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.     

与北极涛动或北半球环状模相关的纬向对称的正规模态

作为一个全球气候变率的重要模态,北极涛动或北半球环状模态(AO/NAM)一般由北半球海平面气压变率的EOF1来表示。但是通常认为EOF1仅有统计学意义而并不能够表明它是否是一种实际的物理模态。另一方面,现有的波-流相互作用理论也仅能给出纬向风的纬向平均状况,而不能够保证沿着某一纬圈的变化,所以它并不意味着一个具有半球尺度的纬向对称的相干结构能够组织起来。因此AO/NAM的形成机制仍然是一个具有争议的问题。文章提出纬向对称的或环状的正规模态与基本气候态附近的线性(进而非线性)动力学之间有着最直接的联系,因而可能在AO/NAM动力学的理解中扮演重要角色。为了深入探讨AO/NAM的动力学本质,使用p-坐标球面原始方程计算了冬季北半球及全球气候基本态附近的线性动力系统纬向对称的正规模态,发现在半球尺度上的纬向对称正规模态具有AO/NAM的经向偶极子及垂直方向近似正压的空间结构特征。而这些纬向对称正规模的时间变化尺度取决于与其他非纬向对称正规模间的相互作用。从而说明了AO/NAM可能是半球尺度上纬向对称正规模态的动力学行为的反映。     

Rotational atmospheric circulation during North Atlantic-European winter: the influence of ENSO

The dominant variability modes of the North Atlantic-European rotational flow are examined by applying a principal component analysis (PCA/EOF) to the 200?hPa streamfunction mid-winter anomalies (Jan?CFeb monthly means). The results reveal that, when this norm is used, the leading mode (EOF1) does not correspond to the traditional North Atlantic Oscillation (NAO, which appears in our analysis as the second leading mode, EOF2) but is the local manifestation of the leading hemispheric streamfunction EOF. The regression of this regional mode onto the global SST field exhibits a clear El Ni?o signature, with no signal over the Atlantic, while the associated upper height anomalies resemble the Tropical/Northern Hemisphere (TNH) pattern. East of North America, this TNH-like wavetrain produces a meridional dipole-like pattern at lower levels. Although in some ways this pattern resembles the NAO (EOF2), the dynamics of these two modes are very different in that only EOF2 is associated with a latitudinal shift of the North Atlantic stormtrack. Thus, the choice of the streamfunction norm in the EOF analysis allows the separation of two different phenomena that can produce similar dipolar surface pressure anomalies over the North Atlantic but that have different impact on European climate. These two modes also differ on their contribution to variability at lower levels: while NAO-EOF2 is mostly confined to the North Atlantic, TNH-EOF1 has a more annular, global character. At upper levels NAO-EOF2 also produces a global pattern but with no annular structure, reminiscent of the ??circumglobal?? teleconnection.     

The variable link between PNA and NAO in observations and in multi-century CGCM simulations

The link between the Pacific/North American pattern (PNA) and the North Atlantic Oscillation (NAO) is investigated in reanalysis data (NCEP, ERA40) and multi-century CGCM runs for present day climate using three versions of the ECHAM model. PNA and NAO patterns and indices are determined via rotated principal component analysis on monthly mean 500?hPa geopotential height fields using the varimax criteria. On average, the multi-century CGCM simulations show a significant anti-correlation between PNA and NAO. Further, multi-decadal periods with significantly enhanced (high anti-correlation, active phase) or weakened (low correlations, inactive phase) coupling are found in all CGCMs. In the simulated active phases, the storm track activity near Newfoundland has a stronger link with the PNA variability than during the inactive phases. On average, the reanalysis datasets show no significant anti-correlation between PNA and NAO indices, but during the sub-period 1973?C1994 a significant anti-correlation is detected, suggesting that the present climate could correspond to an inactive period as detected in the CGCMs. An analysis of possible physical mechanisms suggests that the link between the patterns is established by the baroclinic waves forming the North Atlantic storm track. The geopotential height anomalies associated with negative PNA phases induce an increased advection of warm and moist air from the Gulf of Mexico and cold air from Canada. Both types of advection contribute to increase baroclinicity over eastern North America and also to increase the low level latent heat content of the warm air masses. Thus, growth conditions for eddies at the entrance of the North Atlantic storm track are enhanced. Considering the average temporal development during winter for the CGCM, results show an enhanced Newfoundland storm track maximum in the early winter for negative PNA, followed by a downstream enhancement of the Atlantic storm track in the subsequent months. In active (passive) phases, this seasonal development is enhanced (suppressed). As the storm track over the central and eastern Atlantic is closely related to the NAO variability, this development can be explained by the shift of the NAO index to more positive values.     

On the separability of the North Atlantic oscillation and Arctic oscillation

We address the issue of whether the Arctic (AO), and North Atlantic oscillations (NAO) are inseparable, forming an annular mode in the Northern Hemisphere atmospheric circulation. This annular mode is the leading empirical orthogonal function of hemispheric sea level pressure (SLP) data, explaining the largest amount of its variability. We examine whether the NAO and AO are inseparable spatial modes of the atmospheric circulation using rotated principal component analysis (RPCA), a methodology that identifies simple and unique patterns of spatial dataset variability. RPCA of the spring, summer, and autumn SLP fields from 1946-1998 reveal NAO and AO-like patterns, occurring as separate regional teleconnections forming the first and second principal components respectively. The RPCA-based NAO dipole pattern is like that observed in many prior studies, while the AO-like pattern exhibits high SLP variability over the Kara and Laptev seas. During winter however, and in annual analyses, a distinct AO-like pattern is not obtained and the two patterns may be inseparable using commonly accepted RPCA methods. The RPCA-based AO-like mode is significantly linked to north-central Siberian seasonal air temperatures and to the prevailing direction of motion of the underlying Arctic Ocean in summer, suggesting that the non-winter AO-like pattern, as a stand alone teleconnection separate from the NAO, contributes significantly to high-latitude climate and ocean variability. The winter NAO/AO inseparability is discussed as a possible effect of a shared winter storm track between the northeastern Atlantic and the Arctic.     

冬季北太平洋风暴轴的年代际变化特征及其可能影响机制

利用1958-2002年的ERA-40再分析资料,用谐波变换和EOF方法分析了冬季北太平洋风暴轴在年代际时间尺度上的变化特征,并通过回归分析的方法初步探讨了风暴轴年代际变化的可能影响机制.结果表明,在年代际时间尺度上,北太平洋风暴轴有两种主要模态,第1模态是风暴轴在其气候平均位置增强或减弱的主体一致变化型,其年代际变化受到上游涡旋强迫的影响,北大西洋强(弱)的涡旋活动,使得冬季北太平洋西风急流减弱(增强)、变宽(窄)、北抬(南压),同期北太平洋风暴轴活动偏强(弱),黑潮延续体区海表温度有偏暖(冷)的响应;第2模态是风暴轴中东部在气候平均位置南北两侧振荡的经向异常型,与太平洋年代际振荡(PDO)循环的暖(冷)位相相联系,下垫面海温非绝热加热的作用,激发加强(减弱)大气中类太平洋/北美遥相关型(PNA)的响应,引起大气斜压性异常偏南(北),使得风暴轴整体南压(北抬),且中东部向东南(北)方向移动.因此,冬季北太平洋风暴轴的年代际变化不仅是局地波-流相互作用的结果,还应考虑上游涡旋活动和海温热力强迫的作用.     

Interannual variability of winter precipitation over northwest India and adjoining region: impact of global forcings

The role of El Niño/Southern Oscillation (ENSO) and the mechanism through which ENSO influences the precipitation variability over northwest India and the adjoining (NWIA) region is well documented. In this study, the relative role of North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) and ENSO in modulating the Asian jet stream in the Northern Hemisphere winter and their relative impact on the precipitation variability over the region have been estimated through analysis of observed data. It is seen that interannual variations of NWIA precipitation are largely influenced by ENSO. An empirical orthogonal function (EOF) analysis has been carried out to understand dominant modes of interannual variability of zonal wind at 200 hPa of the Northern Hemisphere. The EOF-1 pattern in the tropical region is similar to that of an ENSO pattern, and the principal component (PC) time series corresponds to the ENSO time series. The EOF-2 spatial pattern resembles that of NAO/AO with correlation of PC time series with AO and NAO being 0.74 and 0.62, respectively. The precipitation anomaly time series over the region of interest has marginally higher correlation with the PC-2 time series as compared to that of PC-1. Regression analysis of precipitation and circulation parameters indicates a larger contribution of the second mode to variability of winds and precipitation over the NWIA. Moisture transport from the Arabian Sea during the active phase of NAO/AO and the presence of a cyclonic anomaly lead to higher precipitation over the NWIA region.     

A comparison of climatological subseasonal variations in the wintertime storm track activity between the North Pacific and Atlantic: local energetics and moisture effect

Distinct differences of the storm track?Cjet relationship over the North Pacific and North Atlantic are investigated in terms of barotropic and baroclinic energetics using NCEP-2 reanalysis data for the period of 1979?C2008. From fall to midwinter the Pacific storm track (PST) activity weakens following the southward shift of the Pacific jet, whereas the Atlantic storm track (AST) activity remains steady in position and intensifies regardless of the slight southward shift of the Atlantic jet. This study is devoted to seeking for the factors that can contribute to this conspicuous difference between the two storm tracks on climatological subseasonal variation by analyzing eddy properties and local energetics. Different eddy properties over the two oceans lead to different contribution of barotropic energy conversion to the initiation of storm tracks. In the North Atlantic, meridionally elongated eddies gain kinetic energy efficiently from stretching deformation of the mean flow in the jet entrance. On the other hand, the term associated with shearing deformation is important for the initiation of PST. Analysis of baroclinic energetics reveals that the intensification of the AST activity in midwinter is mainly attributed to coincidence between location of maximum poleward and upward eddy heat fluxes and that of the largest meridional temperature gradient over slight upstream of the AST. The relatively large amount of precipitable water and meridional eddy moisture flux along baroclinic energy conversion axis likely provides a more favorable environment for baroclinic eddy growth over the North Atlantic than over the North Pacific. In the meantime, the midwinter minimum of the PST activity is attributable to the southward shift of the Pacific jet stream that leads to discrepancy between core region of poleward and upward heat fluxes and that of meridional thermal gradient. Weakening of eddy-mean flow interaction due to eddy shape and reduction of moist effect are also responsible for the weakening of storm track activities in midwinter when the strongest baroclinicity exists over the North Pacific.     

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.     

北太平洋风暴轴对黑潮延伸体系统变异的响应及其能量转换机制

西北太平洋纬向扰动海温经验正交函数（EOF）分解第一和第三模态、第二和第四模态分别代表同期黑潮延伸体和亲潮强弱的配置关系,将两者的典型位相合成,可以分别得到延伸体收缩和扩张状态时的典型模态海温,本文以此及气候态海温作为初始海温强迫场,利用CESM1.2.0模式,讨论了延伸体的系统变异对北太平洋风暴轴的影响及其在不同能量转换过程的主要影响机制,结果表明,延伸体收缩状态下,北太平洋风暴轴强度整体加强,而扩张模态下强度减弱。空间分布上,收缩模态下,风暴轴主要体现为经向方向的变化,中心及其以北强度加强,中心以南减弱;扩张状态下,则主要表现为纬向方向的差异,中心及以西强度减弱明显,中心以东有所增强。对能量转换的诊断分析表明,正压能量转换过程对涡动动能的变化贡献很小,且在风暴轴中心附近,其作用主要为消耗涡动动能,延伸体收缩状态下其消耗作用增强,而扩张状态下消耗作用减弱,这一差异主要是由于不同海温异常强迫下瞬变涡旋的形变不同造成;斜压有效位能释放比正压能量转换大一个量级以上,该过程几乎全部通过基流的经向温度梯度和经向涡动热量输送的相互作用完成,在这一过程中大气斜压性（经向温度梯度）起了关键性作用,大气斜压性异常、基流经向温度梯度异常、斜压有效位能释放异常与风暴轴异常的空间分布均具有较好的对应关系,该过程可能也是延伸体海温异常影响北太平洋风暴轴的主要物理过程;涡动有效位能需要进一步转换为涡动动能才能产生瞬变涡旋运动,涡动有效位能释放的量级与斜压有效位能的释放相当,但数值要小,这一过程通过冷暖空气的上升下沉运动完成,延伸体异常模态下,扰动垂直速度和扰动温度的负相关性的变化与涡动有效位能向涡动动能转换的变化也有较好的对应关系。     

Intraseasonal non-stationarity of the leading modes of atmospheric moisture over Europe during summer

A gridded monthly precipitable water (PW) data for 1979?C2007 from the NCEP/NCAR reanalysis are used to investigate summertime interannual PW variability over Europe and its relation to the key climate parameters in the region. During summer season the first EOF mode of PW, explaining 27?C41% of its total variance, demonstrates significant month-to-month changes in its structure, thus, implying its essential non-stationarity. The second EOF mode of PW is also non-stationary during the summer season. In contrast to precipitation, both leading modes of PW are not associated with the North Atlantic Oscillation (NAO), as well as with other regional teleconnections, suggesting relatively minor role of the atmospheric dynamics in atmospheric moisture variability over Europe during summer season. Analysis of links between leading EOF modes of regional PW and air temperature (AT) has revealed a strong link between PW and AT over Europe, persisting during entire summer season. Locally, these links imply that positive (negative) AT anomalies result in enhanced (decreased) PW over particular region. Revealed links between leading modes of PW and AT highlight important role of thermodynamics in summertime PW variability over Europe. Detected relatively weak and unstable links between leading modes of PW and precipitation over Europe were somewhat expected since in contrast to atmospheric moisture, regional precipitation variability is largely driven by the atmospheric dynamics (particularly, the NAO).     

A shift of the NAO and increasing storm track activity over Europe due to anthropogenic greenhouse gas forcing

 In accordance with a number of other general circulation model experiments, the coupled atmosphere-ocean-GCM ECHAM4+OPYC3 simulates increasing upper air storm track activity over the east Atlantic and Western Europe with rising greenhouse gas forcing. This paper addresses the question to what extent this change is attributable to the variable north Atlantic Oscillation (NAO), which is closely related to the intensity of the Atlantic storm track’s extension into Europe. The NAO index, which is based on sea level pressure fluctuations over the north Atlantic in the 300-y control run of this model, only shows a moderate increase within the 240-y scenario run, so that its long-term trend does not exceed the variability of the control climate before the end of the simulation. In contrast, the steadily growing storm track activity over northwestern Europe already surpasses the standard deviation defined from the control run after about 160 y. This effect is associated with a change of the NAO pattern. A determination of the centres of action for subsequent 10-y periods based on empirical orthogonal functions shows a systematic northeastward shift of the NAO’s northern variability centre from a position close to the east coast of Greenland, where it is also located in the control run, to the Norwegian Sea. Received: 10 September / Accepted: 15 January 1999     

Are band-pass variance statistics useful measures of storm track activity? Re-examining storm track variability associated with the NAO using multiple storm track measures

Over the past few decades, band-pass filtered (BP) variance statistics have been widely used as indicators of storm track activity. A recent study suggested that these statistics may be subjected to biases related to Doppler shifting of variance into and out of the fixed frequency band, in the process perhaps greatly exaggerating the correlation between BP variance and mean flow variability. In this study, BP statistics are examined together with other storm track measures to assess whether BP variances are useful indicators of the patterns of storm track activity. Storm track variability related to the North Atlantic Oscillation, as well as the five leading EOFs of 300 hPa BP filtered variance statistics, have been examined. Results presented here suggest that BP variance statistics are useful measures of the patterns of storm track activity. Firstly, BP variance statistics largely reflect changes in total unfiltered transient eddy variance as well as spatially filtered statistics. Secondly, different BP statistics all give qualitatively similar patterns. Thirdly, patterns derived from BP statistics are well correlated (spatially) with precipitation anomalies. In addition, BP statistics are straightforward to compute, and easily reproducible. Moreover, BP variance and covariance statistics are closely related to how storm tracks interact with the large scale circulation. However, results shown in this paper support the suggestion that some biases may be caused by the Doppler effect, especially for variations such as the NAO which are tightly tied to changes in the background flow speed. Thus it is argued that BP statistics should be examined together with other measures of storm track activity to obtain a broader perspective on storm track variations.     

北大西洋海表面温度锋与大西洋风暴路径及大气大尺度异常的关系研究

运用NCEP、Had ISST再分析资料,北大西洋涛动(NAO)月指数序列,探讨了海表面温度(SST)锋的时空变化特征,揭示了北大西洋SST锋的主要气候变率及其与北大西洋风暴轴和大气大尺度环流异常的关系。研究表明,剔除季节循环后的SST锋显示其最主要变率为锋区的向南/北摆动,其对应的风暴轴发生相应的西南/东北移动,并同时在北大西洋上空对应一个跨海盆的位势高度负/正异常。这种环流异常可引起高纬度海平面气压(SLP)的反气旋/气旋式环流,这有利于增强海表面风对大洋副极地环流的负/正涡度异常输入,进一步减弱/加强了高纬度上层冷水向SST锋区的输送。北大西洋SST锋的另一主要模态为锋区在南北方向的分支和合并。当SST锋异常在40°N~45°N以单支形式加强时,对流层位势高度场和SLP南北梯度增大,对应NAO正位相,此时风暴轴也为单支型;同时SLP异常场促使冰岛附近具有气旋式风应力异常,亚速尔地区具有反气旋式风应力异常,导致副极地环流和副热带环流均加强,增加高纬度冷水和低纬度暖水在锋区的输入,从而进一步增强40°N~45°N附近的SST锋区。当SST锋异常在40°N~45°N纬带南北发生分支时,风暴轴也同时出现北强南弱的南北分支,此时对应了负位相NAO,来自北南的冷暖水输送减弱,SST锋也发生减弱分支。此外,位于大洋内区的SST锋东端也存在一个偶极子型的模态,尽管其解释方差相对较小,但仍与偏东北的NAO型具有显著相关。谱分析表明,北大西洋SST锋与风暴轴具有1~3年和年代际共振,与中高纬大尺度环流也存在周期1~3年的共变信号,其中准一年共变信号体现了SST锋和NAO之间的对应关系。进一步诊断分析表明,SST锋上空的近表层大气斜压性和经向温度梯度随着SST锋的增强而增强,经向热通量的向北输送导致涡动有效位能的增加;海洋的非绝热加热产生更强的垂直热量通量,这有利于涡动有效位能释放成为涡动动能,从而表现为该区域的风暴轴加强,并进一步影响风暴轴中的天气尺度扰动与下游大尺度环流异常的相互作用过程。     

Long-term precipitation variability in Morocco and the link to the large-scale circulation in recent and future climates

Summary ?Monthly precipitation data from the Global Historical Climatology Network for 42 stations in Morocco and its vicinity are investigated with respect to baroclinicity, storm track and cyclone activity, moisture transports, North Atlantic Oscillation (NAO) variations, and different circulation types by means of correlation and composite studies. The results are related to a climate change scenario from an ECHAM4/OPYC3 transient greenhouse gas only (GHG) simulation. Precipitation in northwestern Morocco shows a clear link to the baroclinic activity over the North Atlantic during boreal winter (DJF). In large precipitation months the North Atlantic storm track is shifted southward, more westerly and northwesterly circulation situations occur and moisture transports from the Atlantic are enhanced. The occurrence of local cyclones and upper-level troughs is more frequent than in low precipitation months. The negative correlation to the NAO is relatively strong, especially with Gibraltar as a southern pole (−0.71). The northward shift of the storm track and eastward shift of the Azores High predicted by the ECHAM model for increasing GHG concentrations would therefore be associated with decreasing precipitation and potentially serious impacts for the future water supply for parts of Morocco. In the region south of the Atlas mountains, moisture transports from the Atlantic along the southern flank of the Atlas Mountains associated with cyclones west of Morocco and the Iberian Peninsula can be identified as a decisive factor for precipitation. Northeastern Morocco and Northwestern Algeria, however, is rather dominated by the influence of cyclones over the Western Mediterranean that are associated with a strong northwesterly moisture transport. As both regions appear to be less dependent on the North Atlantic storm track and more on local processes, a straight forward interpretation of the large-scale changes predicted by the ECHAM4/OPYC3 cannot be done without the application of down-scaling methods in the future. Received July 19, 2001; revised May 31, 2002     

Response of Northern Hemisphere storm tracks to Indian-western Pacific Ocean warming in atmospheric general circulation models

With 40 years integration output of two atmospheric general circulation models (GAMIL/IAP and HadAM3/UKMO) forced with identical prescribed seasonally-varying sea surface temperature, this study examines the effect of the observed Indian-western Pacific Ocean (IWP) warming on the Northern Hemisphere storm tracks. Both models indicate that the observed IWP warming tends to cause both the North Pacific storm track (NPST) and the North Atlantic storm track (NAST) to move northward. Such a consistent effect on the two storm tracks is closely associated with the changes in the low-level atmospheric baroclinicity, high-level jet stream and upper-level geopotential height. The IWP warming can excite a wavelike circum-global teleconnection in the geopotential height that gives rise to an anticyclonic anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic. These geopotential height anomalies tend to enhance upper-level zonal westerly winds north of the climatological jet axes and increase low-level baroclinicity and eddy growth rates, thus favoring transient eddy more active north of the climatological storm track axes, responsible for the northward shift of the both storm tracks. The IWP warming-induced northward shift of the NAST is quite similar to the observed, suggesting that the IWP warming can be one of the key factors to cause decadal northward shift of the NAST since the 1980s. However, the IWP warming-induced northward shift of the NPST is completely opposite to the observed, implying that the observed southward shift of the NPST since the 1980s would be primarily attributed to other reasons, although the IWP warming can have a cancelling effect against those reasons.     

The effect of Doppler correction on measures of storm track intensity

In climate research, the intensity of a storm track is often measured by the band pass filtered eddy kinetic energy, geopotential variance or related variables. The interpretation of such fields disregards the influence the advection speed might have on these measures. In this paper the need for a Doppler corrected storm track measure is discussed. A Doppler corrected measure is defined and applied to 10 years of ECMWF reanalysis data, correcting the storm track measure for spatial and temporal variability of the advection speed. The storm track intensity is also calculated correcting only for the temporal variability of the advection speed. It is also related to fluctuations of the NAO teleconnection pattern. The Doppler correction suggests that (1) maximum and minimum baroclinic activity is found somewhat downstream of the locations indicated by non-corrected measures, (2) the storm track activity estimated by conventional measures is much too low in the areas of the eastern ends of the storm tracks, (3) the monthly mean time series of the strength of a storm track, as estimated by conventional measures, is strongly influenced by the variability of the advection speed at times, (4) the strength of the storm track seems to be less strongly connected with teleconnection patterns such as NAO or with the background mean flow speed than usually thought on the basis of conventional Eulerian statistics.     

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。     

Arctic Oscillation and Antarctic Oscillation in Internal Atmospheric Variability with an Ensemble AGCM Simulation

In this study, we investigated the features of Arctic Oscillation （AO） and Antarctic Oscillation （AAO）, that is, the annular modes in the extratropics, in the internal atmospheric variability attained through an ensemble of integrations by an atmospheric general circulation model （AGCM） forced with the global observed SSTs. We focused on the interannual variability of AO/AAO, which is dominated by internal atmospheric variability. In comparison with previous observed results, the AO/AAO in internal atmospheric variability bear some similar characteristics, but exhibit a much clearer spatial structure： significant correlation between the North Pacific and North Atlantic centers of action, much stronger and more significant associated precipitation anomalies, and the meridional displacement of upper-tropospheric westerly jet streams in the Northern/Southern Hemisphere. In addition, we examined the relationship between the North Atlantic Oscillation （NAO）/AO and East Asian winter monsoon （EAWM）. It has been shown that in the internal atmospheric variability, the EAWM variation is significantly related to the NAO through upper-tropospheric atmospheric teleconnection patterns.