Ural Blocking Research Articles

Atmospheric Contributions to the Reversal of Surface Temperature Anomalies Between Early and Late Winter Over Eurasia

AbstractObservations have shown subseasonal reversal of temperature anomalies between early and late winter over Eurasia, which is distinct from the seasonal mean condition. Based on the reanalysis data, the 1800‐year control simulation and the 40‐member ensemble simulations in 1920–2100 from the Community Earth System Model (CESM) Large Ensemble (CESM‐LE), this study reveals that the reversal of surface air temperature (SAT) anomalies between early and late winter is one of the dominant and intrinsic features of the Arctic‐Eurasian winter climate. Such a reversal is characterized by “colder Arctic, warmer Eurasia” in December (January–February) and “warmer Arctic, colder Eurasia” in January–February (December). Robust climate dynamic processes associated with the reversal of SAT anomalies, including subseasonal reversals of anomalies in the Ural blocking, midlatitude westerlies, and stratospheric polar vortex, are found in both reanalysis data and CESM simulations, indicating the important role of internal atmospheric variability. Further analysis reveals that the reversal of Ural blocking anomalies in late December can be a potential precursor for the reversal of SAT anomalies in late winter. The reversal of midlatitude westerly wind anomalies associated with the Ural blocking can affect upward propagation of planetary‐scale waves especially with wavenumber 1, subsequently promoting the contribution of stratospheric polar vortex to the reversal of SAT anomalies in late winter over the Arctic‐Eurasian regions. Such a troposphere‐stratosphere pathway triggered by the perturbation of tropospheric circulations is confirmed by the CESM‐LE simulations, and it may be useful for the prediction of subseasonal reversal of SAT anomalies.

Intraseasonal variability modes of winter surface air temperature over central Asia and their modulation by Greenland Sea ice and central Pacific El Niño–Southern Oscillation

AbstractThe two dominant patterns of the winter (December–February) intraseasonal surface air temperature (SAT) over central Asia is derived by empirical orthogonal function analysis, with the first (mode 1) featuring a regional monopole and the second (mode 2) featuring a northwest–southeast‐orientated dipole. Mode 1 is characterized with warmer (colder) Arctic and colder (warmer) Eurasia along with an anomalous high (low) over the Ural Mountains, while mode 2 is characterized with northwest–southeast dipole over Eurasia along with wave train‐like atmospheric circulation anomalies from the western Atlantic to Asia. Whether the two intraseasonal modes are modulated by lower‐boundary forcing is then investigated, and the results suggest that mode 1 is significantly modulated by sea ice concentration over the Greenland Sea, but by central‐Pacific ENSO for mode 2. Finally, the underlying mechanisms are diagnosed, and the results suggest that the diabatic heating related with decreased sea ice in Greenland Sea induce a positive height anomaly over Urals which favours the formation of Ural blocking and therefore tends to intensify the occurrence of mode 1. Also, central‐Pacific El Niño excites the Pacific–North American teleconnection, which alters North Atlantic subtropical jet extension, favours intensified energy conversion to intraseasonal wave train and subsequently intensifies the occurrence of mode 2. This study highlights the joint role of Arctic sea ice and the SST in the tropical central Pacific.

Contribution of Ural and Kamchatka Blockings to the Amplified Warm Arctic–Cold Eurasia Pattern under Arctic Sea Ice Loss and Eurasian Cooling

Abstract Previous studies have shown that sea ice loss over the Barents–Kara Seas (BKS) in early winter intensifies surface warming and favors more frequent Ural blocking (UB). Using three different blocking detection methods based on reanalysis and model simulation, we find that the increased occurrences of UB and Kamchatka blocking (KB) over Eurasia are concurrent with both the sea ice loss over the BKS and Eurasian cooling, emphasizing the synergic conditions of sea ice and atmospheric internal variability in facilitating the frequent blocking events. Under the sea ice loss and Eurasian cooling conditions, the KB manifests the strongest surface cold anomaly over northern Eurasia. In contrast, the UB shows either a robust or slightly stronger cold anomaly over Eurasia depending on the conditions above. Consequently, the surface impacts of two regional blockings given the combined conditions of BKS sea ice loss and Eurasian cooling contribute the most to the amplified dipole pattern of Eurasian surface air temperature (SAT). The enhanced dipole pattern shows distinct similarities to the warm Arctic and cold Eurasia (WACE) pattern, previously defined as the second principal mode of winter SAT variability over Eurasia. This implies the role of sea ice loss in reinforcing the internal variability of Eurasian SAT via multiple blocking activities, triggering cold extremes toward Eurasia. Climate simulations with Community Atmosphere Model version 5 (CAM5) can reproduce the observed linkage among sea ice, blocking, and the WACE pattern.

The Influence of Extratropical Weather Regimes on Wintertime Temperature Variations in the Arctic during 1979–2019

In this study, the Arctic sea ice cover in the sector 30° W–60° E in February, and the monthly mean temperature (averaged over the polar cap north of 70° N and 700–1000 hPa, Tcap) in winter during 1979–2019 were analyzed using established change-point detection methods. Step changes were found in 2004, with lower sea ice cover and higher air temperature during 2005–2019 than 1979–2004 (with Tcap anomalies of 1.05 K and −0.63 K, respectively). Two combinations of weather regimes were associated with the anomalously warm months (1.61 K): (1) Scandinavian trough and Ural blocking, and (2) Atlantic ridge and Ural blocking. The first causes a “polar express” for the poleward transport of heat and moisture from mid-latitude East Europe. The second causes a “two-stage heat pump” that transports heat and moisture from the subarctic Atlantic. Their opposite combinations were associated with the anomalously cold months (−0.73 K), which occurred more frequently during 1979–2004. These trends in weather regimes could account for 25% of the step-change in Arctic winter temperature, with the remainder likely caused by changes in sea ice cover, ocean heat transport, and concentrations of aerosol and greenhouse gases.

Dominant patterns of winter surface air temperature over Central Asia and their connection with atmospheric circulation

The dominant patterns of the winter (December–February) surface air temperature anomalies (SATAs) over Central Asia (CA) are investigated in this study. The first two leading modes revealed by empirical orthogonal function (EOF) analysis represent the patterns by explaining 74% of the total variance. The positive phase of EOF1 is characterized by a monopole pattern, corresponding to cold SATAs over CA, while the positive phase of EOF2 shows a meridional dipole pattern with warm and cold SATAs over northern and southern CA. EOF1 is mainly modulated by the negative phase of the Arctic Oscillation (AO) in the troposphere, and the negative AO phase may be caused by the downward propagation of the precursory anomalies of the stratospheric polar vortex. EOF2 is mainly influenced by the Ural blocking pattern and the winter North Atlantic Oscillation (NAO). The SATAs associated with EOF2 can be attributed to a dipole-like pattern of geopotential height anomalies over CA. The dipole-like pattern is mainly caused by the Ural blocking pattern, and the NAO can also contribute to the northern part of the dipole.摘要本文利用经验正交函数分解方法 (Empirical orthogonal function, EOF) , 针对1979–2019年冬季 (12月–2月) 中亚地区地面气温异常进行了研究. 结果表明, 中亚地区冬季地面气温异常的前两个EOF模态解释方差总占比可达74%. 其中, 第一模态 (EOF1) 正位相为一致型变化, 对应中亚地区气温冷异常; 第二模态 (EOF2) 正位相则为南北偶极型变化, 对应于中亚地区南冷北暖型气温异常. EOF1可能受到冬季北极涛动 (Arctic Oscillation, AO) 负位相的调制, 而AO的负位相则可能来自于前期平流层极涡正位势高度异常下传. EOF2则可能受到乌拉尔山阻塞及冬季北大西洋涛动 (North Atlantic Oscillation, NAO) 的共同调制. 乌拉尔山阻塞可引起中亚区域南北偶极型气温异常, 而冬季NAO可对该偶极型气温异常的北侧产生贡献.

Influence of Arctic Sea Ice Concentration on Extended‐Range Prediction of Strong and Long‐Lasting Ural Blocking Events in Winter

AbstractIt is traditionally considered that the predictability of atmosphere reaches approximately 2 weeks due to its chaotic features. Considering boundary conditions, the lead prediction time can exceed 2 weeks in certain cases. We find that the Arctic sea ice concentration (SIC) is crucial for extended‐range prediction of strong and long‐lasting Ural blocking (UB) formation. By applying the rotated empirical orthogonal function‐based particle swarm optimization algorithm, the conditional nonlinear optimal perturbation is calculated with the Community Atmosphere Model, version 4, to identify the optimally growing boundary errors in extended‐range prediction of strong and long‐lasting UB formation. It is found that SIC perturbations in the Greenland Sea (GS), Barents Sea (BS), and Okhotsk Sea (OKS) are important for strong and long‐lasting UB formation prediction in four pentads. Further analysis reveals that the SIC perturbations in these areas first influence the local temperature field through the diabatic heating process and further affect the temperature field in the Ural sector mainly by advection and convection processes. Moreover, the zonal winds in the Ural sector are adjusted by the thermal wind balance, thus affecting UB formation. The local characteristics of the SIC perturbations indicate that the GS, BS, and OKS may be sensitive areas in regard to extended‐range prediction of strong and long‐lasting UB formation, which can provide scientific support for the SIC target observations in the future.

Characteristics of Transient Eddies During Ural Blocking Events With Different Lifetime in Winter

Ural blocking high (hereafter UB) is one of the most important weather systems influencing the weather and climate of China, with its onset and development closely connected with transient eddies. Herein, we classified the wintertime UB events during 1979–2015 into three types according to their lifetime, i.e., short, medium, and long lifetime UB events and further analyzed the characteristics of momentum and heat fluxes transported by transient eddies during the three types of UB events. The results show that the eddy momentum and heat transport by transient eddies over the Ural regions is strong, while the westerly is weak, favorable for the establishment and maintenance of the UB. Before the onset of all the UB events there are enhanced lower-level temperature disturbances, decelerated westerly and convergence of transient momentum and heat fluxes. After the onset of the UB, the upper-level jet stream is accelerated and eddy flux convergence is enhanced. Furthermore, it is found that the duration and position of convergence could be one of important factors determining the lifetime of UB event. The long-time strong convergence of transient eddies favors the maintenance of UB. During long UB events, the polar jet and the convergence of eddy momentum fluxes have the strongest intensity and northward shifted position. The convergence is further strengthened until Day +2 and weakened since Day +7, favorable for the long-time maintenance of UB. There is convergence on Day -3 of both short and medium UB events, which disappears on Day +4 during short events, while strengthened after the onset of medium events, leading to a long-time maintenance of UB. Furthermore, among the three types of UB events, the upward propagation of wave activity and E-P flux divergence are the strongest and most long-lasting during long UB events, while the weakest and shortest during short UB events, which can also verify strong atmospheric baroclinicity and long-lasting strong convergence of transient eddies are favorable for long duration of the UB.

Impact of winter blocking on surface air temperature in East Asia: Ural versus Okhotsk blocking

Atmospheric blockings and their impacts on wintertime surface air temperature in East Asia are investigated by using three blocking indices. Blockings in two specific regions that indirectly or directly modulate East Asian climate variability are analyzed: Ural (UR) blocking over East Asia’s northwest and Okhotsk (OK) blocking over East Asia’s northeast. Both lead to cold anomalies in East Asia but through slightly different mechanisms. In the typical case of UR blocking, cold air associated with a mid-level trough develops to the east of the quasi-stationary blocking and slowly moves southeastward as the Siberian high intensifies. Four days after the blocking onset, East Asia experiences significant cooling due to the temperature advection associated with anomalous temperature gradient and adiabatic cooling. During the typical OK blocking event, a preexisting cold anomaly over northern Eurasia is pushed southeastward by the westward expansion of the blocking. The anomalous meridional flow due to blocking-induced dipolar circulation over the North Pacific further reinforces the cold advection over East Asia, with a minor contribution of vertical processes. In both types of blocking, the cold anomaly persists even in the decaying phase, due to an offset between the temperature advection and the diabatic heating.

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.

On the Association of the Summertime Shortwave Cloud Radiative Effect in Northern Russia With Atmospheric Circulation and Climate Over East Asia

AbstractThis study employs ERA5 reanalysis and CERES/EBAF Ed4.1 data to evaluate the dominant features of summer shortwave cloud radiative effect (SWCRE) variability and associated atmospheric circulation anomalies. Our findings suggest that the greatest variability in summer SWCRE occurs over the Barents, Kara, and Laptev seas and northern Eurasia. We also observe a close relationship between summertime SWCRE, particularly in northern Russia, and atmospheric circulation variability over East Asia. Significant positive SWCRE anomalies over northern Russia favor the generation of the Ural blocking, and dynamically trigger the emergence of positive Eurasian (EU) pattern, resulting in positive (negative) precipitation anomalies in northern (southeastern) China and persistent East Asian heatwaves between 20°N and 40°N. In contrast to previous work, which has focused mainly on local atmospheric responses to SWCRE, this study provides a broader perspective, thereby helping bridge summertime circulation between high latitudes and East Asia.

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.

Interaction between Atlantic cyclones and Eurasian atmospheric blocking drives wintertime warm extremes in the high Arctic

Abstract. Atmospheric blocking can influence Arctic weather by diverting the mean westerly flow and steering cyclones polewards, bringing warm, moist air to high latitudes. Recent studies have shown that diabatic heating processes in the ascending warm conveyor belt branch of extratropical cyclones are relevant to blocking dynamics. This leads to the question of the extent to which diabatic heating associated with mid-latitude cyclones may influence high-latitude blocking and drive Arctic warm events. In this study we investigate the dynamics behind 50 extreme warm events of wintertime high-Arctic temperature anomalies during 1979–2016. Classifying the warm events based on blocking occurrence within three selected sectors, we find that 30 of these events are associated with a block over the Urals, featuring negative upper-level potential vorticity (PV) anomalies over central Siberia north of the Ural Mountains. Lagrangian back-trajectory calculations show that almost 60 % of the air parcels making up these negative PV anomalies experience lifting and diabatic heating (median 11 K) in the 6 d prior to the block. Further, almost 70 % of the heated trajectories undergo maximum heating in a compact region of the mid-latitude North Atlantic, temporally taking place between 6 and 1 d before arriving in the blocking region. We also find anomalously high cyclone activity (on average five cyclones within this 5 d heating window) within a sector northwest of the main heating domain. In addition, 10 of the 50 warm events are associated with blocking over Scandinavia. Around 60 % of the 6 d back trajectories started from these blocks experience diabatic heating, of which 60 % undergo maximum heating over the North Atlantic but generally closer to the time of arrival in the block and further upstream relative to heated trajectories associated with Ural blocking. This study suggests that, in addition to the ability of blocks to guide cyclones northwards, Atlantic cyclones play a significant role in the dynamics of high-latitude blocking by providing low-PV air via moist-diabatic processes. This emphasizes the importance of the mutual interactions between mid-latitude cyclones and Eurasian blocking for wintertime Arctic warm extremes.

The modulation of Interdecadal Pacific Oscillation and Atlantic Multidecadal Oscillation on winter Eurasian cold anomaly via the Ural blocking change

Observations reveal distinct interdecadal winter Eurasian cold anomaly (ECA) centered over central Eurasia (40°–60° N, 60°–120° E), with a more southwestward extension during 1965–1976 than during 2002–2013. In this paper, Ural blockings (UB) in association with the Interdecadal Pacific Oscillation (IPO) and Atlantic Multi-decadal Oscillation (AMO) are analyzed to explain the ECA’s decadal change from 1965–1976 to 2002–2013 using reanalysis data. It is found that the 1965–1976 winter ECA is associated with a negative-phase IPO (IPO−) together with negative-phase AMO (AMO−), while the 2002–2013 ECA is related to positive-phase AMO (AMO+) concurring with IPO−. UB mainly related to positive North Atlantic Oscillation is relatively short-lived and rapidly retrograde during both IPO+ and AMO−, but long-lived and shows different longitudinal movements and positions during IPO− and AMO+. During IPO−, UB grows rapidly and decays slowly due to weak westerly winds and small meridional potential vorticity gradient (PVy) over North Atlantic mid-high latitudes and Eurasian high latitudes, and moves slowly westward during its decay stage, causing strong cold anomalies over central Eurasia and its upstream region (30°–50° N, 30°–70° E). AMO− has a similar effect due to the slow decay of retrograde UB. However, during AMO+ UB grows slowly, decays rapidly and shows eastward movement due to strong (weak) westerly winds and large (small) PVy over North Atlantic (Eurasian) high latitudes, causing strong cold anomalies over central Eurasia and its downstream side. Through these UB-induced sub-seasonal changes, the interdecadal IPO−, AMO− and AMO+ help explain the decadal variation of the winter-mean ECA from 1965–1976 to 2002–2013.

Decadal Variability of Winter Warm Arctic‐Cold Eurasia Dipole Patterns Modulated by Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation

AbstractIn recent decades, the winter surface air temperature (SAT) anomaly in the Northern Hemisphere has exhibited a warm Arctic‐cold Eurasia (WACE) dipole pattern and this has undergone significant decadal variation. In this paper, the physical cause of the decadal variability of the WACE pattern is explored, and it is shown to be mediated by the phases of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Although the negative PDO (PDO−) or positive AMO (AMO+) favors the WACE pattern, the meridional structure of the pattern is significantly influenced by whether PDO− or AMO+ dominates. During PDO− (AMO+) phase, the winter‐mean WACE pattern shows an asymmetric dipole with weak (strong) warm anomaly over the Barents‐Kara Seas or BKS and strong (weak) cold anomaly over central Eurasia or Siberia, which corresponds to Ural blocking (UB) concurring with the negative (positive) phase of North Atlantic Oscillation in a winter‐mean field. The winter asymmetric WACE pattern is more strongly related to PDO− than AMO+. It is further found that an asymmetric sub‐seasonal WACE dipole pattern with strong (weak) warm anomaly over BKS and weak (strong) cold anomaly over Siberia is usually formed during the UB episode due to favored (suppressed) sub‐seasonal downward infrared radiation and turbulent heat flux over BKS during AMO+ (PDO−), which leads to a strong asymmetric dipole of the winter WACE pattern.

Subseasonal Predictability of South China Sea Summer Monsoon Onset With the ECMWF S2S Forecasting System

AbstractAccurate forecasting of the South China Sea summer monsoon (SCSSM) onset is critical for water resource management in the rainy season across East Asia. The present work evaluates the skill of the ECMWF subseasonal‐to‐seasonal (S2S) forecasting system in predicting SCSSM onset dates using a hindcast experiment. The model provides accurate predictions of the SCSSM onset date 10 days in advance. The predictability mainly arises from the sea surface temperature anomaly (SSTA) in the northern Indian Ocean and central‐eastern tropical Pacific, which usually coincides with moderate and extreme decaying El Niño, thus creating windows of opportunity for SCSSM onset forecasting. The model can reasonably reproduce the SCSSM‐related SSTA and its induced atmospheric teleconnections. However, when anomalous Ural blocking influences the mid‐high latitudes, the ECMWF model forecasts of the SCSSM onset are trapped in the synoptic timescale because of the limited ability of the model to consider blocking in forecasting.

Distinct Tropospheric and Stratospheric Mechanisms Linking Historical Barents‐Kara Sea‐Ice Loss and Late Winter Eurasian Temperature Variability

AbstractReduced Arctic sea‐ice has been proposed to induce severe Eurasian cold events. However, the physical mechanisms for this connection, particularly the relative importance of tropospheric and stratospheric processes, remain unclear. Using ERA‐Interim reanalysis data and WACCM‐SC simulations, we show that the Eurasian cooling induced by reduced sea‐ice centers over eastern Asia and northern Europe. Tropospheric and stratospheric processes contribute roughly equally to the cooling over eastern Asia, while the stratospheric and tropospheric contributions are 60% and 40%, respectively, over northern Europe. In the tropospheric pathway, weakened meridional temperature gradient due to reduced sea‐ice strengthens the Ural blocking and enhances the Siberian High. The enhanced Siberian High favors two streams of cold air‐mass, reaching northern Europe and eastern Asia. In the stratospheric pathway, enhanced upward‐propagating planetary wave 1 causes a shift of the stratospheric polar vortex toward Eurasia and consequently, tropospheric cyclonic anomalies are induced that enhance surface cold anomalies.

Maintenance and development of the Ural high and its contribution to severe cold wave activities in winter 2020/21

Two successive severe cold waves invaded eastern China from the end of 2020 to early 2021, leading to an extensive, severe, and persistent drop in temperature. The paper investigates the features and formation mechanisms of the two cold waves. The main results are as follows: (1) An anticlockwise turning of the transverse trough was observed in both cold waves. However, a broad ridge was maintained over the Ural area from mid-December 2020 till mid-January 2021. No breakdown or discontinuous westward shift of the blocking high was observed, which is different from typical cold waves in eastern Asia. (2) The maintenance and strengthening of northerly winds in front of the Ural high led to an increase in baroclinicity in-situ. In the downstream region, the gradient of the geopotential height contour in the south of the transverse trough rapidly increased and the advection of cold temperature consistently enhanced and advanced southwards. This in turn caused the intensification and southward expansion of the Siberian high. (3) Energy propagation of the quasi-stationary wave was a reason for the development and persistence of the Ural blocking. Prior to the occurrence of the two cold waves, the energy of the low-frequency stationary wave originating from near 0°E (or even to the west) propagated eastwards, which helped the Ural ridge intensify and maintain. Meanwhile, it also contributed to the development of the trough downstream of the ridge and resulted in the anticlockwise turning of the transverse trough, providing a favorable condition for the southward outbreak of cold air.摘要2020年底至2021年初, 连续两次强寒潮入侵中国东部地区, 导致大范围强降温. 本文研究这两次寒潮的特征和形成机理. 主要结果如下: 从2020年12月中旬到2021年1月中旬, 乌拉尔山地区维持一宽阔的高压脊. 脊前北风的维持和加强导致局地斜压性增加, 导致下游横槽槽底等高线梯度加大, 冷平流加强, 西伯利亚高压加强和向南扩张. 准定常波能量传播是乌拉尔山阻塞发展和持续的主要原因. [Display omitted]

Impact of Ural blocking on sub-seasonal Siberian cold anomalies modulated by the winter East Asian trough

Impact of Ural blocking on sub-seasonal Siberian cold anomalies modulated by the winter East Asian trough

Effects of Barents–Kara Seas ice and North Atlantic tripole patterns on Siberian cold anomalies

Possible physical routes are presented that link the decline in sea-ice concentration (SIC) in the Barents–Kara Seas (BKS), the sea surface temperature anomaly in the pattern of the North Atlantic tripole (NAT), and the Siberian cold anomaly in boreal winter, via mid–high-latitude circulation anomalies mainly characterized by Ural blocking (UB). It is found that the Siberian cold anomalies with BKS SIC loss are mostly related to UB, while those without significant BKS SIC loss are significantly correlated with the North Atlantic Oscillation (NAO). The negative/positive-phase NAT (NAT−/NAT+) exerts its impact on the cold/warm anomalies over northern Eurasia through coupling with the negative/positive-phase NAO (NAO−/NAO+). In particular, a strong NAT− winter tends to cause a cold anomaly shifting southwards from the polar area to mid–high-latitude of Eurasia, producing a severe cold event over Siberia. From the perspective of intraseasonal atmospheric circulation, a UB event under NAT− winter leads to a more intense Siberian cold anomaly than one under NAT+, suggesting a vital role played by the NAO in forming cold conditions in Siberia. Considering the effects of BKS sea-ice variation, it is revealed that the UB-related Siberian cold anomaly tends to achieve its maximum strength in a NAT− winter when the BKS SIC declines. However, for winters with sea ice loss in BKS but a weak NAT+, the UB-related Siberian cold anomaly shows southward displacement. It is also found that strong NAT− coupling with a strong NAO− can cause cold conditions in Siberia without the need for a BKS SIC decline and long-lived UB, and this explains 75% of the cold Siberian winters in the context of increased BKS SIC.

A Connection of Winter Eurasian Cold Anomaly to the Modulation of Ural Blocking by ENSO

AbstractThis study investigates how El Niño‐Southern Oscillation (ENSO) modulates winter cold anomalies over central Eurasia (CE) using reanalysis data during 1950–2019. It is found that ENSO can significantly influence winter air temperature over CE mainly through modulating the strength and location of the cyclonic anomaly of Ural blocking (UB) or long‐lived UB events. A strong (weak) cyclonic anomaly of UB on the southeastern (eastern) side of the Ural Mountains tends to occur over midlatitude (high‐latitude) Eurasia during La Niña (El Niño) winters. Such anomalous circulation leads to a strong (weak) cold anomaly over CE, especially over East Asia, due to enhanced (weakened) cold air advection toward the CE during La Niña (El Niño). The UB‐related cyclonic anomaly in the midlatitude (high latitude) side of Eurasia is shown to be related to weakened (enhanced) eastward extension of strong westerly winds over midlatitude Eurasia during La Niña (El Niño) winters.