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.     

Winter climate anomalies in Europe and their associated circulation at 500 hPa

Regional anomalies of the surface climate over Europe are defined by a simultanous EOF-analysis of the normalized monthly mean sea level pressure, temperature and precipitation fields of 100 winters (December–February, 1887–1986) at 40 stations. The monthly amplitudes of the first EOF (about 25% of the total variance) are used as an index for the monthly winter climate anomaly. They characterize a high (low) pressure cell over central Europe associated with a positive (negative) temperature and precipitation anomaly over northern (central-southern) Europe as indicated by a northward (southward) shift of the tail end of the cross-Atlantic cyclone track. These patterns resemble the phenomenological anticyclonic (cyclonic) Grosswetter classification and the European blocking (enhanced zonal flow) regime. The second EOF is of similar magnitude and gives latitudinal corrections to these two basic flow regimes. The joint probability distribution of both amplitudes shows a weak bimodality mainly associated with the first EOF. Further insight into the underlying physical processes of the climate anomaly patterns in Europe is obtained from the extended Eliassen-Palm flux diagnostics of the barotropic transient eddy-mean flow interaction (Hoskins et al. 1983) and the stationary wave propagation (Plumb 1985). The diagnostics confined to the barotropic components and applied to the regression and the composite anomaly fields of the transient and stationary eddy flows of the 500 hPa geopotential (1946–87, north of 20°N) leads to the following results: (1) The bandpass filtered transient eddy variances of the 500 hPa geopotential show a shift of the cross-Atlantic storm track: In high (low) pressure situations over Europe the cross-Atlantic storm track intensity is enhanced (reduced) and its tail end is shifted northward (remains zonal); the North Pacific storm track extends further (less) eastward and thus closer to the west coast of North America. (2) The extreme high pressure system over Europe tends to be supported by an anomalous transient eddy forcing of the mean flow stream-function: it enhances the zonal wind to its north and generates anticyclonic vorticity about 10° upstream from its center. In the low pressure composite the anomalous cyclonic vorticity is generated reducing the zonal flow to its north. (3) The occurrence (lack) of a strong eastward stationary wave activity flux over the Atlantic is associated with the high (low) pressure situations over Europe. Finally, a positive feedback is conjectured between the stationary wavetrain modifying the tail end of the cross-Atlantic storm track and the transient eddies intensifying this anomaly.     

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.     

瞬变天气涡旋对北大西洋涛动的增强效应

使用NCEP/NCAR再分析资料计算了冬季北大西洋瞬变涡旋活动强度与北大西洋涛动(NAO)逐日指数的时间序列,结果发现:当涡旋活动强度出现峰值后会伴随NAO模态增强现象;而随着NAO的增强,涡旋能量同落.为了判断是否涡流相互作用将天气尺度的能量转换为低频尺度的能量,使用瞬变涡度通量来研究涡度与能量的传输.通过分析瞬变涡...     

1998年夏第二阶段梅雨期乌拉尔山阻塞形势的维持

１９９８年夏季长江流域发生了近５０年来最严重的洪水。洪水形成最直接、最主要的原因是梅雨异常。异常梅雨的形成与东亚夏季风偏弱及热带外环流持续异常有关，其中一个明显特征是乌拉尔山地区长时间维持阻塞形势。本文结合诊断分析和数值试验，从瞬变对基本流的强迫（大气内部强迫）及热带热源强迫（外源强迫）两方面，分析了与第二段梅雨相对应的乌拉尔长时间阻塞的维持机制。利用共轭敏感性分析方法，计算了最有利于乌拉尔阻塞发展和维持的敏感扰动，发现扰动的分布位置，刚好与观测到的，从东大西洋到欧洲区域的异常增强的瞬变活动区相重叠。Ｅ矢量及斜压线性静止波模式的诊断进一步表明，异常期间的增强瞬变活动有利于乌拉尔出现正高度异常。计算了持续异常期间的高空急流及大气加热场，发现北美到大西洋的高空急流及热带加热都出现明显异常。中期天气预报模式 ＩＡＰ Ｔ４２Ｌ９的集合预报试验表明，热带地区的加热异常，尤其是热带中西太平洋和大西洋的加热异常，有利于乌拉尔正高度异常的形成。最后，提出了一种热带异常热源驱动下，瞬变波与定常波双向相互作用的阻塞形成与自维持的可能机制。     

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。     

Exploring the phase-strength asymmetry of the North Atlantic Oscillation using conditional nonlinear optimal perturbation

Negative-phase North Atlantic Oscillation(NAO) events are generally stronger than positive-phase ones, i.e., there is a phase-strength asymmetry of the NAO. In this work, we explore this asymmetry of the NAO using the conditional nonlinear optimal perturbation(CNOP) method with a three-level global quasi-geostrophic spectral model. It is shown that, with winter climatological flow forcing, the CNOP method identifies the perturbations triggering the strongest NAO event under a given initial constraint. Meanwhile, the phase-strength asymmetry characteristics of the NAO can be revealed. By comparing with linear results, we find that the process of perturbation self-interaction promotes the onset of negative NAO events, which is much stronger than during positive NAO onset. Results are obtained separately using the climatological and zonal-mean flows in boreal winter(December–February) 1979–2006 as the initial basic state. We conclude, based on the fact that NAO onset is a nonlinear initial-value problem, that phase-strength asymmetry is an intrinsic characteristic of the NAO.     

On non-linearities in a forced North Atlantic Oscillation

Large ensembles of simulations (ensemble size of 500 members) are performed using a simplified atmospheric general circulation model (AGCM) in order to investigate the non-linearities in the response to composite sea surface temperature (SST) anomaly forcings that are constant in time. The SST composite corresponds to the observed anomaly associated with the atmospheric North Atlantic Oscillation (NAO). The integration length is 90 days for each ensemble (covering January, February and March). A non-linearity is found in the mean response to the SST-forcing, with the negative SST-NAO forcing leading to a stronger and more clear atmospheric NAO response. These non-linearities appear to be due to asymmetries in the heating anomalies induced by the SST-forcing and asymmetries in the transient eddy vorticity forcing. Further non-linearities are due to initial period dependences of the response to the same SST-forcing. As a consequence, a pre-existing negative atmospheric NAO is much more persistent due to SST-feedback than a positive NAO.     

Mechanisms of the atmospheric response to North Atlantic multidecadal variability: a model study

The atmospheric circulation response to decadal fluctuations of the Atlantic meridional overturning circulation (MOC) in the IPSL climate model is investigated using the associated sea surface temperature signature. A SST anomaly is prescribed in sensitivity experiments with the atmospheric component of the IPSL model coupled to a slab ocean. The prescribed SST anomaly in the North Atlantic is the surface signature of the MOC influence on the atmosphere detected in the coupled simulation. It follows a maximum of the MOC by a few years and resembles the model Atlantic multidecadal oscillation. It is mainly characterized by a warming of the North Atlantic south of Iceland, and a cooling of the Nordic Seas. There are substantial seasonal variations in the geopotential height response to the prescribed SST anomaly, with an East Atlantic Pattern-like response in summer and a North Atlantic oscillation-like signal in winter. In summer, the response of the atmosphere is global in scale, resembling the climatic impact detected in the coupled simulation, albeit with a weaker amplitude. The zonally asymmetric or eddy part of the response is characterized by a trough over warm SST associated with changes in the stationary waves. A diagnostic analysis with daily data emphasizes the role of transient-eddy forcing in shaping and maintaining the equilibrium response. We show that in response to an intensified MOC, the North Atlantic storm tracks are enhanced and shifted northward during summer, consistent with a strengthening of the westerlies. However the anomalous response is weak, which suggests a statistically significant but rather modest influence of the extratropical SST on the atmosphere. The winter response to the MOC-induced North Atlantic warming is an intensification of the subtropical jet and a southward shift of the Atlantic storm track activity, resulting in an equatorward shift of the polar jet. Although the SST anomaly is only prescribed in the Atlantic ocean, significant impacts are found globally, indicating that the Atlantic ocean can drive a large scale atmospheric variability at decadal timescales. The atmospheric response is highly non-linear in both seasons and is consistent with the strong interaction between transient eddies and the mean flow. This study emphasizes that decadal fluctuations of the MOC can affect the storm tracks in both seasons and lead to weak but significant dynamical changes in the atmosphere.     

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     

Synoptic eddy feedback and air–sea interaction in the North Atlantic region

This paper explores the role of synoptic eddy feedback in the air-sea interaction in the North Atlantic region, particularly the interaction between the North Atlantic Oscillation (NAO) and the North Atlantic sea surface temperature anomalies (SSTA) tripole. A linearized five-layer primitive equation atmospheric model with synoptic eddy and low-frequency flow (SELF) interaction is coupled with a linearized oceanic mixed-layer model to investigate this issue. In this model, the “climatological” storm track/activity (or synoptic eddy activity) is characterized in terms of spatial structures, variances, decay time scales and propagation speeds through the complex empirical orthogonal function (CEOF) analysis on the observed data, which provides a unique tool to investigate the role of synoptic eddy feedback in the North Atlantic air–sea coupling. Model experiments show that the NAO-like atmospheric circulation anomalies can produce tripole-like SSTA in the North Atlantic Ocean, and the tripole-like SSTA can excite a NAO-like dipole with an equivalent barotropic structure in the atmospheric circulation, which suggests a positive feedback between the NAO and the SSTA tripole. This positive feedback makes the NAO/SSTA tripole-like mode be the leading mode of the coupled dynamical system. The synoptic eddy feedback plays an essential role in the origin of the NAO/SSTA tripole-like leading mode and the equivalent barotropic structure in the atmosphere. Without synoptic eddy feedback, the atmosphere has a baroclinic structure in the response field to the tripole-like SSTA forcing, and the leading mode of the dynamic system does not resemble NAO/SSTA tripole pattern.     

夏季亚洲高空急流纬向非对称变异与北大西洋海温和欧亚陆面热力异常的可能联系

基于1979~2015年ERA-Interim再分析资料,分析了夏季亚洲高空急流纬向非对称变异特征及其可能的外强迫因子。研究发现夏季亚洲200 hPa纬向风异常EOF第二模态（方差贡献为16.4%）主要表现出了急流纬向非对称的空间异常形态,反映了西亚和东亚区域急流南北偏移的反位相变化。通过进一步的诊断分析,我们发现急流纬向非对称变异与北大西洋海表温度（简称海温）和欧亚陆面热力异常可能存在一定的联系。北大西洋三极型海温异常会激发出向下游传播的异常波列,夏季该波列在欧亚大陆上空的异常环流中心与急流纬向非对称相关的异常环流中心对应一致,其中东欧平原的异常反气旋和巴尔喀什湖附近的异常气旋对西亚急流变化存在影响,东亚地区急流的变化与贝加尔湖北部异常气旋和贝加尔湖南部的异常反气旋有关。对比欧亚土壤湿度关键区内垂直环流,陆面热力异常可能会改变局地环流进而影响急流变异,且这种影响存在区域差异。     

The Behaviors of Optimal Precursors during Wintertime Eurasian Blocking Onset

In this paper the optimal precursors for wintertime Eurasian blocking onset are acquired by solving a nonlinear optimization problem whose objective function is constructed based on a blocking index with a triangular T21,three-level,quasi-geostrophic global spectral model.The winter climatological state is chosen as the reference basic state.Numerical results show that the optimal precursors are characterized by a baroclinic pattern with a westward tilt with height,which are mainly located upstream of the blocking region.For an optimization time of 5 days,these perturbations are mainly localized over the Northeast Atlantic Ocean and continental Europe.With the extension of the optimization time to 8 days,these perturbations are distributed more upstream and extensively in the zonal direction.Wave spectrum analysis reveals that the optimal precursors are composed of not only synoptic-scale(wave numbers 5-18) waves,but planetary-scale(wave numbers 0-4) waves as well.The synoptic-scale optimal precursors are mainly located in the mid-latitude area,while the planetary-scale optimal precursors focus primarily on the highlatitude region.The formation of a strong planetary-scale positive blocking anomaly is accompanied by the reinforcement of synoptic-scale perturbations and further fragmentation into two branches,in which the northern branch is generally stronger than the southern one.The eddy forcing arising from the selfinteraction of synoptic-scale disturbances is shown to be crucial in triggering the dipole blocking anomaly,and the planetary-scale optimal precursor provides the initial favorable background conditions for blocking onset.     

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.     

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.     

An Observed Connection Between Wintertime Temperature Anomalies over Northwest China and Weather Regime Transitions in North Atlantic

In this study, the association between wintertime temperature anomalies over Northwest China and the weather regime transitions in North Atlantic on synoptic scale is analyzed by using observational surface air temperature(SAT) data and atmospheric reanalysis data. Daily SAT anomaly and duration time are used in order to define SAT anomaly cases. Differences with regard to the circulation anomalies over the Ural Mountains and the upstream North Atlantic area are evident. It is found that the colder than normal SAT is caused by the enhanced Ural high and associated southward flow over Northwest China. Time-lagged composites reveal possible connections between the SAT anomalies and the different development phases of the North Atlantic Oscillation(NAO). The Ural highs tend to be strengthened during the negative phase of NAO(NAO–) to Atlantic ridge transition, which are closely related to the downstream-propagating Rossby wave activity. The opposite circulation patterns are observed in the warm SAT cases. A cyclonic circulation anomaly is distinctly enhanced over the Urals during the positive phase of NAO(NAO+) to Scandinavian blocking transition, which would cause warmer SAT over Northwest China. Further analyses suggest that the intensified zonal wind over North Atlantic would favor the NAO– to Atlantic ridge transition, while the weakened zonal wind may be responsible for the transition between NAO+ and Scandinavian blocking.     

Contrasting the tropical responses to zonally asymmetric extratropical and tropical thermal forcing

The mechanism is investigated by which extratropical thermal forcing with a finite zonal extent produces global impact. The goal is to understand the near-global response to a weakened Atlantic meridional overturning circulation suggested by paleoclimate data and modeling studies. An atmospheric model coupled to an aquaplanet slab mixed layer ocean, in which the unperturbed climate is zonally symmetric, is perturbed by prescribing cooling of the mixed layer in the Northern Hemisphere and heating of equal magnitude in the Southern Hemisphere, over some finite range of longitudes. In the case of heating/cooling confined to the extratropics, the zonally asymmetric forcing is homogenized by midlatitude westerlies and extratropical eddies before passing on to the tropics, inducing a zonally symmetric tropical response. In addition, the zonal mean responses vary little as the zonal extent of the forced region is changed, holding the zonal mean heating fixed, implying little impact of stationary eddies on the zonal mean. In contrast, when the heating/cooling is confined to the tropics, the zonally asymmetric forcing produces a highly localized response with slight westward extension, due to advection by mean easterly trade winds. Regardless of the forcing location, neither the spatial structure nor the zonal mean responses are strongly affected by wind–evaporation–sea surface temperature feedback.     

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.     

冬季北大西洋风暴轴的东西变化及其能量诊断

利用NCEP/NCAR再分析资料,定义一个风暴轴经度指数,基于这个指数做合成分析,对冬季北大西洋风暴轴63 a(1948-2010年)的东西变化特征及其能量平衡差异进行了诊断。主要结论如下:(1)北大西洋风暴轴存在明显地东扩和西退。当风暴轴向东扩展时,天气尺度瞬变波可以向下游发展至乌拉尔山以东的亚洲上空;风暴轴西退时,天气尺度瞬变波活动范围向西收缩到15°W以西的大洋上空。(2)能量诊断表明,当风暴轴向东扩展时,涡动动能在高纬度的大西洋东部及西欧上空明显增强。在0°以西的区域,涡动动能的增强主要归因于能量斜压转换过程的增强;而在0°以东区域,涡动动能的增强可能与涡动非地转位势通量引起的"下游发展效应"增强有关。风暴轴向西收缩时,变化相反。