Changes in Atmospheric Circulation during the Winter Regional Extreme Cold Events over China since 1960

The number of days with occurrence of winter regional extreme cold events (RECEs) in China was found more during 1960/1961-1985/1986 (period 1), less during 1986/1987-2005/2006 (period 2), but more again during 2006/2007-2017/2018 (period 3). So far, the differences in the atmospheric circulation favoring RECEs among these three periods are unclear. In this paper, changes in atmospheric circulation during the RECEs over China are examined by using composite analysis based on the station observed temperature data and NCEP-NCAR reanalysis data in winters of 1960/1961-2017/2018. The results show: (1) the stratospheric polar vortex was more active and tended to split before the outbreak of RECEs in period 3 than that in other two periods. The shift of the stratospheric polar vortex to Eurasia helped the upper Arctic cold air to affect the lower latitudes. (2) The troposphere was characterized by a typical or significant three-wave pattern before the outbreak of RECEs in period 2, in contrast to a weakened three-wave pattern in period 1. Compared to periods 1 and 2, the Okhotsk blocking high was stronger in period 3, contributing to the inverted omega-shaped circulation pattern in East Asia-North Pacific section and a shift of global pattern from three-wave to two-wave. The weakened three-wave or two-wave circulation pattern was manifested by the stronger Ural/Okhotsk blocking high, conducive to the strengthening of the meridional circulation and the occurrence of RECEs in East Asia. (3) The Siberian high was the strongest in period 3, followed by period 1, and it was the weakest in period 2. Before the outbreak of RECEs, the Siberian high in period 3 began to intensify one week earlier than that in periods 1 and 2. Thus, the accumulation time of cold air mass in period 3 was the longest. In summary, the synergism of atmospheric circulation at high and low levels in periods 1 and 3 was more conducive to more and strong RECEs than that in period 2. Moreover, the split of the stratospheric polar vortex may have played an important role on the formation of tropospheric two-wave pattern in period 3. The results obtained herein may provide a better understanding of the mechanisms for occurrences of RECEs in China.