An Isentropic Mass Circulation View on the Extreme Cold Events in the 2020/21 Winter

Three extreme cold events successively occurred across East Asia and North America in the 2020/21 winter.This study investigates the underlying mechanisms of these record-breaking persistent cold events from the isentropic mass circulation(IMC)perspective.Results show that the midlatitude cold surface temperature anomalies always co-occurred with the high-latitude warm anomalies,and this was closely related to the strengthening of the low-level equatorward cold air branch of the IMC,particularly along the climatological cold air routes over East Asia and North America.Specifically,the two cold surges over East Asia in early winter were results of intensification of cold air transport there,influenced by the Arctic sea ice loss in autumn.The weakened cold air transport over North America associated with warmer northeastern Pacific sea surface temperatures(SSTs)explained the concurrent anomalous warmth there.This enhanced a wavenumber-1 pattern and upward wave propagation,inducing a simultaneous and long-lasting stronger poleward warm air branch(WB)of the IMC in the stratosphere and hence a displacement-type Stratospheric Sudden Warming(SSW)event on 4 January.The WB-induced increase in the air mass transported into the polar stratosphere was followed by intensification of the equatorward cold branch,hence promoting the occurrence of two extreme cold events respectively over East Asia in the beginning of January and over North America in February.Results do not yield a robust direct linkage from La Ni?a to the SSW event,IMC changes,and cold events,though the extratropical warm SSTs are found to contribute to the February cold surge in North America.     

Circulation Characteristics of Persistent Cold Spells in Central–Eastern North America

The circulation patterns of persistent cold weather spells with durations longer than 10 days in central–eastern North America(United States and Canada; 32°–52°N, 95°–65°W) are investigated by using NCEP reanalysis data from 1948 to 2014. The criteria for the persistent cold spells are:(1) three-day averaged temperature anomalies for the regional average over the central–eastern United States and Canada must be below the 10th percentile, and(2)such extreme cold spells must last at least 10 days. The circulation patterns associated with these cold spells are examined to find the common signals of these events. The circulation anomaly patterns of these cold spells are categorized based on the El Ni?o–Southern Oscillation, Arctic Oscillation(AO), and other climate indices. The atmospheric circulation patterns that favor the cold spells are identified through composites of geopotential height maps for the cold spells. Negative AO phases favor persistent cold spells. Phases of sea surface temperature(SST) modes that are associated with warm SSTs in the eastern extratropical Pacific also favor persistent cold events in the study region.Stratospheric polar vortex breakdown alone is not a good predictor for the regional extreme cold spells in central–eastern North America. The meridional dispersions of quasi-stationary Rossby waves in the Pacific–North America sector in terms of cut-off zonal wavenumber modulated by background flow are analyzed to provide insight into the difference in evolution of the cold spells under different mean AO phases. The waveguide for AO 1 is in a narrow latitudinal band centered on 40°N, whereas the waveguide for AO –1 is in a broader latitudinal band from 40° to 65°N. The circulation patterns and lower boundary conditions favorable for persistent cold spells identified by this study can be a stepping-stone for improving winter subseasonal forecasting in North America.     

Relationship between Solar Wind-Magnetosphere Energy and Eurasian Winter Cold Events

The profound impact of solar irradiance variations on the decadal variability of Earth' s climate has been investigated by previous studies.However,it remains a challenge to quantify the energetic particle precipitation(EPP) influence on the surface climate,which is an emerging research topic.The solar wind is a source of magnetospheric EPP,and the total energy input from the solar wind into Earth' s magnetosphere(E_(in)) shows remarkable interdecadal and interannual variability.B ased on the new E_(in) index,this study reveals a significant interannual relationship between the annual mean E_(in)and Eurasian cold extremes in the subsequent winter.Less frequent cold events are observed over Eurasia(primarily north of 50°N) following the higher-than-normal E_(in) activity in the previous year,accompanied by more frequent cold events over northern Africa,and vice versa.This response pattern shows great resemblance to the first empirical orthogonal function of the variability of cold extremes over Eurasia,with a spatial correlation coefficient of 0.79.The pronounced intensification of the positive North Atlantic Oscillation events and poleward shift of the North Atlantic storm track associated with the anomalously higher E_(in) favor the anomalous extreme atmospheric circulation events,and thus less frequent extreme cold temperatures over northern Eurasia on the interannual time scale.It is further hypothesized that the wave-mean flow interaction in the stratosphere and troposphere is favorable for the connection of E_(in) signals to tropospheric circulation and climate in the following winter.     

A Possible Mechanism for Winter Sea Ice Decline over the Bering Sea and Its Relationship with Cold Events over North America

In this study, the mechanism for the sea ice decline over the Bering Sea and its relationship with cold events over North America are investigated based on the daily ERA-Interim data during the winter(December–February) of1979–2016. The results show that the sea ice decline over western(eastern) Bering Sea is mainly contributed by(1)the strengthened southerly(southeasterly) wind near the surface, which possibly pushes the sea ice to move northward, and(2) the intensified downward infrared radiation(IR), which is closely related to the local increasing surface air temperature(SAT) and the intensified moisture convergence mostly induced by the anomalous southeasterly wind associated with an anticyclonic anomaly over the Alaska Bay. During the sea ice decline over the Bering Sea, a cold SAT anomaly is simultaneously found over North America. It is proved that the occurrence of such a cold event is driven by the atmospheric internal variation, but not the forcing of sea ice decline over the Bering Sea. This study deepens our understanding of sea ice decline and its relationship with contemporary cold events in winter.     

Northern Hemisphere Sudden Stratospheric Warming and Its Downward Impact in Four Chinese CMIP6 Models

Using the World Meteorological Organization definition and a threshold-based classification technique,simulations of vortex displacement and split sudden stratospheric warmings(SSWs)are evaluated for four Chinese models(BCC-CSM2-MR,FGOALS-f3-L,FGOALS-g3,and NESM3)from phase 6 of the Coupled Model Intercomparison Project(CMIP6)with the Japanese 55-year reanalysis(JRA-55)as a baseline.Compared with six or seven SSWs in a decade in JRA-55,three models underestimate the SSW frequency by~50%,while NESM3 doubles the SSW frequency.SSWs mainly appear in midwinter in JRA-55,but one-month climate drift is simulated in the models.The composite of splits is stronger than displacements in both the reanalysis and most models due to the longer pulse of positive eddy heat flux before onset of split SSWs.A wavenumber-1-like temperature anomaly pattern(cold Eurasia,warm North America)before onset of displacement SSWs is simulated,but cold anomalies are mainly confined to North America after displacement SSWs.Although the lower tropospheric temperature also displays a wavenumber-1-like pattern before split SSWs,most parts of Eurasia and North America are covered by cold anomalies after split SSWs in JRA-55.The models have different degrees of fidelity for the temperature anomaly pattern before split SSWs,but the wavenumber-2-like temperature anomaly pattern is well simulated after split SSWs.The center of the negative height anomalies in the Pacific sector before SSWs is sensitive to the SSW type in both JRA-55 and the models.A negative North Atlantic Oscillation is simulated after both types of SSWs in the models,although it is only observed for split SSWs.     

A DIAGNOSTIC ANALYSIS OF AIR TEMPERATURE ANOMALY MODE OVER CHINA IN 2009/2010 WINTER BASED ON GENERALIZED EQUILIBRIUM FEEDBACK ASSESSMENT (GEFA) METHOD

By using the observed monthly mean data over 160 stations of China and NCAR/NCEP reanalysis data, the generalized equilibrium feedback assessment(GEFA) method, combined with the methods of EOF analysis, correlation and composite analysis, is used to explore the influence of different SST modes on a wintertime air temperature pattern in which it is cold in the northeast and warm in the southwest in China. The results show that the 2009/2010 winter air temperature oscillation mode between the northern and southern part of China is closely related to the corresponding sea surface temperature anomalies(SSTA) and its associated atmospheric circulation anomalies. Exhibiting warming in Northeast China and cooling in Southwest China, the mode is significantly forced by the El Nio mode and the North Atlantic SSTA mode, which have three poles. Under the influence of SSTA modes, the surface northerly flow transported cold air to North and Northeast China, resulting in low temperatures in the regions. Meanwhile, the mid-latitude westerlies intensify and the polar cold air stays in high latitudes and cannot affect the Southwest China, resulting in the warming there.     

Extreme Cold Events from East Asia to North America in Winter 2020/21: Comparisons,Causes, and Future Implications

Three striking and impactful extreme cold weather events successively occurred across East Asia and North America during the mid-winter of 2020/21.These events open a new window to detect possible underlying physical processes.The analysis here indicates that the occurrences of the three events resulted from integrated effects of a concurrence of anomalous thermal conditions in three oceans and interactive Arctic-lower latitude atmospheric circulation processes,which were linked and influenced by one major sudden stratospheric warming(SSW).The North Atlantic warm blob initiated an increased poleward transient eddy heat flux,reducing the Barents-Kara seas sea ice over a warmed ocean and disrupting the stratospheric polar vortex(SPV)to induce the major SSW.The Rossby wave trains excited by the North Atlantic warm blob and the tropical Pacific La Nina interacted with the Arctic tropospheric circulation anomalies or the tropospheric polar vortex to provide dynamic settings,steering cold polar air outbreaks.The long memory of the retreated sea ice with the underlying warm ocean and the amplified tropospheric blocking highs from the midlatitudes to the Arctic intermittently fueled the increased transient eddy heat flux to sustain the SSW over a long time period.The displaced or split SPV centers associated with the SSW played crucial roles in substantially intensifying the tropospheric circulation anomalies and moving the jet stream to the far south to cause cold air outbreaks to a rarely observed extreme state.The results have significant implications for increasing prediction skill and improving policy decision making to enhance resilience in“One Health,One Future”.     

Response of the North Pacific Storm Track Activity in the Cold Season to Multi-scale Oceanic Variations of Kuroshio Extension System: A Statistical Assessment

In this paper, a statistical method called Generalized Equilibrium Feedback Analysis(GEFA) is used to investigate the responses of the North Pacific Storm Track(NPST) in the cold season to the multi-scale oceanic variations of the Kuroshio Extension(KE) system, including its large-scale variation, oceanic front meridional shift, and mesoscale eddy activity.Results show that in the cold season from the lower to the upper troposphere, the KE large-scale variation significantly weakens the storm tr...     

Northern Hemisphere Stratospheric Polar Vortex Extremes in February under the Control of Downward Wave Flux in the Lower Stratosphere

Using ECWMF ERA-40 and Interim reanalysis data, the planetary wave fluxes associated with the February extreme stratospheric polar vortex were studied. Using the three-dimensional Eliassen-Palm (EP) flux as a measure of the wave activity propagation, the authors show that the unusual warm years in the Arctic feature an anomalous weak stratosphere-troposphere coupling and weak downward wave flux at the lower stratosphere, especially over the North America and North Atlantic (NANA) region. The extremely cold years are characterized by strong stratosphere-troposphere coupling and strong downward wave flux in this region. The refractive index is used to examine the conception of planetary wave reflection, which shows a large refractive index (low reflection) for the extremely warm years and a small refractive index (high reflection) for the extremely cold years. This study reveals the importance of the downward planetary wave propagation from the stratosphere to the troposphere for explaining the unusual state of the stratospheric polar vortex in February.     

Record Arctic Ozone Loss in Spring 2020 is Likely Caused by North Pacific Warm Sea Surface Temperature Anomalies

Record ozone loss was observed in the Arctic stratosphere in spring 2020. This study aims to determine what caused the extreme Arctic ozone loss. Observations and simulation results are examined in order to show that the extreme Arctic ozone loss was likely caused by record-high sea surface temperatures(SSTs) in the North Pacific. It is found that the record Arctic ozone loss was associated with the extremely cold and persistent stratospheric polar vortex over February–April, and the extremely cold vortex was a result of anomalously weak planetary wave activity. Further analysis reveals that the weak wave activity can be traced to anomalously warm SSTs in the North Pacific. Both observations and simulations show that warm SST anomalies in the North Pacific could have caused the weakening of wavenumber-1 wave activity, colder Arctic vortex, and lower Arctic ozone. These results suggest that for the present-day level of ozone-depleting substances, severe Arctic ozone loss could form again, as long as certain dynamic conditions are satisfied.     

欧亚北部2004年以来频繁冷冬的特征分析及机理初探

利用1961—2013年NCEP/NCAR发布的月平均全球再分析资料,分析了欧亚北部(40°65°N,50°-120°E)2004年以来频繁冷冬的异常特征及形成机理。结果表明:欧亚北部2004年以来冷冬频繁发生,但温度异常的空间分布,尤其中心冷区的位置有显著差异,主要表现为全区偏冷型(2005、2009、2010、2012年)和南部偏冷型(2004、2007、2011年)。全区偏冷年主要由北极涛动(AO)显著负位相所致,对应海表温度特征为北大西洋高、中、低纬度成东北-西南走向的"+、-、+"带状分布,该分布有利于北极涛动/北大西洋涛动(AO/NAO)负位相维持和增强;南部偏冷年大气内部活动异常为乌拉尔-贝加尔湖阻塞高压偏强,北极涛动/北大西洋涛动以弱正位相为主,对应主要海表温度特征为北大西洋中部偏高,其次则为太平洋年代际振荡(PDO)负位相下"类拉尼娜事件",上述海表温度异常均可促进类似欧亚遥相关的罗斯贝波列形成,有利于乌拉尔贝加尔湖阻塞高压偏强、亚洲中部多低槽活动。2004年以来欧亚北部两种类型冷冬的大气环流与海表温度均表现出与历史典型年相类似的特征。     

“拉尼娜”非典型影响下的新疆2021/2022年冬季气温

2021/2022年冬季新疆大部气温偏高，影响新疆冷空气活动频次少，并非拉尼娜事件对新疆冬季气温的典型影响特征。成因分析表明：2021/2022年冬季北极涛动（AO）为正位相，欧亚中高纬度500 h Pa位势高度西高东低，新疆上空700 hPa南风距平优势明显。2021/2022年冬季“拉尼娜”非典型影响新疆气温的原因主要来自于东部型弱拉尼娜事件对北大西洋涛动（NAO）和亚洲极涡面积的非典型影响，而对亚洲极涡面积的非典型影响是来自东部型拉尼娜事件对冬季AO的不确定性影响，冬季AO正位相有利于新疆气温正距平。2月300 hPa区域纬向风异常偏弱，削弱了NAO正位相对新疆气温的影响；70°E以东中纬度高度场负距平是冬季气温阶段性变化的原因之一。     

DECADAL VARIATIONS IN CLIMATE ASSOCIATED WITH THE NORTH ATLANTIC OSCILLATION

Large changes in the wintertime atmospheric circulation have occurred over the past two decades over the ocean basins of the Northern Hemisphere, and these changes have had a profound effect on regional distributions of surface temperature and precipitation. The changes over the North Pacific have been well documented and have contributed to increases in temperatures across Alaska and much of western North America and to decreases in sea surface temperatures over the central North Pacific. The variations over the North Atlantic are related to changes in the North Atlantic Oscillation (NAO). Over the past 130 years, the NAO has exhibited considerable variability at quasi-biennial and quasi-decadal time scales, and the latter have become especially pronounced the second half of this century. Since 1980, the NAO has tended to remain in one extreme phase and has accounted for a substantial part of the observed wintertime surface warming over Europe and downstream over Eurasia and cooling in the northwest Atlantic. Anomalies in precipitation, including dry wintertime conditions over southern Europe and the Mediterranean and wetter-than-normal conditions over northern Europe and Scandinavia since 1980, are also linked to the behavior of the NAO. Changes in the monthly mean flow over the Atlantic are accompanied by a northward shift in the storm tracks and associated synoptic eddy activity, and these changes help to reinforce and maintain the anomalous mean circulation in the upper troposphere. It is important that studies of trends in local climate records, such as those from high elevation sites, recognize the presence of strong regional patterns of change associated with phenomena like the NAO.     

两类ENSO对中国北方冬季平均气温和极端低温的不同影响

利用1961-2012年观测、再分析资料以及全球大气环流模式数值试验,探讨了中国北方冬季平均气温对于不同类型（即东部型和中部型）ENSO事件的气候响应,并分析了不同类型ENSO对极端低温事件的可能影响,重点关注了北大西洋涛动（NAO）在其中的桥梁作用。结果表明,ENSO信号能通过调制北大西洋地区的大气环流改变欧亚中高纬度地区的纬向温度平流输送和西伯利亚高压的强度,进而影响中国北方冬季气温,由于不同类型ENSO事件海温分布的差异,这种影响具有明显的非线性特征。在两类厄尔尼诺和东部型拉尼娜事件冬季,北大西洋涛动均呈现负位相,不利于北大西洋的暖湿空气向欧亚大陆输送,西伯利亚高压偏强,因而中国北方地区较气候态偏冷。中部型厄尔尼诺和东部型拉尼娜事件冬季气温负异常的显著区域分别位于东北大范围地区、内蒙古河套附近;东部型厄尔尼诺事件冬季显著的冷异常信号仅局限于黑龙江北部与大兴安岭地区;而中部型拉尼娜事件冬季虽伴随北大西洋涛动正位相,但其空间结构向西偏移,对下游中国北方地区气温的直接影响并不显著,可能受局地信号干扰较大。数值试验再现了北大西洋涛动以及中国北方冬季气温对不同类型ENSO的响应,进一步佐证了上述结论。此外,两类厄尔尼诺事件冬季中国东北地区日平均气温容易偏低,极端低温事件的发生频次增多;而两类拉尼娜事件对极端低温的影响较弱。      

冬季与北大西洋涛动相关Rossby波列传播特征及对下游气候的影响

利用再分析数据,以在北半球冬季与北大西洋涛动(North Atlantic Oscillation,NAO)相关的向下游传播的准定常波列在欧洲地区是否发生反射为标准,将1957/1958年至2001/2002年这45个冬季分为高纬型和低纬型两类冬季,分别简称为在H型和L型冬季。在H(L)型冬季,和NAO相联系的向下游传播的Rossby波列主要沿高纬度(低纬度)路径传播。对比了在两种类型冬季NAO与同期大气环流、近地面温度(Surface Air Temperature,SAT)、海表面温度(Sea Surface Tempertaure,SST)和降水的关系。结果表明:大气环流方面,在H型冬季,300 hPa位势高度异常在西-西伯利亚和中-西伯利亚西部与NAO呈现正相关,而在L型冬季300 hPa位势高度异常在亚洲东海岸(约40°N)和北太平洋呈现正相关,在H型冬季与NAO相关的经向风异常在中纬度形成波列,而在L型冬季与NAO相关的经向风异常在副热带形成波列;SAT方面,在H型冬季SAT异常在欧亚大陆腹地高纬度地区与NAO呈现正相关,而在L型冬季与NAO相关的SAT异常在欧亚大陆腹地的高纬度地区相对较弱,但NAO造成的SAT异常可以扩展到亚洲东北部;降水方面,H型冬季与L型冬季主要区别在中国南方,在H型冬季降水异常与NAO的关系相对较弱,而在L型冬季降水异常与NAO呈现正相关关系;SST方面,同期SST异常在北大西洋中纬度海域与NAO呈现正相关,而在L型冬季与NAO相关的SST异常在北大西洋中纬度地区相对较弱,在北大西洋北部和南部较强。总体而言,在H型和L型冬季,NAO具有不同下游影响。     

Can Eurasia Experience a Cold Winter under a Third-Year La Ni?a in 2022/23?

The Northern Hemisphere(NH) often experiences frequent cold air outbreaks and heavy snowfalls during La Ni?a winters. In 2022, a third-year La Ni?a event has exceeded both the oceanic and atmospheric thresholds since spring and is predicted to reach its mature phase in December 2022. Under such a significant global climate signal, whether the Eurasian Continent will experience a tough cold winter should not be assumed, despite the direct influence of mid-to high-latitude,large-scale atmospheric ...     

On the yearly phase delay of winter intraseasonal mode in the western United States

In the western United States, persistent and recurrent flow patterns not only modulate precipitation events but also result in prolonged surface inversion episodes. In this region, the frequency of persistent ridge/trough events ranges between 20 and 40 days, well within the intraseasonal timescale. Based on NCEP reanalysis data starting at 1949, with a focus on the interior West, we observed that episodes of prolonged ridge/trough events appear to occur about a week later every year and resets every 5–7 years—a previously undocumented phenomenon examined herein. Diagnostic analyses indicate that the interplay between regional intraseasonal flow patterns and the North Atlantic Oscillation (NAO) alternates the preferred timeframe for the persistent ridge/trough events to occur. This may result from different phases of the NAO shifting the winter mean ridge and such shifts modulate the occurrence and timing of persistent ridge/trough events. When the timing changes evolve around the quasi-6 years cycle of the NAO, the resultant evolution forms what appears to be a steady phase delay in the ridge/trough events year after year. These results are a further step in disclosing the multiple-scale interaction between intraseasonal and interannual modes and its regional climate/weather impact.     

Extending North Atlantic Oscillation reconstructions back to 1500

Monthly (1659–1995) and seasonal (1500–1658) North Atlantic Oscillation (NAO) indices were estimated using instrumental and documentary proxy predictors from Eurasia. Uncertainty estimates were calculated for the reconstructions, and the variability of the 500-year winter NAO has been assessed. The late twentieth century NAO extremes are within the range of variability during earlier centuries.     

江南地区持续性暴雨过程的月内环流异常和形成机制分析

利用1951—2009年NCAR/NCEP再分析资料,使用多种统计分析方法识别出对江南地区持续性暴雨过程发生、发展有重要影响的月内环流异常特征,并结合天气分析方法对其影响机制进行初步分析。结果表明,过程发生前3—1周,环流异常特征在中纬度的欧亚地区、北太平洋和南半球印度洋集中分布,其中欧亚和南半球环流异常特征反映的是沿高空急流东传的Rossby波列。正是上述环流异常特征传播和生消所对应的环流异常调整,使得副热带高压（副高）、阻塞高压、梅雨槽等关键环流系统分阶段逐步发展到位,形成有利于过程发生的大尺度环流背景“锁相”特征。      

Possible relationship between NAO and tropical cyclone genesis frequency in the western North Pacific

This study examined a strong positive correlation between the North Atlantic Oscillation (NAO) index during June and the total tropical cyclone (TC) genesis frequency in the western North Pacific during July and August. To investigate a possible cause for this relation, the mean difference between highest positive NAO years and lowest negative NAO years was analyzed by dividing into when the El Niño and La Niña years were included and when the El Niño and La Niña years were not included.When the El Niño and La Niña years were included, for positive NAO years, the TCs mostly occurred in the northwestern region of tropical and subtropical western Pacific, and showed a pattern that migrate from the sea northeast of the Philippines, pass the East China Sea, and move toward the mid-latitudes of East Asia. In contrast, for negative NAO years, the TCs mostly occurred in the southeastern region of tropical and subtropical western Pacific, and showed a pattern that migrate westward from the sea southeast of the Philippines, pass the South China Sea, and move toward the southern coast of China and Indochinese peninsula. These two different TC migration patterns affect the recurving location of TC, and for positive NAO years, the recurving of TC was averagely found to take place in the further northeast. In addition, the migration patterns also affect the TC intensity, and the TCs of positive NAO years had stronger intensity than the TCs of negative NAO years as sufficient energy can be absorbed from the ocean while moving north in the mid-latitudes of East Asia. The TCs of negative NAO years showed weak intensity as they weaken or disappear shortly while landing on the southern coast of China and Indochinese peninsula. On the other hand, the above result of analysis is also similarly observed when the El Niño and La Niña years were not included.