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Apr.  2022

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# Seasonal Cumulative Effect of Ural Blocking Episodes on the Frequent Cold events in China during the Early Winter of 2020/21

• Starting in mid-November, China was hit by several cold events during the early winter of 2020/21. The lowest temperature observed at Beijing station on 7 January reached −19.6°C. In this paper, we show that the outbreak of the record-breaking extreme cold event can be attributed to a huge merging Ural blocking (UB) ridge over the Eurasian region. The sea-ice cover in the Kara and East Siberia Seas (KESS) in autumn was at its lowest value since 1979, which could have served as a precursor signal. Further analysis shows that several successive UB episodes occurred from 1 September 2020 to 10 January 2021. The persistent UB that occurred in late September/early October 2020 may have made an important contribution to the October historical minimum of sea ice in the KESS region. Our results also show that, after each UB episode in winter, significant upward propagation of wave activity occurred around 60°E, which resulted in weakening the stratospheric vortex. Meanwhile, each UB episode also caused a significant reduction in sea-ice extent in KESS and a significant weakening of the westerly jet in mid–high-latitude Eurasia. Results suggest that the Arctic vortex, which is supposed to enhance seasonally, became weaker and more unstable than the climatic mean under the seasonal cumulative effects of UB episodes, KESS warming, and long-lasting negative-phase North Atlantic Oscillation (NAO–). Those seasonal cumulative effects, combined with the impact of La Niña winter, led to the frequent occurrence of extreme cold events.
摘要: 2020/21年初冬，我国遭遇了几次大范围的极寒天气过程。1月7日，北京站观测到的最低气温达到-19.6℃。研究表明，破纪录的极寒事件的爆发可归因于欧亚地区上空巨大的乌拉尔阻塞脊的发展和合并过程。秋季喀拉海和东西伯利亚海的海冰覆盖率为1979年以来的最低值，这可能是一个前兆信号。进一步的分析表明，从2020年9月1日到2021年1月10日，发生了几次连续的乌拉尔阻塞事件。2020年9月底/10月初发生的持续的乌拉尔阻塞可能对喀拉海和东西伯利亚海10月份的海冰历史最低值有重要贡献。结果还显示，冬季每次乌拉尔阻塞发生后，60°E附近区域对流层大气会发生明显的能量向上传播，导致平流层涡旋的减弱。同时，每次UB事件也会导致喀拉海和东西伯利亚海的海冰明显减少，欧亚大陆中高纬度地区的西风急流明显减弱。总得来说，在乌拉尔阻塞事件、喀拉海和东西伯利亚海变暖和持续的负位相北大西洋涛动的季节性累积效应下，本应季节性增强的北极涡旋变得比气候平均值更弱和更加不稳定。这些季节性的累积效应，再加上拉尼娜的协同影响，导致了极端寒冷事件的频繁发生。
• Figure 1.  Spatial patterns of time-averaged (6–8 January 2021) SAT anomalies (units: K) relative to corresponding daily means for 1981–2010 of 2414 stations across China.

Figure 2.  Evolutions of daily spatial distributions of 500-hPa geopotential height (contours, CI = 80 gpm) and SAT (shading, units: K) anomalies from 29 December 2020 to 9 January 2021.

Figure 3.  Spatial patterns of daily cold air mass (shading; in hPa) and its flux vector (arrows; in hPa m s−1) from 29 December 2020 to 9 January 2021. Cold air mass, ${\text{DP}} = {P_{\text{S}}} - P({\theta _{\text{T}}})$, is defined as the pressure difference between the 280-K potential temperature surface $P({\theta _{\text{T}}})$ and the earth’s surface ${P_{\text{S}}}$, and its flux vector can be expressed as $\int_{{p_{\text{S}}}}^{p({\theta _{\text{T}}})} {{\boldsymbol{v}}{\rm{d}}p}$, ${\boldsymbol{v}} = (u,v)$.

Figure 4.  Spatial patterns of SIC anomalies (units: %) for (a, d) September, (b, e) October, and (c, f) November 2020, based on monthly SIC data from (a–c) the Met Office Hadley Centre and (d–f) NSIDC. The region marked by the black frame is KESS (75°–85°N， 60°E–180°).

Figure 5.  Time series during 1980–2020 of the spatially averaged SIC anomaly (units: %) over (a, c) the whole Arctic and (b, d) KESS (75°–85°N, 60°E–180°). The red, blue, and purple dashed lines represent the SIC variations in September, October, and November, respectively, and the black solid line represents the autumn mean. The monthly SIC dataset used here is derived from (a, b) the Met Office Hadley Centre and (c, d) NSIDC.

Figure 6.  Time series of (a) the 300-hPa zonal wind (units: m s−1) averaged over the Ural region (45°–65°N, 30°–120°E), where the red solid line represents autumn/winter 2020, the red dashed line represents the corresponding daily mean during 1980–2020, and the blue line is the normalized NAO index, (b) the KESS (75°–85°N, 60°E–180°) SIC derived from the monthly data of NSIDC (red line) and ERA5 (blue line) and mean SAT of 2414 stations nationwide (black line). The black (blue) dashed boxes indicate the sharp SAT (SIC) decline associated with UB. The gray shading represents the UB episodes and the vertical gray line represents the Lag-0 day for each UB episode.

Figure 7.  Horizontal patterns of vaTa (units: K m s−1) at the 100-hPa pressure level for (a) 16 November 2020, (b) 10 December 2020, and (c) 1 January 2021.

Figure 8.  Time–pressure evolutions of the (a) domain-mean (50°–80°N, 30°–120°E) zonal wind tendency (shading, units: m s−1 d−1) and vertical component of EP flux (contours, × 106 m2 s−2 Pa), and (b) zonal mean (90°–150°E) westerly wind anomaly at 55°N. Similar to Taguchi and Hartmann (2006), the variables in (a) are scaled by a factor inversely proportional to pressure $\sqrt {1000/P}$ . The vertical dashed line represents the Lag-0 days of several UB events.

Figure 9.  Spatial distributions of December SST (units: K) for the 1979–2020 mean climatology (contours) and anomaly pattern in 2020 (shading).

Figure 10.  Schematic diagram of the physical processes leading to the cold events in early winter 2020/21. The thick gray solid line over the Eurasian continent represents the flow line of the UB. The letter H (L) represents the anticyclonic (cyclonic) center of the UB. Red (blue) shading indicates the warming (cooling) caused by UB. KESS marks the Kara and East Siberia Seas with abnormal sea-ice cover. The upward-pointing arrows (a) indicate the propagation of EP fluxes from the troposphere to the stratosphere (weakening of the polar vortex) associated with several UB episodes. The downward-pointing arrows (b) highlight the break-up of the polar vortex and its associated several cold air outbreaks in East Asia. The light blue shading in the central-eastern Pacific represents the cold SST anomaly due to La Niña. NAO− indicates the negative NAO phase starting from December 2020. The dashed lines represent the several UBs and associated processes that occur across seasons (from Sep 2020 to Jan 2021), which is referred to as the seasonal cumulative effect.

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## Manuscript History

Manuscript revised: 17 August 2021
Manuscript accepted: 07 September 2021
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Seasonal Cumulative Effect of Ural Blocking Episodes on the Frequent Cold events in China during the Early Winter of 2020/21

###### Corresponding author: Yao YAO, yaoyao@tea.ac.cn;
• 1. Key Laboratory of Regional Climate-Environment for Temperate East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 2. University of Chinese Academy of Sciences, Beijing 100029, China
• 3. Institute of Urban Meteorology, China Meteorological Administration, Beijing 100029, China

Abstract: Starting in mid-November, China was hit by several cold events during the early winter of 2020/21. The lowest temperature observed at Beijing station on 7 January reached −19.6°C. In this paper, we show that the outbreak of the record-breaking extreme cold event can be attributed to a huge merging Ural blocking (UB) ridge over the Eurasian region. The sea-ice cover in the Kara and East Siberia Seas (KESS) in autumn was at its lowest value since 1979, which could have served as a precursor signal. Further analysis shows that several successive UB episodes occurred from 1 September 2020 to 10 January 2021. The persistent UB that occurred in late September/early October 2020 may have made an important contribution to the October historical minimum of sea ice in the KESS region. Our results also show that, after each UB episode in winter, significant upward propagation of wave activity occurred around 60°E, which resulted in weakening the stratospheric vortex. Meanwhile, each UB episode also caused a significant reduction in sea-ice extent in KESS and a significant weakening of the westerly jet in mid–high-latitude Eurasia. Results suggest that the Arctic vortex, which is supposed to enhance seasonally, became weaker and more unstable than the climatic mean under the seasonal cumulative effects of UB episodes, KESS warming, and long-lasting negative-phase North Atlantic Oscillation (NAO–). Those seasonal cumulative effects, combined with the impact of La Niña winter, led to the frequent occurrence of extreme cold events.

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