The different stratospheric influence on cold-extremes in Eurasia and North America
The different stratospheric influence on cold-extremes in Eurasia and North America
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DOI：
10.1038/s41612-018-0054-4
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发表时间：
2018-01-01
影响因子：
9
通讯作者：
Coumou, Dim
中科院分区：
文献类型：
--
作者：
Kretschmer, Marlene;Cohen, Judah;Coumou, Dim
平流层极涡可以影响对流层环流，从而影响中纬度地区的冬季天气。通常与平流层突然变暖（SSW）有关的弱涡旋状态已被证明会增加寒流的风险，特别是在欧亚大陆，但它对北美冬季的作用尚不清楚。通过聚类分析，我们发现平流层下部存在两种主要的极帽高度增加模式。这两种模式都代表一个弱极涡，但它们与不同的波动机制和不同的区域对流层影响有关。第一种模式是纬向对称的，与吸收的向上传播的波活动有关，导致北大西洋涛动（NAO）的负相和欧亚大陆北部的冷空气爆发。这种耦合机制在文献中得到了充分的证明，并且与北环模（NAM）的向下迁移一致。第二种模式是纬向不对称的，与加拿大上空向下反射的行星波有关，随后是西太平洋涛动（WPO）的负相，以及加拿大中部和大湖区的寒流。因果效应网络（CEN）分析证实了与这种不对称模式相关的大气路径。此外，我们的研究结果表明，反射机制对上升波活动通量的精确区域敏感，并且依赖于涡旋强度的状态。确定在周至月时间尺度上运行的因果路径可以为改进中纬度地区寒流的分季节到季节性预报铺平道路。
The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes.