Abstract:During the winter of 2020/2021, the climate over China was characterized by “cold and dry” in the first half of winter and “warm and wet” in the latter half of winter. In the cold and warm two periods, the amplitude of surface air temperature (SAT) fluctuated extremely, and the observed daily SATs at many stations in China broke the lowest and highest records, respectively. From 1 December 2020 to 10 January 2021, it was cold and dry over most of China, but turned sharply to warm and wet condition during the period from 13 January to 28 February 2021. The overturning changes of the large-scale circulation systems, including the Ural blocking, Siberian high, East Asian winter monsoon (EAWM), the position of the polar cold air and the western Pacific subtropical high, provided the direct reason for the cold-warm transition in China during this winter. Further analysis shows that the La Ni〖AKn~D〗a event, coupled with the reduced Arctic ice and the warmer North Atlantic currents in the mid-latitudes, met the conditions for the “warm Arctic, cold Eurasian” effect in the earlier winter. They intensified the meridional circulation with stronger Ural blocking, Siberian high and EAWM, and further led to the cold and dry climate in China. In the latter half of winter, the Arctic experienced a sudden stratospheric warming event, favoring the long-lasting negative Arctic Oscillation. The main body of the polar vortex shifted to the Western Hemisphere, while at the same time, the Ural blocking collapsed and the EAWM weakened. As a result, the temperature in most parts of China suddenly increased to be above normal.