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# Influence of Major Stratospheric Sudden Warming on the Unprecedented Cold Wave in East Asia in January 2021

• An unprecedented cold wave intruded into East Asia in early January 2021 and led to record-breaking or historical extreme low temperatures over vast regions. This study shows that a major stratospheric sudden warming (SSW) event at the beginning of January 2021 exerted an important influence on this cold wave. The major SSW event occurred on 2 January 2021 and subsequently led to the displacement of the stratospheric polar vortex to the East Asian side. Moreover, the SSW event induced the stratospheric warming signal to propagate downward to the mid-to-lower troposphere, which not only enhanced the blocking in the Urals–Siberia region and the negative phase of the Arctic Oscillation, but also shifted the tropospheric polar vortex off the pole. The displaced tropospheric polar vortex, Ural blocking, and another downstream blocking ridge over western North America formed a distinct inverted omega-shaped circulation pattern (IOCP) in the East Asia–North Pacific sector. This IOCP was the most direct and impactful atmospheric pattern causing the cold wave in East Asia. The IOCP triggered a meridional cell with an upward branch in East Asia and a downward branch in Siberia. The meridional cell intensified the Siberian high and low-level northerly winds, which also favored the invasion of the cold wave into East Asia. Hence, the SSW event and tropospheric circulations such as the IOCP, negative phase of Arctic Oscillation, Ural blocking, enhanced Siberian high, and eastward propagation of Rossby wave eventually induced the outbreak of an unprecedented cold wave in East Asia in early January 2021.
摘要: 2021年1月初一次超强寒潮过程侵袭东亚地区，导致我国大范围地区出现极端低温或破纪录低温。研究表明，2021年1月初发生的一次强平流层爆发性增温事件对此次超强寒潮的爆发产生了重要影响。这次平流层爆发性增温事件发生在2021年1月2日，之后平流层增暖信号向下传播至对流层，不仅增强了乌拉尔-西伯利亚地区的阻塞高压也加强了负位相的北极涛动，减弱了对流层极涡，使得对流层极涡中心偏向东亚-北太平地区。偏向东亚-北太平地区的对流层极涡、乌拉尔阻塞和北美西部的下游阻塞高压脊在东亚-北太平洋地区共同形成了一个倒欧米伽形的环流型。倒欧米伽环流型是引起东亚超强寒潮最直接和影响最大的环流系统。此外，倒欧米伽环流型的出现也激发了一个经向环流圈，环流圈的上升支位于东亚，而下沉支位于西伯利亚地区。经向环流圈增强了西伯利亚高压和东亚地区低空的偏北风，进一步有利于超强寒潮影响东亚地区。因此，平流层爆发性增温事件和对流层的环流系统，如倒欧米伽环流型、负位相的北极涛动、乌拉尔阻塞、西伯利亚高压以及Rossby波传播的下游效应共同导致了2021年1月初东亚地区超强寒潮的爆发。
• Figure 1.  Spatiotemporal evaluation of the RELTE in East Asia from 5 to 10 January 2021. The color shading shows the daily minimum temperature anomaly relative to the extreme low-temperature thresholds (units: °C). The blue dots indicate the regions affected by this event where daily minimum temperatures fell below the extreme low-temperature thresholds defined in section 2.2.1.

Figure 2.  (a) Distribution of the minimum value of the daily minimum temperature (units: °C) over China from 6 to 8 January 2021. (b) Sites at which the daily minimum temperature broke records (red dots) or reached historical extreme values (blue dots) over China from 6 to 8 January 2021. (c) Daily variation of the minimum temperature (units: °C) averaged for the 383 stations shown in Figure (b) from 10 December 2020 to 10 January 2021 (red line) and its climatology for the time period 1981–2010 (black dashed line).

Figure 3.  (a) Temporal evolution of the 10-hPa zonal-mean temperature gradient (units: K) between 90°N and 60°N from 1 December 2020 to 31 January 2021. (b) Temporal evolution of the 10-hPa zonal-mean zonal wind at 60°N (units: m s–1). (c) Time–height cross section of the temperature of the polar cap (65°–90°N) (shading; units: K) and zonal winds (contours; red/blue contours indicate positive/negative values and black contours indicate zero line; interval 5 m s–1) from 1 December 2020 to 31 January 2021. (d) Time–height cross section of the potential vorticity (shading; units: PVU) over the polar region (75°–90°N). (e) Distribution of the geopotential height (contours; units: dagpm) and its anomalies (shading; units: dagpm) at 50 hPa averaged from 2 to 5 January 2021.

Figure 4.  Height–latitude cross section of the Eliassen–Palm flux (vectors; units: m2 s–2) and its divergence (contours; interval 100 m2 s–2) (a) averaged from 28 to 31 December 2020 and (b) averaged from 2 to 5 January 2021. The vectors are scaled by$\sqrt{\text{1000}/{p}}$ at all levels and are magnified by a scale factor of five above 100 hPa for a better visualization of the smaller vectors in the stratosphere. This scaling method is applied after Eq. (3).

Figure 5.  (a) Time–height cross section of the geopotential height anomaly (units: dagpm) over the polar cap from 1 December 2020 to 31 January 2021. (b) Arctic Oscillation index from 1 December 2020 to 31 January 2021. (c) Siberian high index (units: hPa) from 1 December 2020 to 31 January 2021. The climatological mean period is defined as 1981–2010. (d) Time–height cross section of geopotential height anomaly (units: dagpm) averaged over 55°–75°N, 80°–110°E from 1 December 2020 to 31 January 2021.

Figure 6.  (a) Height–latitude cross section of the potential vorticity (shading; units: PVU) and 315-K potential temperature contour (red dotted contour; units: K) averaged along 115°–135°E on 6 January 2021. (b) Latitude–time cross section of the potential vorticity (shading; units: PVU) and temperature (contours; units: °C) at the 315-K isentropic surface averaged along 115°–135°E from 1 to 10 January 2021.

Figure 7.  (a) Geopotential height (shading; units: dagpm) and TN wave activity flux (vectors; units: m2 s–2) at 500 hPa averaged from 5 to 8 January 2021. The bold black 528-dagpm isoline in the East Asia–North Pacific sector indicates the location of the IOCP. The green dotted lines (the left line: 60°, 67.5°, 70°, 72.5°, 75°, 77.5°, 80°, 85°, 87.5°, 90°, 92.5°, 95°, 97.5°, 97.5°, 100°, 100°, 102.5°, 102.5°, 105°, 105.0°, 105°, 105°, 107.5°, 107.5°, 107.5°, 107.5°, 107.5°, 107.5°, 107.5°E for latitudes from 80°N to 10°N, and the right line: 120°E for latitudes from 80°N to 10°N) indicate the transect location in Fig. 9. (b) Geopotential height anomalies at 500 hPa (shading; units: dagpm) averaged from 5 to 8 January 2021.

Figure 8.  The potential vorticity (shading; units: PVU), temperature (black contours; units: °C) at the 315-K isentropic surface, and 528 dagpm geopotential height contour at 500 hPa (red dashed contours) on alternate days from 2 to 8 January 2021.

Figure 9.  (Left panel) Height–latitude cross section of the potential temperature (dashed red contours; units: K), potential vorticity (solid blue contours; units: PVU) on the current day, and the change of potential vorticity (shading; units: PVU) between the current day and preceding two days averaged between the two green lines in Fig. 7a from 4 to 7 January 2021. (Right panel) Same as left panel, but for the height–latitude cross section of the change in the geopotential height (shading; units: dagpm) and vertical velocity (contours; units: Pa s–1).

Figure 10.  Schematic diagram of the influence of the major SSW event on the extreme East Asian cold wave in January 2021. SSW, stratospheric sudden warming; SPV, stratospheric polar vortex; AO, Arctic Oscillation; PV, potential vorticity; SH, Siberian high; UB, Ural blocking; Merid., Meridional; IOCP, inverted omega-shaped circulation pattern.

Figure A1.  Geopotential height (contours; units: dagpm) and its anomalies (shading; units: dagpm) at 10 hPa on (a) 1 and (b) 3 January 2021.

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## Manuscript History

Manuscript revised: 31 December 2021
Manuscript accepted: 18 January 2022
###### 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

## Influence of Major Stratospheric Sudden Warming on the Unprecedented Cold Wave in East Asia in January 2021

###### Corresponding author: Dong SI, sidong@mail.iap.ac.cn;
• 1. National Climate Center, China Meteorological Administration, Beijing 100081, China
• 2. Climate Change Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

Abstract: An unprecedented cold wave intruded into East Asia in early January 2021 and led to record-breaking or historical extreme low temperatures over vast regions. This study shows that a major stratospheric sudden warming (SSW) event at the beginning of January 2021 exerted an important influence on this cold wave. The major SSW event occurred on 2 January 2021 and subsequently led to the displacement of the stratospheric polar vortex to the East Asian side. Moreover, the SSW event induced the stratospheric warming signal to propagate downward to the mid-to-lower troposphere, which not only enhanced the blocking in the Urals–Siberia region and the negative phase of the Arctic Oscillation, but also shifted the tropospheric polar vortex off the pole. The displaced tropospheric polar vortex, Ural blocking, and another downstream blocking ridge over western North America formed a distinct inverted omega-shaped circulation pattern (IOCP) in the East Asia–North Pacific sector. This IOCP was the most direct and impactful atmospheric pattern causing the cold wave in East Asia. The IOCP triggered a meridional cell with an upward branch in East Asia and a downward branch in Siberia. The meridional cell intensified the Siberian high and low-level northerly winds, which also favored the invasion of the cold wave into East Asia. Hence, the SSW event and tropospheric circulations such as the IOCP, negative phase of Arctic Oscillation, Ural blocking, enhanced Siberian high, and eastward propagation of Rossby wave eventually induced the outbreak of an unprecedented cold wave in East Asia in early January 2021.

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