An Isentropic Mass Circulation View on the Extreme Cold Events in the 2020/21 Winter

Abstract

Three extreme cold events successively occurred across East Asia and North America in the 2020/21 winter. This study investigates the underlying mechanisms of these record-breaking persistent cold events from the isentropic mass circulation (IMC) perspective. Results show that the midlatitude cold surface temperature anomalies always co-occurred with the high-latitude warm anomalies, and this was closely related to the strengthening of the low-level equatorward cold air branch of the IMC, particularly along the climatological cold air routes over East Asia and North America. Specifically, the two cold surges over East Asia in early winter were results of intensification of cold air transport there, influenced by the Arctic sea ice loss in autumn. The weakened cold air transport over North America associated with warmer northeastern Pacific sea surface temperatures (SSTs) explained the concurrent anomalous warmth there. This enhanced a wavenumber-1 pattern and upward wave propagation, inducing a simultaneous and long-lasting stronger poleward warm air branch (WB) of the IMC in the stratosphere and hence a displacement-type Stratospheric Sudden Warming (SSW) event on 4 January. The WB-induced increase in the air mass transported into the polar stratosphere was followed by intensification of the equatorward cold branch, hence promoting the occurrence of two extreme cold events respectively over East Asia in the beginning of January and over North America in February. Results do not yield a robust direct linkage from La Niña to the SSW event, IMC changes, and cold events, though the extratropical warm SSTs are found to contribute to the February cold surge in North America.

摘要

2020/21年冬季, 东亚和北美先后发生了三次极端低温事件. 本研究从等熵大气经向质量环流 (IMC) 的观点揭示了这些破纪录极端低温事件背后的可能机制. 结果表明, 中纬度地面温度冷异常往往伴随高纬度暖异常, 这与IMC的低层向赤冷支的加强, 尤其是在气候态冷空气路径——东亚和北美的冷空气经向输送的加强, 密切相关. 发生在初冬东亚的两次低温事件是由秋季北极海冰显著偏低从而加强东亚路径冷空气经向输送的结果, 而同期北美异常偏暖, 与东北太平洋异常暖海温引起的冷空气经向输送减弱有关. 此时, 增强的一波波动上传, 持续加强IMC的平流层向极暖支, 导致在1月4日发生了偏心型平流层爆发性增温 (SSW) 事件. 偏强的平流层向极暖支引起极地平流层空气质量增多, 使得地面高压加强南伸、 低层向赤道冷支加强, 有利于1月初东亚极端低温事件和2月北美极端低温事件的发生. 本研究还指出了温带暖海温对2月北美极端低温事件的重要贡献, 但并未发现拉尼娜与SSW事件、 IMC变化和极端低温事件之间存在显著的直接联系.