1月北大西洋东部-乌拉尔山阻塞高压频次年际尺度偶极子模态特征

基于观测和再分析资料,本文研究了近几十年来1月北大西洋东部-乌拉尔山阻塞高压频次年际变异主导模态特征及与其相联的大气背景场。结果表明,1980—2019年冬季(12月—次年2月)该地区阻塞高压频次年际变异的主导模态存在明显月际差异:12月表现为北大西洋东部-欧洲西部地区阻塞高压频次的显著同位相变化,1月为北大西洋东部-欧洲西部与乌拉尔山地区阻塞高压频次的具有显著的反位相变化即偶极子模态,2月则为北大西洋东部-乌拉尔山阻塞高压频次的显著同位相变化。进一步研究表明:1月北大西洋东部-乌拉尔山阻塞高压频次偶极子模态与同期局地纬向西风、纬向风垂直切变、经向位涡梯度等大气背景场异常偶极型变化相联系。当乌拉尔山地区关键大气背景场为负异常,而北大西洋东部-欧洲西部为正异常时,两个地区阻塞高压频次分别增加和减少,呈现偶极子模态;反之亦然。阻塞高压频次偶极子模态及相关的背景环流异常可通过调节水平温度平流、垂直运动和水汽输送等来影响1月欧亚北部气温和降水,以及它们的极端事件频次。当阻塞高压频次偶极子模态处于正位相时,乌拉尔山北部(南部)和欧洲南部增温(降温),极端暖、冷事件频次分别增加(减少)和减少(增加),斯堪的纳维亚半岛北部降温且极端暖事件减少,乌拉尔山及东北亚地区降水和极端降水频次减少而欧洲大陆部分地区增加;反之亦然。此外,在冬季仅1月阻塞高压频次年际变异主导模态表现为偶极子模态可能与关键大气背景场气候态及其变异的月际差异密切相关。     

北半球冬季阻塞环流与NAO之间的关系

首先根据Oman关于北大西洋涛动（NAO）指数冬季平均值的年际变化挑选出强NAO年,然后利用NCEP/NCAR逐日再分析资料,分析了在强NAO年北半球冬季阻塞发生频率和生命期的统计特征,最后通过对强NAO年大气斜压性进行冬季气候平均,合成得到了三个区域（北大西洋、欧洲和北太平洋）对流层平均大气斜压性随纬度的分布情况。结果发现：北大西洋在NAO负位相时下游阻塞发生频率更高,持续时间更长;NAO正位相则有利于欧洲长生命阻塞的发生和维持;北太平洋阻塞在NAO负位相发生频率明显更高,在NAO正位相阻塞的平均持续时间更长。大气斜压性随纬度的分布与阻塞的发生有较好的对应关系,过强的大气斜压性会抑制阻塞的发生。     

乌拉尔山阻塞与北大西洋涛动的关系及其对中国冬季天气的影响

通过对2008年1-2月中国南方一次严重低温雨雪冰冻天气的分析,发现期间的两次乌拉尔山阻塞过程都对其有重要影响,且均伴随着北大西洋涛动正位相事件(North Atlantic Oscillation~+,NAO~+),但由于其发展演变位置的不同对中国的温度和降水造成了截然不同的影响。因此利用ERA-Interim再分析资料计算了1979-2014年冬季乌拉尔山阻塞的平均活动中心,将NAO~+相关乌拉尔山阻塞根据位置变化分为偏北型、偏南型、偏东型和偏西型四类阻塞,研究其对中国冬季天气的影响。结果表明,偏南型和偏东型的乌拉尔山阻塞更容易引起中国冬季的异常降温;研究还发现与NAO~+相关的乌拉尔山阻塞的发展演变总是滞后NAO~+事件3~6天,其位置的变化主要受前期NAO~+期间的纬向风异常分布及急流位置和强度的影响;另外,对1979-2014年冬季乌拉尔山阻塞和NAO的统计结果显示,绝大部分的乌拉尔山阻塞发生时伴随了NAO事件,NAO~+期间比NAO负位相(North Atlantic Oscillation-,NAO-)期间更容易产生乌拉尔山阻塞,但伴随NAO-事件的阻塞强度更大,引起中国冬季的降温也更明显;进一步研究表明,单一的NAO事件期间引起中国冬季温度的变化非常微弱,因此,乌拉尔山阻塞可以作为NAO事件影响中国寒冷天气的媒介。     

欧洲地区夏季热浪的特征及其与阻塞环流的联系

选取了一个热浪指数,利用地面2 m气温场和500 h Pa位势高度场的美国环境预报中心和国家大气研究中心(NCEP/NCAR)再分析资料,通过聚类分析发现欧洲大陆容易产生6类热浪:西欧型(WE)、俄罗斯型(RU)、东欧型(EE)、斯堪的纳维亚半岛型(SC)、北海型(NS)、伊比利亚半岛型(IB);这些热浪事件都与欧洲大陆阻塞的位置有关。同时我们发现这6类热浪发生的频率出现明显的年代际变化,特别在20世纪80年代以后欧洲大陆热浪发生频率明显的增多趋势可能与欧洲大陆增暖背景有关,而欧洲大陆热浪发生频率的年代际变化可能是夏季北大西洋涛动(NAO)的年代际变化的结果。夏季NAO偶极子通过欧洲地区的阻塞异常对欧洲大陆气温有重要的调制作用。当夏季NAO指数处于正位相阶段时,欧洲大陆容易产生高纬度热浪,反之则容易产生低纬度热浪,并且欧洲大陆增暖趋势并不影响NAO对欧洲气温的调制作用。同时还发现:大西洋夏季NAO事件可以是欧洲热浪发生的前期条件,欧洲大陆阻塞异常落后于NAO事件1~5 d,其中IB型和WE型与NAO同期相关,其余4类型热浪对应阻塞落后于NAO 4~5 d。另外,也发现大西洋—欧洲大陆定常波列正距平的位置通过对欧洲阻塞的影响,而影响欧洲热浪发生的频率和位置。     

近11年中国寒潮频发的机理分析

利用全国840个站点观测资料以及ERA5再分析资料,分析了过去63 a(1960—2022年)冬半年(前一年10月—当年4月)全国性寒潮频次的年代际变化特征。统计结果表明:在全球变暖背景下,寒潮频次的下降趋势在2012年发生了转折,2012—2022年呈现显著上升趋势。2012—2022年,乌拉尔山阻塞频率与全国性寒潮发生频次呈显著负相关。乌拉尔山阻高会抑制高空急流发生发展,大气经向环流减弱,北极内冷气团南下受阻,从而减少寒潮发生的频次。同时乌拉尔山阻高与东亚中高纬地区地面2 m温度经向分布联系紧密,乌拉尔山阻塞的频繁发生往往会出现更多的北极增温、中纬度地区降温的天气尺度现象。此外,北极增暖效应本身也与我国寒潮频次有显著的负相关,北极增暖会减小中高纬温度差,西风带减弱,进而减弱冷空气的输送。近11 a,乌拉尔山阻塞频率呈显著减少趋势,相应地,等熵面位涡经向梯度具有线性增加趋势,不利于阻塞高压的维持,伴随北极增暖效应也呈现出减弱态势,这样的大气环流配置有利于近年来我国寒潮频次的增加。     

冬季北大西洋风暴轴的变化及其对西伯利亚高压的影响

采用1948—2010年NCEP/NCAR逐日再分析资料,计算大西洋风暴轴特征指数和西伯利亚高压特征指数,研究了冬季北大西洋风暴轴的变化及其对西伯利亚高压的影响。结果表明:风暴轴经度指数与西伯利亚高压纬度、强度、面积指数显著相关。风暴轴经度指数正异常月,大西洋天气尺度瞬变波活动向东扩展到欧洲乃至乌拉尔山以东,西伯利亚及东亚地区反气旋式波破碎加强,瞬变波对月平均气流的反馈作用使得欧亚大陆上空50°N以北西风加速、50°N以南西风减速;中纬度经向环流加强,气候态的西欧沿岸脊、欧洲东部槽、青藏高原北部脊均加强,东亚大槽减弱东移;在西伯利亚地区,暖空气向北输送加强,来自极区的冷空气南下减弱,致使西伯利亚及东亚地区温度偏高,西伯利亚高压面积减少、纬度偏南、强度减弱。风暴轴经度指数负异常月的情况则反之。     

近30年夏季亚欧大陆中高纬度阻塞高压的统计特征

文中利用1970～2001年NCEP再分析500 hPa逐日高度场资料,根据阻塞高压的天气学定义,采用客观统计方法检索出近32 a亚欧中高纬度392个阻塞高压个例,对其进行了气候学分析.结果表明,亚欧中高纬地区夏季阻塞高压活动频繁,10 d以下的过程占绝对多数,地理分布主要集中在45～70°N之间,纬向上可划分5个高发区,其中乌拉尔山和贝加尔湖东部阻高活动频次最高,同时,每个区域中又存在着相应的阻高活跃区.亚欧中高纬地区夏季阻高活动具有明显的季节内变化特征.6月份,阻塞活动多发生在乌拉尔山地区和鄂霍次克海地区,以双阻为主要形势;7月份,欧洲区和贝加尔湖地区的阻塞活动有所增多,尤其贝加尔湖东部地区增多明显,而乌拉尔山地区的阻塞形势明显减少,鄂霍次克海地区的阻塞活动位置向北移动,多发生在60°N以北,双阻形势逐渐减弱,贝加尔湖地区的中阻形势有所增强.8月份,阻塞形势主要存在于贝加尔湖东西两区,中阻形势占据主导地位.亚欧中高纬地区夏季阻塞高压活动年际变化特征也很突出,且这种年际振荡有明显的地理差异.另外,研究表明亚洲北部的阻高活动多以稳定型为主,移动型阻高个例仅占6.6%.移动型阻高以起源于乌拉尔山地区最多,移距最长,生命期最长.偶极子类阻高多集中在贝加尔湖东部与乌拉尔山地区,约占该地区总阻高频次的62.0%和49.7%,平均生命期分别达到7 d/次和9 d/次以上.     

不同热盐环流平均强度下北大西洋气候响应的差异

基于美国大气研究中心的CCSM3（Community Climate System Model version3）模式,对淡水扰动试验中不同热盐环流（thermohline circulation,THC）平均强度下,北大西洋气候响应的差异进行研究。结果表明：1）在不同平均强度下,北大西洋海洋、大气要素的气候态差异显著。相对于高平均强度,在低平均强度下,北大西洋地区海表温度（sea surface temperature,SST）、海表盐度（sea surface salinity,SSS）、海表密度（sea surface density,SSD）、表面气温（surface air temperature）异常减弱,最大负异常位于GIN（Greenland sea--Iceland sea--Norwegiansea）海域;海平面气压（sealev—elpressure,SLP）异常升高,相应于北大西洋海域降温,表现为异常冷性高压的响应特征;海冰分布区域向南扩大;北大西洋西部热带海域降水减少,导致热带辐合带（intertropical convergence zone,ITCZ）南移。2）在不同THC平均强度下,SST、SSS和SSD年际异常最显著的区域不同;在高平均强度下,最显著区域位于GIN海域,而在低平均强度下则位于拉布拉多海海域。3）在高平均强度下,北大西洋SST主导变率模态的变率极大区域位于GIN海,而在低平均强度下该极大区域不存在;北大西洋SLP的主导变率模态表现为类NAO型,但在高平均强度下,类NAO型表现得更明显。     

冬季乌拉尔山月平均阻塞形势的统计分析和数值试验

根据冬季（１２，１，２月）乌拉尔山地区月平均阻塞形势集中出现的范围和强度定义了一个阻塞指数。相关分析表明：乌拉尔山月平均阻塞与中国月平均气温有较大的负相关关系；它和全球海表温度之间存在三个明显的高相关海区；北太平洋热带东部海区，北大西洋热带和中纬度海区以及阿留申群岛西南方的海区。应用两层原始方程谱模式，在以上３个海区上空加入理想的非绝热孤立热源强迫分别做数值试验。结果表明：３个海区上空的非绝热孤立     

20世纪北大西洋温盐环流的年代际变化试评估

根据相对丰富的大气器测资料,综合前人对有限的海洋资料的诊断分析,从北大西洋涛动(NAO)变率、表层海温(SST)变率、格陵兰海和拉布拉多海的深对流活动长期变化等不同角度,对20世纪大洋温盐环流(Thermohaline Circulation,THC)变率进行了试评估.结果表明:(1)19世纪末以来,大西洋温盐环流的变化可分为4个时期:1900年以前的一段时期,THC较强;1904年到1930年,THC较弱;1931年到1972年,THC较强;1973年至1995年,THC较弱,目前则又有所增强.(2)与THC的变化相联系,大西洋主要气候要素的变化,相互间存在着某种协调关系,THC强,NAO弱,北大西洋北部SST升高,格陵兰海的对流活动增强,拉布拉多海的对流活动则减弱.     

An Asymmetric Spatiotemporal Connection between the Euro-Atlantic Blocking within the NAO Life Cycle and European Climates

This paper examines an asymmetric spatiotemporal connection and climatic impact between the winter atmospheric blocking activity in the Euro-Atlantic sector and the life cycle of the North Atlantic Oscillation(NAO) during the period 1950–2012. Results show that, for positive NAO(NAO+) events, the instantaneous blocking(IB) frequency exhibits an enhancement along the southwest–northeast(SW–NE) direction from the eastern Atlantic to northeastern Europe(SW–NE pattern, hereafter), which is particularly evident during the NAO+decaying stage. By contrast, for negative NAO(NAO-)events, the IB frequency exhibits a spatially asymmetric southeast–northwest(SE–NW) distribution from central Europe to the North Atlantic and Greenland(SE–NW pattern, hereafter). Moreover, for NAO-(NAO+) events, the most marked decrease(increase) in the surface air temperature(SAT) in winter over northern Europe is in the decaying stage. For NAO+events, the dominant positive temperature and precipitation anomalies exhibit the SW–NE-oriented distribution from western to northeastern Europe, which is parallel to the NAO+-related blocking frequency distribution. For NAO-events, the dominant negative temperature anomaly is in northern and central Europe, whereas the dominant positive precipitation anomaly is distributed over southern Europe along the SW–NE direction. In addition, the downward infrared radiation controlled by the NAO's circulation plays a crucial role in the SAT anomaly distribution. It is further shown that the NAO's phase can act as an asymmetric impact on the European climate through producing this asymmetric spatiotemporal connection with the Euro-Atlantic IB frequency.     

Temporal Structures of the North Atlantic Oscillation and Its Impact on the Regional Climate Variability

In this study, the temporal structure of the variation of North Atlantic Oscillation （NAO） and its impact on regional climate variability are analyzed using various datasets. The results show that blocking formations in the Atlantic region are sensitive to the phase of the NAO. Sixty-seven percent more winter blocking days are observed during the negative phase compared to the positive phase of the NAO. The average length of blocking during the negative phase is about 11 days, which is nearly twice as long as the 6-day length observed during the positive phase of the NAO. The NAO-related differences in blocking frequency and persistence are associated with changes in the distribution of the surface air temperature anomaly, which, to a large extent, is determined by the phase of the NAO. The distribution of regional cloud amount is also sensitive to the phase of the NAO. For the negative phase, the cloud amounts are significant, positive anomalies in the convective zone in the Tropics and much less cloudiness in the mid latitudes. But for the positive phase of the NAO, the cloud amount is much higher in the mid-latitude storm track region. In the whole Atlantic region, the cloud amount shows a decrease with the increase of surface air temperature. These results suggest that there may be a negative feedback between the cloud amount and the surface air t.emperature in the Atlantic region.     

Do European Blocking Events Precede North Atlantic Oscillation Events?

Using a two-dimensional blocking index, the cause and effect relationship between European blocking (EB) events and North Atlantic Oscillation (NAO) events is investigated. It is shown that the EB event frequency is enhanced over Northern (Southern) Europe for negative (positive) phases of the NAO. Enhanced EB events over Northern Europe precede the establishment of negative phase NAO (NAO-) events, while the enhanced frequency of EB events over Southern Europe lags positive phase NAO (NAO+) events. The physical explanation for why enhanced EB events over Northern (Southern) Europe lead (lag) NAO- (NAO+) events is also provided. It is found that the lead-lag relationship between EB events in different regions and the phase of NAO events can be explained in terms of the different latitudinal distribution of zonal wind associated with the different phases of NAO events. For NAO+ events, the self-maintained eastward displacement of intensified midlatitude positive height anomalies owing to the intensified zonal wind can enhance the frequency of EB events over Southern Europe, thus supporting a standpoint that EB events over Southern Europe lag NAO+ events. Over Northern Europe, EB events lead NAO- events because NAO- events arise from the self-maintained westward migration of intensified blocking anticyclones due to the weakened zonal wind in higher latitudes.     

Dynamics of eddy-driven North Atlantic Oscillations in a localized shifting jet: zonal structure and downstream blocking

In a weakly nonlinear model how an initial dipole mode develops to the North Atlantic Oscillation (NAO) in a localized shifting jet under the prescribed eddy forcing is examined. It is found that the zonal structure of the eddy-driven NAO anomaly is not only dominated by the longitudinal distribution of the preexisting Atlantic storm track, but also by the initial condition of the NAO anomaly itself associated with the interaction between a localized shifting jet and a topographic standing wave over the Atlantic basin. When both the initial NAO anomaly and the eddy vorticity forcing in the prior Atlantic storm track are more zonally localized, the subsequent eddy-driven NAO anomaly can be more zonally isolated and asymmetric. But, it seems that the shape of the initial NAO anomaly associated with the latitudinal shift of a prior Atlantic jet plays a more important role in producing the zonal asymmetry of subsequent NAO patterns. The zonal asymmetry of the NAO anomaly can be enhanced as the height of topography increases. In addition, it is further found that blocking events occur easily over the Europe continent through the decaying of positive-phase NAO events. However, prior to the positive-phase NAO life cycle the variability in each of three factors: the Atlantic jet, the eddy vorticity forcing in the Atlantic storm track and the initial NAO anomaly can result in a variation in the blocking activity over the Europe sector in strength, duration, position and pattern.     

Atmospheric blocking: space-time links to the NAO and PNA

In the Northern hemisphere, regions characterized by an enhanced frequency of atmospheric blocking overlap significantly with those associated with the major extra-tropical patterns of large-scale climate variability—namely the North Atlantic Oscillation (NAO) and the Pacific North American (PNA) pattern. There is likewise an overlap in the temporal band-width of blocks and these climate patterns. Here the nature of the linkage between blocks and the climate patterns is explored by using the ERA-40 re-analysis data set to examine (1) their temporal and spatial correlation and (2) the interrelationship between blocks and the NAO/PNA. It is shown that a strong anti-correlation exists between blocking occurrence and the phase of the NAO (PNA) in the North Atlantic (western North Pacific), and that there are distinctive inter-basin differences with a clear geographical (over North Atlantic) and quantitative (over North Pacific) separation of typical blocking genesis/lysis regions during the opposing phases of the climate patterns. An Empirical Orthogonal Function (EOF) analysis points to a significant influence of blocking upon the NAO pattern (identifiable as the leading EOF in the Euro-Atlantic), and a temporal analysis indicates that long-lasting blocks are associated with the development of negative NAO/PNA index values throughout their life-time. In addition an indication of a cause-and effect relationship is set-out for the North Atlantic linkage.     

Physical mechanisms of European winter snow cover variability and its relationship to the NAO

Annual snow cover in the Northern Hemisphere has decreased in the past two decades, an effect associated with global warming. The regional scale changes of snow cover during winter, however, vary significantly from one region to another. In the present study, snow cover variability over Europe and its connection to other atmospheric variables was investigated using Cyclostationary Empirical Orthogonal Function (CSEOF) analysis. The evolution of atmospheric variables related to each CSEOF mode of snow cover variability was derived via regression analysis in CSEOF space. CSEOF analysis clearly shows that the North Atlantic Oscillation (NAO) is related to European snow cover, particularly in January and February. A negative NAO phase tends to result in a snow cover increases, whereas a positive NAO phase results in snow cover decreases. The temporal changes in the connection between the NAO and European snow cover are explained by time-dependent NAO-related temperature anomalies. If the NAO phase is negative, the temperature is lower in Europe and snow cover increases; by contrast, when the NAO phase is positive, the temperature is higher and snow cover decreases. Temperature and snow cover variations in Europe are associated with the thermal advection by anomalous wind by NAO. CSEOF analysis also shows an abrupt increase of snow cover in December and January and a decrease in February and March since the year 2000, approximately. This abrupt change is associated with sub-seasonal variations of atmospheric circulation in the study region.     

Patterns of low-frequency variability in a three-level quasi-geostrophic model

 To assess the extent to which atmospheric low-frequency variability can be ascribed to internal dynamical causes, two extended runs (1200 winter seasons) of a three level quasi-geostrophic model have been carried out. In the first experiment the model was forced by an average forcing field computed from nine winter seasons; in the second experiment we used a periodically variable forcing in order to simulate a seasonal cycle. The analysis has been focused on the leading Northern Hemisphere teleconnection patterns, namely the Pacific North American (PNA) and the North Atlantic Oscillation (NAO) patterns, and on blocking, both in the Euro-Atlantic and Pacific sectors. The NAO and PNA patterns are realistically simulated by the model; the main difference with observations is a westward shift of the centres of action of the NAO. Related to this, the region of maximum frequency of Atlantic blocking is shifted from the eastern boundary of the North Atlantic to its central part. Apart from this shift, the statistics of blocking frequency and duration compare favourably with their observed counterparts. In particular, the model exhibits a level of interannual and interdecadal variability in blocking frequency which is (at least) as large as the observed one, despite the absence of any variability in the atmospheric energy sources and boundary conditions on such time scales. Received: 30 January 1997 / Accepted: 17 June 1997     

On the Connection between Upper Atmospheric Dynamics and Tropospheric Parameters: Correlations between Mesopause Region Winds and the North Atlantic Oscillation

The middle- and high-latitude stratospheric and mesospheric wind field in winter is dominated by the stratospheric polar vortex, which reaches up into the mesopause region and leads to westerly winds there in winter. The tropospheric mean winter wind field is also connected with the stratospheric polar vortex, which thus can be considered as extending from the lower up to the upper atmosphere. We found that the January and February zonal winds of the mesopause region, as measured at the Collm Observatory of the University of Leipzig, are closely connected with the North Atlantic Oscillation (NAO) which can be considered as a measure for part of the northern hemispheric mean circulation in the Atlantic and Europe domain, since it influences the circulation over that domain. The NAO itself is considered to be a measure for Central Europe winter winds and temperatures, since large NAO indices are connected with stronger westerlies in Central Europe. Thus, the mesopause region winds are also positively correlated to the Central European winter surface temperatures.     

On the misinterpretation of the North Atlantic Oscillation in CMIP5 models

The representation of the wintertime North Atlantic Oscillation (NAO) and its relationship with atmospheric blocking and the Atlantic jet stream is investigated in a set of CMIP5 models. It is shown that some state-of-the-art climate models are unable to correctly simulate the physical processes connected to the NAO. This is especially true for models with a strongly underestimated frequency of high-latitude blocking over Greenland. In these models the first empirical orthogonal function (EOF1) of the Euro-Atlantic sector can represent at least three different categories of dominant modes of variability associated with different prevalent regions of blocking occurrence and jet stream displacements. It is therefore possible to show that such “biased NAOs” are connected with different dynamical processes with respect to the canonical NAO seen in observations. Since the NAO is a widely used concept in scientific community, the consequent “dynamical misinterpretation” of the NAO that can result when climate models are analyzed may have important implications for the NAO-related studies. This may be especially relevant for the ones involving climate scenarios, since these modeled NAOs may react differently to greenhouse gas forcing.