模态分解(EEMD)方法,精准捕捉了平流层极涡的动态年循环特征,分析了与极涡建立和崩溃日相关的海冰关键区,并从热力和动力双重维度,揭示了秋季海冰影响平流层极涡演变的物理过程和机理。结果表明:平流层极涡建立和崩溃时间均存在显著的年际变化,其中崩溃时间的年际差异更为显著。伴随极涡的建立偏晚(早)或崩溃偏早(晚),均有盛冬极涡偏强(弱)的特征;且极涡建立偏晚还与晚冬-早春平流层极区温度暖异常有关,这可能为极涡的提前崩溃提供有利条件。10月巴伦支海区域的SIC与极涡建立时间呈正相关,可能通过其对秋季行星波的调控和短波辐射的反射作用影响极区温度,但显著性均较低,表明海冰在极涡建立中作用有限。而10月东西伯利亚海的SIC与极涡崩溃时间呈显著负相关,前者影响后者的主要机制为:秋季异常偏低的东西伯利亚海海冰,通过抑制行星一波的上传,使得次极地平流层有Eliassen-Palm(E-P)通量辐散异常,平流层绕极西风加强,缓变的非绝热过程起主导作用,使得极涡崩溃时间偏晚;而秋季异常偏高的海冰,通过增强行星一波的上传,使得次极地平流层有E-P通量辐合异常,平流层绕极西风减弱,快变的动力作用起主导作用,使得极涡崩溃时间偏早。

     

    Abstract: This paper, based on the ERA5 reanalysis dataset and the NOAA sea ice concentration (SIC) data, accurately extracted the varying annual cycle of the stratospheric polar vortex (SPV) using the Ensemble Empirical Mode Decomposition (EEMD) method. We further identified key sea ice regions associated with the interannual changes in the build-up and break-up date of the SPV, and investigated how autumn sea ice in the key region influences the evolution of SPV through both thermal and dynamic processes. Results show that there are significant interannual variations in both the SPV build-up and break-up dates, with the variation in break-up dates being more pronounced. Years with anomalously late (early) SPV build-up and early (late) break-up tend to have an anomalously strong (weak) SPV in mid-winter. Late SPV build-up is linked to warm temperature anomalies in the polar stratosphere during late winter to early spring, creating favorable conditions for SPV early break-up events. Further investigation shows a positive correlation between October SIC in the Barents Sea and the SPV build-up date, likely due to its influence on planetary wave modulation and reflected shortwave radiation in autumn. However, the statistical significance is relatively low, indicating a limited role of sea ice in the SPV build-up. Conversely, October SIC in the East Siberian Sea shows a significant negative correlation with the SPV break-up date. The primary mechanism behind this relationship is that anomalously low SIC in the East Siberian Sea suppresses the upward propagation of planetary waves. This leads to Eliassen-Palm (E-P) flux divergence anomalies, stronger stratospheric circumpolar westerlies. Thereby, SPV break-up is delayed, which is dominated by slow diabatic processes. In contrast, high SIC enhances planetary wave propagation, resulting in E-P flux convergence, weaker circumpolar westerlies. Therefore, SPV break up earlier, which is due to rapid dynamic processes.

     

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