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

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 Nina 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 Nina) 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 Nina 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.

Automated summary

This report highlights that the frequent cold surges in the winter of 2020/21 were mainly caused by large-scale atmospheric circulation anomalies in the mid-high latitudes of the Northern Hemisphere, with the synergistic effect of the warm Arctic and La Nina intensifying these anomalies.