Two Types of Arctic Oscillation and Their Associated Dynamic Features

In this paper,the dynamical evolutions of two types of Arctic Oscillation (AO),the stratospheric (S) and tropospheric (T) types,have been investigated on an intermediate time scale in terms of transient eddy feedback forcing and three-dimensional Rossby wave propagation.S-Type (T-type) events are characterized by an anomalous stratospheric polar vortex that is in phase (out of phase) with its tropospheric counterpart.Approximately onethird of AO events,both positive and negative,are T-type events.For the positive phase of a T-type event,the formation and maintenance of stratospheric positive anomalies over the polar cap are associated with an upward propagation of Rossby wave packets originating from the near-tropopause altitude over northeastern Asia.However,such upward propagating features are not found for S-type events.In the troposphere,transient eddy feedback forcing is primarily responsible for the meridional seesaw structure of both the S-and T-type events,with an additional contribution from Rossby wave propagation.     

A two-way stratosphere-troposphere coupling of submonthly zonal-mean circulations in the Arctic

This paper examines the dominant submonthly variability of zonally symmetrical atmospheric circula- tion in the Northern Hemisphere （NH） winter within the context of the Northern Annular Mode （NAM）, with particular emphasis on interactive stratosphere-troposphere processes. The submonthly variability is identified and measured using a daily NAM index, which concentrates primarily on zonally symmetrical circulation. A schematic lifecycle of submonthly variability is developed that reveals a two-way coupling pro- cess between the stratosphere and troposphere in the NH polar region. Specifically, anomalous tropospheric zonal winds in the Atlantic and Pacific sectors of the Arctic propagate upwards to the low stratosphere, disturbing the polar vortex, and resulting in an anomalous stratospheric geopotential height （HGT） that subsequently propagates down into the troposphere and changes the sign of the surface circulations. From the standpoint of planetary-scale wave activities, a feedback loop is also evident when the anoma- lous planetary-scale waves （with wavenumbers 2 and 3） propagate upwards, which disturbs the anomalous zonally symmetrical flow in the low stratosphere, and induces the anomalous HGT to move poleward in the low stratosphere, and then propagates down into the troposphere. This increases the energy of waves at wavenumbers 2 and 3 in the low troposphere in middle latitudes by enhancing the land-sea contrast of the anomalous HGT field. Thus, this study supports the viewpoint that the downward propagation of stratospheric NAM signals may not originate in the stratosphere.     

北半球环状模波流相互作用动力学研究进展

总结了国内外学者对于北半球环状模（NAM：Northern Hemisphere Annular Mode）及其活动中心形成原因的研究成果。主要从NAM的天气、气候影响,波流相互作用原理对NAM形成的解释,NAM在北太平洋、北大西洋和北极3个区域活动中心的天气尺度波和行星尺度波活动等方面论述。NAM在对流层的变化与天气尺度波有关,北太平洋和北大西洋两个活动中心是天气尺度波活跃的区域,其峰值区表现为风暴轴,其中北大西洋天气尺度波破碎过程会使得NAM指数急剧变化。NAM在平流层的变化和准定常行星波关系密切,冬季准定常行星波会上传并与高纬平流层纬向流发生相互作用,从而引起北极极涡发生改变。准定常行星波将NAM 3个活动中心有机联系起来：对流层准定常行星波的纬向传播会影响北太平洋风暴轴的位置,而风暴轴的变化会影响下游北大西洋波破碎过程,同时准定常行星波的上传可以影响极涡活动。     

平流层异常下传对2009年12月北半球大范围降雪过程的影响

2009年12月北半球中纬度出现大范围持续低温、暴风雪等天气。采用NCEP／NCAR再分析资料研究了平流层AO（Arctic Oscillation,北极涛动）异常信号下传的特征及其对本次极端气候事件的影响,并讨论了与平流层异常信号下传相关的行星波活动。结果表明：1）与此次极端气候事件相联系的负位相A0异常11月首先发生在平流层,维持将近1个月后于12月初开始下传,并且迅速传至地面。12月整个对流层的位势高度及温度在极区附近出现强的正异常,而中纬度地区则为负异常。2）平流层AO异常信号下传后,地面出现有利于低温降雪过程的环流异常。12月上旬,亚洲大陆东部及北美大陆西部出现异常偏北风,造成了俄罗斯、北美西部大面积负的温度异常;12月中下旬,欧洲大陆盛行偏西北气流,同时蒙古高压增强,欧亚大陆北部包括中国北方出现大片负的温度异常。3）在此次极端气候事件之前,北半球高纬度地区有异常强的行星波上传至平流层,导致平流层出现负位相的AO异常,并维持了一个月;随后,上传到平流层的行星波减弱,同时平流层负位相的AO异常迅速传至地面,导致了有利于低温降雪的环流异常。     

夏季欧亚中高纬环流持续异常事件的Rossby波传播特征

利用西风波导结构以及波作用通量, 探讨了夏季欧亚一类中高纬持续异常环流所对应的Rossby波的能量频散特征。中高纬度对流层上层存在结构较为复杂的弱波导, Rossby波能量频散过程基本上与该弱波导结构一致。Rossby波传播特征在不同时期以及两种环流型 （E型和C型) 之间存在显著差异: (1) 在梅雨前期, 与E型环流对应, Rossby波从南欧气旋式异常环流中心传播到乌拉尔山正高度异常中心, 并且波作用通量在乌拉尔山西侧辐合, 形成该地区正高度异常环流。乌拉尔山持续异常中心东侧重新激发出Rossby波, 并传播至贝加尔湖和鄂霍次克海地区, 维持对应的异常环流。与C型异常环流对应, Rossby波活动非常活跃。该型三个活动中心呈现高纬－中纬－高纬的分布特征, 这与波导结构密切相关。 (2) 在梅雨期, Rossby波的传播对两类持续异常环流的作用更加明显, 其传播路径基本上在处于极区和偏向中纬度一侧的两个“波障碍区”之间的带状西风波导区中。Rossby波从乌拉尔山活动中心向东传播, 最终形成贝加尔湖和鄂霍次克海地区的持续异常环流。在C型维持过程中则还存在另一种强迫因子。在C型中, Rossby波从乌拉尔山活动中心向中纬度传播, 并在亚洲急流中向东传播至东亚地区。 (3) 在后汛期, 在欧亚大陆上纬向“波障碍区”的增加使得Rossby波活动减弱。E型异常环流型的鄂霍次克海活动中心向东扩展到北太平洋, 但来自上游的Rossby波传播只作用于该活动中心的西北侧部分。C型中Rossby波的传播在乌拉尔山活动中心地区变弱。在夏季各个时期, E和C型持续异常环流对应着不同位相的EAP (或PJ) 型, 但并没有Rossby波从中纬度向北传播至鄂霍次克海地区的现象。     

The COWL Pattern Identified with a Large AO Index and Its Impact on Annular Surface Temperature Anomalies

In this study, the cold ocean/warm land (COWL) pattern was identified from the leading empirical orthogonal function (EOF) of the monthly 1000-hPa geopotential height field poleward of 20 N. Traditionally, the leading EOF has been recognized as the Arctic Oscillation (AO), or Northern Annular Mode (NAM), which causes annular surface air temperature (SAT) anomalies over high-latitude regions of the Northern Hemisphere. A new finding of the present study is that the total AO events defined by the large AO index actually include a distinct type of events that are characterized by a less-annular spatial structure, i.e., the COWL pattern, which shows an NAO-like distribution in the Atlantic sector and a center of action over the North Pacific with the same sign as that over the Arctic. In addition, unlike canonical AO events, the COWL events also show a less-annular pattern in the stratosphere. Statistically, at least one-third of the AO events can be categorized as the COWL events. The SAT anomalies associated with the COWL pattern have an annular distribution over the high-latitude region of the two continents in the Northern Hemisphere. In contrast, if the COWL events are removed from the total AO events, the remainder shows less annular SAT anomalies. Thus, the typical annular SAT anomalies associated with AO events are in large part due to the contribution of the COWL pattern. Furthermore, the monthly variability and the interannual variability of all the AO events are equally important.     

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE--Ⅱ.VERTICAL PROPAGATION OF PLANETARY WAVES

A linear,hemispheric and stationary spectral model with multilayers in the vertical was employed to simulate the vertical propagation of waves triggered by mountains.Results show that,in cooperation with the East Asia zonal mean flow,Tibetan Plateau can excite a strong wavenumber 1 perturbation in the stratosphere with its ridge and trough located over the Pacific and Atlantic Oceans respectively.On the other hand,the stratospheric wavenumber 1 perturbation caused by the mechanical forcing of the Rocky Mountains in cooperation with the North America zonal mean flow is very weak.Calculations from observational data of the vertical profile of critical wavenumber for vertically propagating waves imply that the tropospheric wavenumber 1 perturbation can hardly penetrate the North America tropopause upwards,whereas it can freely propagate through the East Asia tropopause into the stratosphere.Two-dimensional E-Pcross-sections obtained from both observational data and simulated results also demonstrate that waves excited by the Rocky Mountains are refracted towards low latitudes in the troposphere during their upward propagation:whereas,in addition to the above mentioned equatorward leaning branch,the wavenumber 1 and 2 planetary waves excited by the Tibetan Plateau possess another branch which is refracted to high latitudes during upward propagation and penetrates the tropopause into the stratosphere.It is therefore concluded that the difference in the horizontal and vertical wave propagations in the two hemispheres is a result of the different dynamical forcing induced by the two main mountains in the Northern Hemisphere.     

2011～2012年冬季欧亚大陆低温严寒事件与平流层北极涛动异常下传的影响

利用NCEP-NCAR 再分析资料分析了2011～2012 年冬季发生在欧亚大陆的一次异常低温严寒事件的大气环流演变过程以及可能的成因。这次低温事件,主要出现在2012 年1 月下旬至2 月上旬,持续大约3 周左右,非常强的低温异常覆盖了几乎整个欧洲以及东亚的西伯利亚、蒙古国和我国东北、华北等地。这次低温事件的演变与对流层北极涛动(AO)由正位相转变为负位相的时间相匹配,意味着AO 可能发挥重要作用。进一步分析表明,前期行星波的异常上传导致平流层发生爆发性增温现象,极夜急流减弱,AO 位相首先在平流层由正变负;在2～3 周左右的时间内,平流层AO 异常信号逐渐下传,使得对流层AO 也转为负位相;随后,乌拉尔山阻塞高压异常发展,极区的冷空气不断向南爆发,先后在东亚和欧洲造成剧烈的降温,导致低温严寒事件。因此,考虑平流层环流的异常可能有助于提高欧亚大陆冬季低温严寒事件的预测能力。     

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE——Ⅱ.VERTICAL PROPAGATION OF PLANETARY WAVES

A linear,hemispheric and stationary spectral model with multilayers in the vertical was employed to simulate thevertical propagation of waves triggered by mountains.Results show that,in cooperation with the East Asia zonal meanflow,Tibetan Plateau can excite a strong wavenumber 1 perturbation in the stratosphere with its ridge and trough lo-cated over the Pacific and Atlantic Oceans respectively.On the other hand,the stratospheric wavenumber 1 perturbationcaused by the mechanical forcing of the Rocky Mountains in cooperation with the North America zonal mean flow isvery weak.Calculations from observational data of the vertical profile of critical wavenumber for vertically propagatingwaves imply that the tropospheric wavenumber 1 perturbation can hardly penetrate the North America tropopause up-wards,whereas it can freely propagate through the East Asia tropopause into the stratosphere.Two-dimensional E-Pcross-sections obtained from both observational data and simulated results also demonstrate that waves excited by theRocky Mountains are refracted towards low latitudes in the troposphere during their upward propagation:whereas,inaddition to the above mentioned equatorward leaning branch,the wavenumber 1 and 2 planetary waves excited by theTibetan Plateau possess another branch which is refracted to high latitudes during upward propagation and penetratesthe tropopause into the stratosphere.It is therefore concluded that the difference in the horizontal and vertical wavepropagations in the two hemispheres is a result of the different dynamical forcing induced by the two main mountains inthe Northern Hemisphere.     

On the Differences and Climate Impacts of Early and Late Stratospheric Polar Vortex Breakup

The stratospheric polar vortex breakup (SPVB) is an important phenomenon closely related to the seasonal transition of stratospheric circulation. In this paper, 62-year NCEP/NCAR reanalysis data were employed to investigate the distinction between early and late SPVB. The results showed that the anomalous circulation signals extending from the stratosphere to the troposphere were reversed before and after early SPVB, while the stratospheric signals were consistent before and after the onset of late SPVB. Arctic Oscillation (AO) evolution during the life cycle of SPVB also demonstrated that the negative AO signal can propagate downward after early SPVB. Such downward AO signals could be identified in both geopotential height and temperature anomalies. After the AO signal reached the lower troposphere, it influenced the Aleutian Low and Siberian High in the troposphere, leading to a weak winter monsoon and large-scale warming at mid latitudes in Asia. Compared to early SPVB, downward propagation was not evident in late SPVB. The high-latitude tropospheric circulation in the Northern Hemisphere was affected by early SPVB, causing it to enter a summer circulation pattern earlier than in late SPVB years.     

北半球冬季平流层强、弱极涡事件演变过程的对比分析

利用1958～2017年逐日的NCEP/NCAR再分析资料对北半球冬季平流层强、弱极涡事件的演变过程进行了对比分析,同时比较了有平流层爆发性增温（SSW）和无SSW发生的两类弱极涡事件的环流演变和动力学特征。结果表明,强极涡的形成存在着缓慢发展和快速增强的过程,而弱极涡事件的建立非常迅速;和强极涡事件相比,弱极涡事件的峰值强度更强,异常中心的位置更高。此外,强、弱极涡事件的产生与波流相互作用的正反馈过程密切相关。对于强极涡事件,发展阶段的太平洋—北美（PNA）型异常削弱了行星波一波;当平流层西风达到一定强度,上传的行星波受到强烈抑制,使得极涡迅速增强达到峰值。而对于弱极涡事件,发展阶段一波型的异常增强了行星波上传,通过对纬向流的拖曳作用使得平流层很快处于弱西风状态,更多行星波进入平流层导致极涡急剧减弱甚至崩溃。针对有、无SSW发生的两类弱极涡事件的对比分析表明,有SSW发生的弱极涡事件发展阶段,平流层出现强的向上的一波Eliassen-Palm（EP）通量异常,通过正反馈过程使得一波和二波上传同时增强而导致极涡崩溃;无SSW发生的弱极涡事件发展阶段,平流层缺乏向上的一波通量,二波活动起到重要作用,其总的行星波上传远弱于有SSW发生的弱极涡事件。对于无SSW发生的弱极涡事件,其发展和成熟阶段对流层上部出现类似欧亚（EU）型的高度异常,伴随着强的向极的EP通量异常,导致对流层有极强的负北极涛动（AO）型异常。而有SSW发生的弱极涡事件发展阶段对流层上部主要表现为北太平洋上空来自低纬的波列异常,其后期的对流层效应更加滞后也不连续,对流层AO异常的强度明显弱于无SSW发生的弱极涡事件。     

Low-Frequency CISK-Rossby Wave and Stratospheric QBO in the Tropical Atmosphere

1.IntroductionThelow~frequencyoscillation(LFO)isaveryimPOrtantweatherphenomenonintheatmosphere.The30--50--dayandquasichiweeklyoscillationsinthetropicalatmospherearemostintensivelystudied,andcomParativelyspeaking,anotherkindofLFOconcernillgtheQBOofthestratosphericzonalwindsismuchlessstudied,which,althoughoccurringinthestratosphere,bearsacloserelationtothetroposphericactivitiesandtheevolutionoflow--latitudecirculationssothatitisworthwhiletoexploreindepththephysicalmechanismfortheQBOoccurre…     

Variation of Air-Sea Heat Fluxes over the Western Pacific Warm Pool Area and Its Relationship with the South China Sea Summer Monsoon Onset

Based on oceanic and atmospheric parameters retrieved by satellite remote sensing using a neural network method, air-sea heat fluxes over the western Pacific warm pool area were calculated with the advanced the advanced Coupled Ocean-Atmosphere Response Experiment 3.0 (COARE3.0) bulk algorithm method. Then, the average annual and interannual characteristics of these fluxes were analyzed. The rela- tionship between the fluxes and the South China Sea (SCS) summer monsoon onset is highlighted. The results indicate that these fluxes have clear temporal and spatial characteristics. The sensible heat flux is at its maximum in the Kuroshio area, while the latent heat flux is at its maximum in the North Equatorial Current and Kuroshio area. The distribution of average annual air-sea heat fluxes shows that both sensible and latent heat fluxes are maximized in winter and minimized in summer. The air-sea heat fluxes have obvious interannual variations. Correlation analysis indicates a close lag-correlation between air-sea heat fluxes in the western Pacific warm pool area and at the SCS summer monsoon onset. The lagcorrelation can therefore predict the SCS summer monsoon onset, providing a reference for the study of precipitation related to the monsoon.     

北半球准定常行星波气候平均态的资料分析和数值模拟

利用NCEP/NCAR再分析资料和大气环流模式(CCSR/NIES AGCM Ver 5.6),对北半球准定常行星波的气候平均态分布进行分析和模拟.再分析资料分析的结果表明:北半球冬季,准定常行星波沿两支波导向上传播,其中一支在对流层上层转向中低纬度传播,另外一支折向高纬度,通过极地波导上传到平流层.其中,1波和2波可以上传到平流层,因而其振幅分布除在中低纬的对流层上层出现一个次大值外,在高纬度平流层中上层会出现一个最大值,3波则主要限制在对流层,其振幅分布除在副热带对流层上层出现一个次大值外,最大值出现在中纬度对流层上层.北半球夏季,整个平流层为东风环流,极地波导不存在,行星波不能上传到平流层,在对流层活动也较弱,1波、2波、3波的传播情况大致相似,表现为在对流层上层由中纬度向赤道地区的传播.相应的振幅分布是,对1波和2波而言,最大值出现在中低纬对流层顶附近,同时在中高纬对流层上层出现一个次大值,而3波的振幅分布正好相反,最大值出现在中高纬对流层上层,次大值则在中低纬对流层顶附近.利用大气环流模式进行的数值模拟表明,模式可以比较好地模拟冬夏季准定常行星波的传播路径,但模拟的北半球冬季沿极地波导向平流层的传播明显偏弱,其结果是对1波、2波而言,高纬度平流层中上层的振幅最大值明显小于再分析资料的数值.文中还讨论了数值模拟与资料分析中行星波的差异可能对大气环流模拟的影响.     

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.     

大气动力学中三种Rossby波作用通量的特征差异和适用性比较

大气动力学诊断Rossby波的传播时,通常用波作用通量来表示。常用的三种波作用通量分别为Plumb波作用通量,T-N波作用通量和局地E-P通量。本文详细讨论了这三种方法的特征差异,并结合2016年1月的一次寒潮事件,比较了三种方法在该事件中的适用性。结果表明:1)Plumb波作用通量的纬向分量较大而经向分量较小,适用于振幅较小的纬向均匀的西风带Rossby长波的诊断。2)T-N波作用通量是对Plumb波作用通量的改进,经向分量得以增强,能更好地描述纬向非均匀气流中的较大振幅的西风带Rossby长波扰动。T-N波作用通量计算时,背景场取多年平均的当月气候场较合适,能更好地反映当前季节内的Rossby波传播异常。3)局地E-P通量可以诊断一段时间内天气尺度瞬变波对背景场(定常波)总的调控作用,但无法直接反映Rossby长波的逐时演变(T-N波或Plumb波作用通量则可以)。     

Atmospheric Circulation Changes in Response to an Observed Stratospheric Zonal Ozone Anomaly

Abstract

In a sensitivity study, the influence of an observed stratospheric zonal ozone anomaly on the atmospheric circulation was investigated using the Fifth Generation European Centre Hamburg Model (ECHAM5) which is a general circulation model. The model was run from 1960 to 1999 (40 years) with a mean seasonal cycle of zonally symmetric ozone. In order to isolate the induced dynamical influence of the observed zonally asymmetric part of the three-dimensional stratospheric ozone, a second run was performed for the boreal extratropics using prescribed monthly means from the 40-year reanalysis dataset from the European Centre for Medium-range Weather Forecasts (ERA-40). The main findings are the interdecadal westward shift of the polar vortex at about 65°N and a significant increase in the number of stratospheric sudden warmings during the 1980–99 period. Under the action of zonally asymmetric ozone a decrease in the Arctic Oscillation was identified between the mid-1980s and the mid-1990s. The lag correlation between the mean Arctic Oscillation at the surface and the daily stratospheric northern annular mode increased in mid-winter. Furthermore, we examined the influence of the stratospheric zonal ozone anomaly on Rossby wave breaking in the upper troposphere and found a significant westward shift of poleward Rossby wave breaking events over western Europe in the winter. By this we show that the stratospheric zonal ozone anomaly has a strong influence on the tropospheric circulation as a result of enhanced dynamical coupling processes.     

The Role of Stationary and Transient Planetary Waves in the Maintenance of Stratospheric Polar Vortex Regimes in Northern Hemisphere Winter

Using 1958-2002 NCEPNCAR reanalysis data, we investigate stationary and transient planetary wave propagation and its role in wave-mean flow interaction which influences the state of the polar vortex (PV) in the stratosphere in Northern Hemisphere (NH) winter. This is done by analyzing the Eliassen-Palm (E-P) flux and its divergence. We find that the stationary and transient waves propagate upward and equatorward in NH winter, with stronger upward propagation of stationary waves from the troposphere to the stratosphere, and stronger equatorward propagation of transient waves from mid-latitudes to the subtropics in the troposphere. Stationary waves exhibit more upward propagation in the polar stratosphere during the weak polar vortex regime (WVR) than during the strong polar vortex regime (SVR). On the other hand, transient waves have more upward propagation during SVR than during WVR in the subpolar stratosphere, with a domain of low frequency waves. With different paths of upward propagation, both stationary and transient waves contribute to the maintenance of the observed stratospheric PV regimes in NH winter.     

Signature of the Antarctic oscillation in the northern hemisphere

Using the ECWMF daily reanalysis data, this paper investigates signatures of the Antarctic Oscillation (AAO) in the upper troposphere of the northern hemisphere. It is found that during boreal winter, a positive (negative) phase of the AAO is associated with anomalous easterlies (westerlies) in middle-low latitudes (~30–40°N) and anomalous westerlies (easterlies) in middle-high latitudes (~45–65°N) of the upper troposphere about 25–40 days later. While there is also a response in zonal wind in the tropics, namely over the central-eastern Pacific, to some extent, these tropical zonal wind anomalies can trigger a Pacific/North American teleconnection patterns (PNA)-like quasi-stationary Rossby waves that propagate into the Northern Hemisphere and gradually evolve into patterns which resemble North Atlantic teleconnection patterns. Furthermore, these quasi-stationary Rossby waves might give rise to anomalous eddy momentum flux convergence and divergence to accelerate anomalous zonal winds in the Northern Hemisphere.