对流层上层定常波异常遥相关型的维持机制

用ＥＣＭＷＦ１９８０－１９８８年的资料和主成份分析方法研究了北半球冬季定常波月平均异常的空间结构和时间变化，分析了对应主要异常型的由辐散风和瞬变涡度通量的异常引起的涡度源异常，并用正压模式研究了涡度源异常在这些异常型维持中的作用。结果表明，瞬变的涡度通量辐合的异常有抵消辐散风所产生的涡度源异常的趋势；单独考虑辐散风异常作为强迫所产生的环流异常与遥相关型的结构和强度相差很大，但如果将瞬变涡度通量辐合的异常也包含到强迫源中，则强迫出来的环流异常会有很大改善     

辐散风和瞬变的涡度通量对定常波的强迫

本文从理论分析、资料诊断和正压模式数值试验三个方面研究了瞬变的涡度通量的辐合与辐散风产生的涡度源之间的关系及它们在对流层上层定常波维持中的作用。结果表明， 瞬变涡度通量的辐合对辐散风产生的涡度源有抵消或耗散的趋势；定常波是在两种强迫力的共同作用下维持的。同时还研究了不同地区散度场对定常波的作用， 结果表明，中纬度东亚地区的辐合场对维持东亚大槽是重要的，并且这一辐合场与位于印度尼西亚地区的辐散场是维持西太平洋上副热带反气旋系统的主要强迫源。     

夏季沿亚洲副热带西风急流Rossby波传播及其与我国降水异常的联系

利用1960—2015年夏季(6—8月)NCEP 2.5°×2.5°全球逐日再分析资料,采用涡度源方程和Eliassen-Palm通量,对夏季沿亚洲副热带西风急流Rossby波活动的波源、能量传播及其与我国降水异常的关系进行了分析和研究。结果表明:夏季200 hPa大气准静止行星波产生的源地主要集中在地中海地区,亚洲副热带西风急流(ASWJ)中的准静止Rossby波在此激发并沿急流向东传播,东传过程中在急流轴南侧波流相互作用相对活跃。波作用通量的辐合辐散中心沿副热带西风急流交替分布,波流相互作用是ASWJ上西风强弱交替变化的动力机制。沿ASWJ交替分布的五个波作用通量辐合辐散关键区散度具有较强的关联性,表现为同一Rossby波列的不同部分,其中波源处Rossby波动能量的传播对其下游ASWJ的强弱影响最大,而急流关键区内纬向风的大小也与波作用通量散度场的强弱和分布密切相关。波源处以及位于我国青藏高原东部至黄土高原上空的波作用通量散度指数WFD-Ⅰ和WFC-Ⅱ与我国南、北方降水相关性较为显著,在WFD-Ⅰ为正异常年时,对应南方关键区降水偏多年份占比为62.5%,在WFD-Ⅰ为负异常年时,对应北方关键区降水偏多年份占比为80%;在WFC-Ⅱ为正异常年时,对应南方关键区降水偏少年份占比为66.7%,在WFC-Ⅱ为负异常年时,对应北方关键区降水偏少年份占比为81.8%。研究夏季波源处WFD-Ⅰ异常年导致我国降水异常的环流成因发现,WFD-Ⅰ为正异常年时,由上游波源地区激发的Rossby波向下游地区的能量频散偏强,位置偏南,波流相互作用导致我国南方上空高空急流加强,高低空辐散辐合配置加强,垂直上升运动增强,易造成我国南方地区降水异常偏多。WFD-Ⅰ为负异常年时,Rossby波能量的经向传播较强而纬向传播较弱,北方降水关键区受波通量辐散控制,高空西风急流加强,高低空辐散辐合配置和垂直上升运动增强,有利于我国北方地区降水的发展。     

正压大气中基态定常波对强迫响应的影响

本文利用正压涡度方程模式研究了热带不同经度位置异常强迫激发的中纬响应类型与北半球冬季基本气流定常波结构的关系。发现对热带印度洋和太平洋的大部分地区,中纬的响应类型主要由基态定常波的结构决定,并且基态定常波通过向扰动转换动能维持了中纬的响应。通过这些结果解释了GCM研究中发现的中综异常响应类型对El Nino成熟期海表面温度异常位置变化的不敏感性。此外还讨论了观测和GCM中出现的热带印度洋——太平洋地区低频尺度上的纬向偶极强迫源对GCM响应结果的作用。     

定常辐散场产生的涡源和定常波能量传播

根据ECMWF1980～1988年的全球资料，确定了南北两半球对流层上层定常的辐散风所形成的定常涡源的位置、定常波能量传播路径及它们的位置关系，讨论了涡源的形成机制。由此揭示了热带西太平洋积云对流加热、风暴路径上的潜热释放、欧亚大陆冷源及青藏高原热力和动力作用等主要外源在冬夏南北半球定常波形成中的作用。     

夏季沿西亚急流Rossby波活动异常的波源和能量传播及转换特征

用1958-2003年NCEP/NCAR再分析资料,利用涡度源方程、Eliassen-Palm通量(EP通量)和非绝热效应的波能方程,分析了夏季沿西亚急流Rossby波活动(WAJRA)异常的波源、能量传播和转换特征,从大气动力学内部机制上进一步认识WAJRA异常的成因,提高对中高纬大气环流异常机理的理解.研究表明,对流层高层位于地中海和北大西洋-斯堪的纳维亚半岛的负涡度源区和EP通量强辐散区为夏季WAJRA异常的波源区.当波源区位置和强度出现异常时,波源所激发东传的Rossby波活动也出现异常,从而导致WAJRA强弱变化.WAJRA强(弱)年冰岛-斯堪的那维亚半岛(斯堪的那维亚半岛以东)EP通量强辐散区激发Rossby波并沿2条路径向东传播,一支向东传播在乌拉尔山附近转向东南并在里海、咸海-新疆上空进入亚洲副热带西风急流传播增强(减弱),另一支直接向东南方向传播在地中海东部-黑海附近进入亚洲西风急流增强(减弱),此外,地中海上空EP通量辐散也增强(减弱),它们共同作用使得WAJRA增强(减弱).沿西亚地区副热带西风急流(简称西亚急流,指亚洲副热带西风急流的15°-60°E部分)非绝热加热产生扰动有效位能远大于基本气流动能向扰动动能的转换和基本气流有效位能向扰动有效位能的转换.西亚急流Rossby波活动强年(弱年)伊朗高原及其北侧的西亚地区非绝热加热产生的有效位能增强(减弱)显著,是WAJRA增强(减弱)的能量源.     

汉江流域暴雨的Eliassen-Palm通量特征及潜势预报

利用1980—2018年ERA-Interim全球再分析资料,对汉江上游汉中地区暴雨过程纬向风场及瞬变波活动Eliassen-Palm(下简称EP)通量特征进行诊断分析。结果表明:(1)尝试将瞬变波EP通量特征分析应用到汉江流域典型暴雨过程动力特征研究中,天气尺度瞬变波EP通量的辐散可以表征天气尺度扰动的强迫作用,根据热成风原理,角动量和热量的沉积耗散会进一步影响纬向环流的变化。大气中高层瞬变波活动特征可作为提前量反映汉江流域暴雨的发生。(2)暴雨发生前,30°N附近200 hPa有纬向风减速中心,对应200 hPa为EP通量辐散区,EP通量辐散区扩展加强有利于暴雨的发生,这种大气波动的垂直分布特征及其变化对暴雨发生有指示意义。     

初夏东北冷涡活动异常与北半球环流低频变化

本文在典型个例分析的基础上, 揭示了初夏东北冷涡活动显著异常与北半球环流低频变化之间的联系及其可能机理。主要结果为: (1) 初夏东北冷涡活动显著异常与上游乌拉尔山附近和下游日本附近的持续性异常环流相对应。冷涡异常多年, 在向下游频散Rossby波能量和瞬变涡动强迫过程的共同作用下, 乌拉尔山附近盛行阻塞型环流。它一方面使极地冷空气向东北地区移动, 另一方面通过Rossby波能量的传播, 使东北冷涡活动加强。冷涡异常少年的环流情况则基本相反。 (2) 谐波分析和低频振荡分析表明, 在冷涡异常多 (少) 年的5～6月, 东北地区正 (负) 涡度位相占优, 这与北太平洋超长波槽西退 (东进) 相对应。长波在北太平洋-北美-大西洋地区呈驻波型, 与Rossby波能量传播匹配, 亦有利于乌拉尔山附近的异常环流得以长时间维持。 (3) 5月份, 乌拉尔山附近的异常环流信号对初夏东北冷涡活动的显著异常具有前兆性意义。     

高频瞬变涡动反馈强迫对东亚/太平洋事件演变过程的作用

本文通过位势倾向方程研究了高频瞬变涡动反馈强迫(TEFF)对东亚/太平洋型在低频时间尺度上(8天以上)的演变过程(EAP事件)的影响。一个重要的发现是,无论正位相还是负位相的EAP事件,在对流层中低层上,TEFF对其中、高纬度两个异常中心的演变均起着重要的作用。尤其是高纬度中心,在对流层中层,TEFF异常在盛期前可解释其增强部分的50%左右;盛期之后则起着减缓其减弱的作用。TEFF异常可分解为由高频涡动热量通量的辐合辐散引起的TEFF_heat异常和高频涡动涡度通量引起的TEFF_vor异常。两者在对流层上层相互抵消、下层相互叠加,导致TEFF异常在对流层上层(低层)的作用要弱(强)于对流层中层。就TEFF_heat和TEFF_vor的相对贡献而言,正位相的EAP事件中,TEFF_vor的贡献较大;但在负位相EAP事件中,TEFF_heat的贡献则增大至与TEFF_vor相当的程度。这种TEFF_heat作用在负事件中的增强可能是与贝加尔湖以东地区对流层低层上斜压性的加强有关。     

季节内西太平洋副高异常进退的诊断研究

采用ＥＣＭＷＦ１９８０年和１９８３年７、８月份逐日２．５°×２．５°网格点资料对两次季节内西太平洋副高的异常进退进行了诊断研究。揭示出：季节内西太平洋副高异常进退是整个北太平洋副高异常进退的结果；这种异常进退表现为东太平洋副高的活动，相应西太平洋副高也有一次活动过程，并表现为北太平洋地区存在一纬向波列，低频波沿此波列西传以实现东、西太平洋副高异常进退的遥相关关系；而东太平洋刻高的异常进退则是被南亚季风区到太平洋信风区的异常加热造成的东太平洋对流层上部辐散风场汇合下沉区的变化所激发。     

THE FORCING OF ANOMALOUS DIVERGENT WIND AND TRANSIENT VORTICITY FLUXES TO THE ANOMALY OF STATIONARY WAVES

The forcing of anomalous divergent wind and transient vorticity fluxes to the anomaly ofstationary waves of DJF (December—January—February) 1982/1983 is studied.The results showthat the anomaly of the transient vorticity flux convergence tends to cancel or dissipate the anomalyof the vorticity sources generated by divergent wind.The stationary wave anomalies are maintainedby both of these forcings.It is also found that,for the DJF 1982/1983 case,both of the vorticitysource anomaly over the tropical and equatorial eastern Pacific and that over the midlatitude NorthPacific are important to maintain the atmospheric circulation anomaly over the Pacific/NorthAmerica region.     

MAINTENANCE MECHANISMS OF THE STATIONARY WAVE ANOMALY TELECONNECTION PATTERNS IN THE UPPER TROPOSPHERE

The spatial structure and temporal variations of the stationary wave anomalies in the uppertroposphere during the Northern Hemisphere winter are studied with the ECMWF data of 1980—1988 and PCA(principal component analysis)method.The vorticity source anomalies of the mainteleconnection patterns are analyzed,and their roles in the maintenance of the stationary waveanomaly teleconnection patterns are investigated with the harotropic model.The results show thatthe anomalies of transient vorticity flux convergences tend to cancel the anomaly of vorticitysources generated by divergent winds.The stationary wave anomaly patterns caused by divergentwind anomalies only are very different from observations.However,if the anomalous transientvorticity flux convergences are also included in the vorticity sources,the simulated patterns will begreatly improved.     

Variability patterns of Rossby wave source

Rossby waves (RW) propagation due to a local forcing is one of the mechanisms responsible for wave trains configurations known as teleconnections. The term teleconnection refers to anomalies patterns that are correlated in several regions of the world, causing large-scale changes in atmospheric waves patterns and temperature and precipitation regimes. The aim of teleconnections studies is to provide a better understanding of atmospheric variability and their mechanisms of action in order to identify patterns that can be tracked and predicted. The purpose of this study was to characterize seasonal and spatial variability of atmospheric RW sources. The RW source at 200?hPa was calculated for the four seasons with reanalysis data of zonal and meridional wind. In the Northern Hemisphere (NH), the RW sources were located on East Asia, North America, North Atlantic and Pacific. The main RW sources in the Southern Hemisphere (SH) were located over Intertropical, South Pacific, South Atlantic and South Indian Convergence Zones. Extratropical regions were also identified, mainly to the south of Australia. The vortex stretching term (S1) and the advection of absolute vorticity by the divergent wind (S2) were analyzed to discuss the physical mechanisms for RW generation. In the NH, the source at East Asia in DJF changed to a sink in JJA, related to the dominance of S1 term in DJF and S2 term in JJA. In the SH, the vortex stretching term had the dominant contribution for RW source located to the south of Australia. The main forcing for RW sources at east of Australia was the advection of absolute vorticity by divergent flow. Over South America, both terms contributed to the source in DJF. The main modes of RW source variability were discussed by using empirical orthogonal functions analysis. RW variability was characterized by wave trains configurations in both hemispheres over regions of jet streams and storm tracks, associated with favorable and unfavorable areas for RW generation.     

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.     

1982年夏季热带大气环流异常与ElNio事件发展机制的探讨

本文利用1980—1983年四个夏季(以7月平均为代表)的ECMRWF资料,分析了1982—1983年El Nino事件强烈发展前的1982年夏季热带大气环流的异常.从一系列显著异常的观测事实分析中得出:1982年夏季异常强的东亚高空东风急流是1982～1983年El Nino事件发展的一个可能促发机制;1982年澳大利亚冬季冷空气的异常堆积导致澳大利亚至东太平洋纬向热力对比的增强,是赤道太平洋偏东信风迅速减弱的一个重要原因.