北半球冬季平流层强、弱极涡事件演变过程的对比分析

利用1958～2017年逐日的NCEP/NCAR再分析资料对北半球冬季平流层强、弱极涡事件的演变过程进行了对比分析，同时比较了有平流层爆发性增温（SSW）和无SSW发生的两类弱极涡事件的环流演变和动力学特征。结果表明，强极涡的形成存在着缓慢发展和快速增强的过程，而弱极涡事件的建立非常迅速；和强极涡事件相比，弱极涡事件的峰值强度更强，异常中心的位置更高。此外，强、弱极涡事件的产生与波流相互作用的正反馈过程密切相关。对于强极涡事件，发展阶段的太平洋—北美（PNA）型异常削弱了行星波一波；当平流层西风达到一定强度，上传的行星波受到强烈抑制，使得极涡迅速增强达到峰值。而对于弱极涡事件，发展阶段一波型的异常增强了行星波上传，通过对纬向流的拖曳作用使得平流层很快处于弱西风状态，更多行星波进入平流层导致极涡急剧减弱甚至崩溃。针对有、无SSW发生的两类弱极涡事件的对比分析表明，有SSW发生的弱极涡事件发展阶段，平流层出现强的向上的一波Eliassen-Palm（EP）通量异常，通过正反馈过程使得一波和二波上传同时增强而导致极涡崩溃；无SSW发生的弱极涡事件发展阶段，平流层缺乏向上的一波通量，二波活动起到重要作用，其总的行星波上传远弱于有SSW发生的弱极涡事件。对于无SSW发生的弱极涡事件，其发展和成熟阶段对流层上部出现类似欧亚（EU）型的高度异常，伴随着强的向极的EP通量异常，导致对流层有极强的负北极涛动（AO）型异常。而有SSW发生的弱极涡事件发展阶段对流层上部主要表现为北太平洋上空来自低纬的波列异常，其后期的对流层效应更加滞后也不连续，对流层AO异常的强度明显弱于无SSW发生的弱极涡事件。

Comparative Analysis of the Evolution Processes of the Strong and Weak Stratosphere Polar Vortex Events in Boreal Winter

Based on the NCEP/NCAR daily reanalysis data for the period of 1958–2017, this study comparatively analyzes the stratospheric and tropospheric evolutions during the lifecycle of both strong and weak stratosphere polar vortex events (SPV and WPV events, respectively). Moreover, the atmospheric circulation and dynamical characteristics of two types of WPV events, namely events with and without stratospheric sudden warming (SSW), were also analyzed. The results show that the formation of SPV events follow a slow development and then a rapid intensification stage, while the WPV events are established dramatically. Compared with the SPV events, the WPV events are stronger and have a higher anomaly center when they reach a peak. Moreover, the occurrence of SPV and WPV events is closely related to the positive feedback of wave–mean flow interaction. For the SPV events, a Pacific–North American teleconnection-like pattern weakens the wave-1 of planetary waves during the growth stage. When the stratospheric westerly winds are strengthened to a certain extent, upward propagating planetary waves are greatly suppressed; thus, the polar vortex is intensified rapidly and reaches the peak stage. For the WPV events, a wave-1 pattern enhances the upward propagating planetary waves in the growth stage, which soon leads to weak westerly winds in the stratosphere by exerting a drag on the zonal flow. More planetary waves then propagate into the stratosphere, and thus, the polar vortex is dramatically weakened and even broken down. In addition, for the WPV events with SSW, enhanced upward wave-1 Eliassen-Palm (EP) flux in the stratosphere occurs in the growth stage. Through the positive feedback of wave–mean flow interaction, both the upward propagating wave-1 and wave-2 EP fluxes are increased, which leads to the breakdown of the polar vortex. For the WPV events without SSW, the upward propagating wave-1 EP flux is weak in the growth stage, while the wave-2 flux plays an important role. Hence, the total upward propagating planetary waves are much smaller than the WPV events with SSW. For the WPV events without SSW, a Eurasian (EU) teleconnection-like pattern in the height field appears in the upper troposphere during the growth and peak stages, accompanied by strong anomalous poleward EP flux, which leads to extreme negative Arctic oscillation (AO) in the troposphere. For the WPV events with SSW, a wave train from the lower latitude over the North Pacific in the height field appears in the upper troposphere mainly during the growth stage. In the later stages, the tropospheric influence of the WPV events with SSW is relatively delayed and not robust, and the magnitude of AO index is much smaller than that for the WPV events without SSW.