Tropical Upper-Tropospheric Potential Vorticity Intrusions during Sudden Stratospheric Warmings Journal of the Atmospheric Sciences

The intrusion of lower-stratospheric extratropical potential vorticity into the tropical upper troposphere in the weeks surrounding the occurrence of sudden stratospheric warmings (SSWs) is examined. The analysis reveals that SSW-related PV intrusions are significantly stronger, penetrate more deeply into the tropics, and exhibit distinct geographic distributions compared to their climatological counterparts. While climatological upper-tropospheric and lower-stratospheric (UTLS) PV intrusions are generally attributed to synoptic-scale Rossby wave breaking, it is found that SSW-related PV intrusions are governed by planetary-scale wave disturbances that deform the extratropical meridional PV gradient maximum equatorward. As these deformations unfold, planetary-scale wave breaking along the edge of the polar vortex extends deeply into the subtropical and tropical UTLS. In addition, the material PV deformations also reorganize the geographic structure of the UTLS waveguide, which alters where synoptic-scale waves break. In combination, these two intrusion mechanisms provide a robust explanation describing why displacement and split SSWs-or, more generally, anomalous stratospheric planetary wave events-produce intrusions with unique geographic distributions: displacement SSWs have a single PV intrusion maximum over the Pacific Ocean, while split SSWs have intrusion maxima over the Pacific and Indian Oceans. It is also shown that the two intrusion mechanisms involve distinct time scales of variability, and it is highlighted that they represent an instantaneous and direct link between the stratosphere and troposphere. This is in contrast to higher-latitude stratosphere-troposphere coupling that occurs indirectly via wave-mean flow feedbacks.

本研究针对平流层突然增温（sudden stratospheric warmings, SSWs）发生前后数周内，中纬度平流层下层位涡向热带对流层上层的侵入过程展开考察。分析结果显示，与SSWs相关的位涡侵入相较于气候态情形，强度显著更强、向热带穿透深度更深，且具备独特的地理分布特征。尽管气候态对流层上层-平流层下层（Upper Troposphere Lower Stratosphere, UTLS）位涡侵入通常归因于天气尺度罗斯比波破碎，但本研究发现，与SSWs相关的位涡侵入由使中纬度向赤道侧的极区位涡经向梯度极大值发生变形的行星尺度波动扰动主导。随着此类变形过程发展，沿极涡边缘发生的行星尺度波破碎会深入延伸至亚热带与热带UTLS区域。此外，物质位涡变形还会重构UTLS波导的地理结构，进而改变天气尺度波破碎的发生位置。综合而言，这两类侵入机制可稳健解释为何位移型与分裂型SSWs——更宽泛而言，即异常平流层行星波动事件——会产生具有独特地理分布的位涡侵入：位移型SSWs仅在太平洋上空存在单个位涡侵入极值区，而分裂型SSWs则在太平洋与印度洋上空均存在侵入极值区。研究还表明，两类侵入机制具有不同的变率时间尺度，且二者构成了平流层与对流层之间直接且瞬时的联系，这与通过波-平均流反馈间接发生的高纬度平流层-对流层耦合过程形成鲜明对比。