The 2020/21 Extremely Cold Winter in China Influenced by the Synergistic Effect of La Niña and Warm Arctic

Abstract

In the first half of winter 2020/21, China has experienced an extremely cold period across both northern and southern regions, with record-breaking low temperatures set in many stations of China. Meanwhile, a moderate La Niña event which exceeded both oceanic and atmospheric thresholds began in August 2020 and in a few months developed into its mature phase, just prior to the 2020/21 winter. In this report, the mid−high-latitude large-scale atmospheric circulation anomalies in the Northern Hemisphere, which were forced by the negative phase of Arctic Oscillation, a strengthened Siberian High, an intensified Ural High and a deepened East Asian Trough, are considered to be the direct reason for the frequent cold surges in winter 2020/21. At the same time, the synergistic effect of the warm Arctic and the cold tropical Pacific (La Niña) provided an indispensable background, at a hemispheric scale, to intensify the atmospheric circulation anomalies in middle-to-high latitudes. In the end, a most recent La Niña prediction is provided and the on-coming evolution of climate is discussed for the remaining part of the 2020/21 winter for the purpose of future decision-making and early warning.

摘要

在2020/21年冬季的前半段 (2020年12月1日-2021年1月10日), 影响我国的冷空气活动持续偏强 (一次全国型强冷空气, 两次全国型寒潮), 导致我国中东部大部地区气温异常偏低, 部分站点最低气温甚至突破历史极值. 同时, 一次中等强度的拉尼娜事件从2020年8月开始, 并于2020/21年冬季发展到成熟阶段. 北半球中高纬大尺度大气环流的持续性异常是今冬前期强寒潮频繁发生的最直接原因: 北极涛动持续负相位、 西伯利亚高压持续偏强、 乌拉尔山高压脊偏强和东亚大槽偏深导致的环流经向度持续偏大. 本报告强调偏暖的北极和偏冷的热带太平洋 (拉尼娜) 两者的协同作用为上述大气环流异常提供了必不可少的背景条件 (半球尺度的南北温度梯度减弱). 最后, 报告还提供了最新的拉尼娜预测及今冬后期的气候趋势预判, 供将来决策和预警之用.