东亚高空纬向风的季节内振荡对登陆中国大陆热带气旋活动的影响

使用1979—2015年欧洲长期天气预报中心所提供的ERA-Interim再分析资料和中国气象局上海台风研究所整编的西北太平洋热带气旋（TC）最佳路径资料,分析了7—8月东亚高空纬向风的季节内振荡（ISO）信号特征及其与登陆中国大陆热带气旋（TC）的关系。结果表明:（1）200 hPa纬向风在副热带、中纬度地区季节内振荡显著,尤其是在纬向西风带中,有两个南北分布的大值中心,方差贡献均超过50%。（2）基于东亚高空纬向风的ISO和EOF典型空间模定义的西风指数(EAWI),可以用来描述东亚高空纬向西风在ISO尺度上的经向移动。（3）在西风指数的ISO负位相期间,登陆中国东南沿岸22 °N以北的TC增多;反之减少。在西风急流出口南侧的副热带区域,200 hPa ISO纬向风向北移动,使纬向西风位置偏北,出现东风异常,从而使西风减弱;TC引导气流为向西的异常,有利于TC登陆中国大陆偏北沿岸;同时有异常的ISO纬向异常东风切变,有利于TC登陆过程的维持。（4）在西风指数的ISO负位相期间,在对流层高层西风急流出口区向南输送的天气尺度的E矢量,在TC登陆地区,出现异常扰动涡度通量的辐合,引起了该区域的西风减弱。      

Weakening of Interannual Variability in the Summer East Asian Upper-tropospheric Westerly Jet since the Mid-1990s

In this study,we found that the intensity of interannual variability in the summer upper-tropospheric zonal wind has significantly weakened over Northeast Asia and the subtropical western North Pacific(WNP) since the mid-1990s,concurrent with the previously documented decrease of the westerly jet over North China and Northwest China.Corresponding to this weakening of zonal wind variability,the meridional displacement of the East Asian westerly jet(EAJ) manifested as the leading mode of zonal wind variability over the WNP and East Asia(WNP-EA) before the mid-1990s but not afterward.The energetics of the anomalous pattern associated with the meridional displacement of the EAJ suggests that barotropic energy conversion,from basic flow to anomalous patterns,has led to the weakening of the variability in the EAJ meridional displacement and to a change in the leading dominant mode since the mid-1990s.The barotropic energy conversion efficiently maintained the anomalies associated with the variability in the EAJ meridional displacement during 1979-1993 but acted to dampen the anomalies during 1994-2008.A further investigation of the energetics suggests that the difference in the patterns of the circulation anomaly associated with either the first leading mode or the meridional displacement of the EAJ,i.e.,a southwest-northeast tilted pattern during 1979-1993 and a zonally oriented pattern during 1994-2008,has contributed greatly to the change in barotropic energy conversion.     

东亚高层纬向风季节内振荡与盛夏登陆中国大陆热带气旋频数年际变化的联系

利用1979-2017年欧洲中期天气预报中心提供的ERA-Interim再分析数据与中国气象局-上海台风研究所(China Meteorological Administration-Shanghai Typhoon Research Institute,CMA-STI)、美国联合台风警报中心(Joint Typhoo...     

东西风廓线特征与大气运动平衡态定常波的相关关系及共振条件的研究

本文采用简化数学模型探讨了东、西风廓线特征与外源强迫下大气环流平衡态定常波结构的相关关系。文中采用实际观测资料研究了西风廓线特征与大气环流型季节特征之间的联系。研究表明,西风廓线冬、夏季节差异与中、高纬度西风槽“冬三夏四”波数差气候特征有关,且低纬强东风切变可作为赤道东风波周期振荡成因之一。冬夏西风廓线季节特征可导致大地形强迫效应、海陆加热因子影响作用的显著季节差异。研究还揭示了大气环流型优势波转换的基流特征影响效应,并导出了流场与纬向加热强迫源共振状态的西风廓线特征函数与临界曲线。     

Intraseasonal modulation of tropical cyclogenesis in the western North Pacific: a case study

The temporal clustering of the western North Pacific tropical cyclogenesis and its modulation by the Madden–Julian oscillation (MJO) during the 1991 summer were examined based on the tropical cyclone best track, outgoing longwave radiation, and NCEP/NCAR reanalysis datasets. The wavelet analysis shows that convective activities around the monsoon trough in the western North Pacific possessed a distinct MJO with a period of 20–60 days. Two or more tropical cyclones were observed to form successively during each active phase of the MJO, and tropical cyclones tended to generate around the southeastern part of the maximum vorticity of the low-frequency cyclonic circulation during the developing and peak stages of the active MJO phase. But tropical cyclogenesis scarcely occurred during inactive MJO phases. Thus the MJO was a major agent in modulating repeated development of tropical cyclones in the western North Pacific during the 1991 summer. The MJO in circulation was characterized by a huge anomalous cyclone (anticyclone) in the lower troposphere existing alternately over the western North Pacific, leading to an enhanced (weakened) monsoon trough. An examination of the meridional gradient of absolute vorticity associated with the zonal flow indicates that the zonal flow in the monsoon trough region satisfied the necessary conditions for barotropic instability, with both zonal flow and the meridional gradient of absolute vorticity varying on the similar MJO timescale. The intraseasonal oscillation of such an unstable zonal flow might thus be an important mechanism for temporal clustering of tropical cyclogenesis in the western North Pacific. The barotropic conversion could provide a major energy source for the formation and growth of tropical cyclones in the western North Pacific during active MJO phases, with the eddy kinetic energy generation being dominated by both terms of eddies interacting with zonal and meridional gradients of the basic zonal flow.     

Re-examination of tropical cyclone formation in monsoon troughs over the western North Pacific

The monsoon trough(MT) is one of the large-scale patterns favorable for tropical cyclone(TC) formation over the western North Pacific(WNP). This study re-examines TC formation by treating the MT as a large-scale background for TC activity during May–October. Over an 11-year(2000–10) period, 8.3 TC formation events on average per year are identified to occur within MTs, accounting for 43.1% of the total TC formation events in the WNP basin. This percentage is much lower than those reported in previous studies. Further analysis indicates that TC formation events in monsoon gyres were included at least in some previous studies. The MT includes a monsoon confluence zone where westerlies meet easterlies and a monsoon shear line where the trade easterlies lie north of the monsoon westerlies. In this study, the large-scale flow pattern associated with TC formation in the MT is composited based on the reference point in the confluence zone where both the zonal and meridional wind components are zero with positive vorticity. While previous studies have found that many TCs form in the confluence zone, the composite analysis indicates that nearly all of the TCs formed in the shear region, since the shear region is associated with stronger low-level relative vorticity than the confluence zone. The prevailing easterly vertical shear of zonal wind and barotropic instability may also be conducive to TC formation in the shear region, through the development of synoptic-scale tropical disturbances in the MT that are necessary for TC formation.     

关于西风急流在强迫扰动中作用的数值试验

本文利用非线性水波方程谱模式模拟了不同基本气流对低纬和中纬地区涡源扰动的强迫响应，进而讨论西风急流在大尺度流场强迫扰动中的作用。试验分析表明，中纬度西风急流对来自低纬的扰动波列有增哟的阻 阻滞的作用，并能影响强迫扰动的波数和路径。在存在西风急流的情况下，基本气流对低纬扰源的响应比对中纬扰源的响应要强得多。     

Some Aspects of the Characteristics of Monsoon Disturbances Using a Combined Barotropic－Baroclinic Model

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快速西行进入南海台风的统计特征

通过对快速西行进入南海的 1 3个台风快速西行期间西风带环流系统、副热带高压活动和台风环境流场的特征以及这些台风进一步的移向、移速进行统计分析 ,结果表明 ,台风快速西行进南海期间西风带的特点是中纬从中亚到东亚有高压脊或以纬向环流为主 ,西风槽偏北 ;西北太平洋副热带高压一直强大且呈带状分布或不断加强与西伸 ,造成副热带高压南侧与台风之间梯度加大 ,使偏东引导气流加强。研究还发现 ,如果台风南北两侧均为偏东环境流场 ,将使台风西行更快。     

Quasi-40-day oscillation and its teleconnection structure together with the possible dependence on conversion of barotropic unstable energy of temporal mean flow

A study is made of the distribution of the diagnostic quantity vector E and the teleconnection structure of 30-50 (quasi-40) day oscillation, together with the dependence on the conversion of barotropic unstable energy of mean flow in terms of ECWMF daily 500 hPa grid data in winter, indicating that the energy transportation is closely associated with the westerly jet position, with zonal (meridional) propagation in the strong (weak) wind region, that considerable conversion of barotropic energy occurs at the jet exit region where low-frequency oscillation gains energy from the mean flow, leading to maximum kinetic energy for the oscillation observed there, which is marked by evident barotropy in striking contrast to the baroclinicity at low latitudes and that the teleconnection core is related to the center of action in the atmosphere and bound up with the pattern of the west wind.     

Maintenance and oscillation mechanisms of summer tropical upper-tropospheric easterlies

The mechanisms of the maintenance and oscillation of 1982 summer tropical 200-hPa mean easterly flow and extra-long waves are investigated in terms of the energy equations in wavenumber-frequency space. Calculation results show that the difference in heating between land and sea and the boundary effect serve as the main source of energy; frictional dissipation as the sink; the conversion of available potential energy into kinetic takes place dominantly in the waves of number 1–2 such transformation is accomplished in just a small amount in zonal mean flow and therefore can be ignored because of the value. In the interaction between wave and zonal mean flow, the latter loses its available potential and gains kinetic energy. The tropical easterly belt over 20°N-5°S is found barotropically stable and that over 10°-5°S, unstable. The waves of number 2 and 1 manifest themselves a primary source and sink of kinetic energy, respectively, in the interplay between waves and between zonal mean flow and wave. It is found that zonal mean flow and the waves of number 1-2 have a roughly 40-and 20-day oscillational period of kinetic energy, respectively, whose primary mechanism is the transfer of barotropic energy, the conversion of baroclinic energy, and the boundary effect.     

MAINTENANCE AND OSCILLATION MECHANISMS OF SUM-MER TROPICAL UPPER-TROPOSPHERIC EASTERLIES

The mechanisms of the maintenance and oscillation of 1982 summer tropical 200-hPa mean easterly flow and extra-long waves are investigated in terms of the energy equations in wavenumber-frequency space. Calculation results show that the difference in heating between land and sea and the boundary effect serve as the main source of energy; frictional dissipation as the sink; the conversion of available potential energy into kinetic takes place dominantly in the waves of number 1-2; such transformation is accomplished in just a small amount in zonal mean flow and therefore can be ignored because of the value.In the interaction between wave and zonal mean flow, the latter loses its available potential and gains kinetic energy. The tropical easterly belt over 20oN-5oS is found barotropically stable and that over 10oN-5oS, unstable. The waves of number 2 and 1 manifest themselves a primary source and sink of kinetic energy, respectively, in the interplay between waves and between zonal mean flow and wave.It is found that zonal mean flow and the waves of number 1-2 have a roughly 40-and 20-day oscillational period of kinetic energy, respectively, whose primary mechanism is the transfer of barotropic energy, the conversion of baroclinic energy, and the boundary effect.     

热带基本气流对冬季北半球中高纬度准定常行星波的影响

本文利用一个包括Rayleigh摩擦、Newton冷却及水平涡旋热力扩散的准地转34层球坐标模式来研究冬季北半球地形与热源强迫所产生的准定常行星波与热带基本气流的关系。 计算结果表明,冬季热带平流层基本气流是西风时,其北半球中高纬度平流层波数2准定常行星波的振幅偏大;而当冬季热带平流层基本气流是东风时,其北半球中高纬度平流层波数2准定常行星波的振幅偏小,这与实际结果比较一致。 计算结果还表明冬季热带对流层基本气流对中高纬度准定常波的影响要比平流层基本气流的影响大。     

INFLUENCE OF THE BASIC FLOW IN THE TROPICS ON THE STATIONARY PLANETARY WAVES AT MIDDLE AND HIGH LATITUDES DURING THE NORTHERN HEMISPHERE WINTER

The relationship between the quasi-stationary planetary waves forced by topography and heat sourceduring the Northern Hemisphere winter is investigated by means of a quasi-geostrophic,34-level,sphericalcoordinate model with the Rayleigh friction,the Newtonian cooling and the horizontal eddy thermal diffu-sion.The calculated results show that when the basic flow is the westerly in the tropical stratosphere,theamplitude of quasi-stationary planetary wave for zonal wavenumber 2 at middle and high latitudes is largerduring the Northern Hemispheric winter;while when the basic flow is the easterly,it is smaller.This is inagreement with the observed results.The calculated results also show that influence of the basic flow in the tropical troposphere on the quasi-stationary planetary waves is larger than that of the basic flow in the tropical stratosphere on the quasi-stationary planetary waves.     

热带东、西风界面附近的扰动流场

在正压无辐散的条件下,讨论了东、西风不连续面上的正压不稳定性,得出:在这种情形下,东、西风界面上的扰动总是不稳定的。文中用小参数的方法,对界面附近扰动发展的非线性问题进行了计算,算到二级近似。结果表明,非线性过程引进短波长的谐波。界面的变形和扰动随时间增长较快,经过0.5天,界面有很小的变形,附近只有很弱的闭合环流;到1.5天,界面变形显著,环流范围增长达300公里,而且短波占主要部分,界面北侧东风气流中的气旋式环流进一步发展,风速也显著增强。     

Modulation of Western North Pacific Tropical Cyclone Genesis by Intraseasonal Oscillation of the ITCZ: A Statistical Analysis

The present study investigates modulation of western North Pacific(WNP) tropical cyclone(TC) genesis in relation to different phases of the intraseasonal oscillation(ISO) of ITCZ convection during May to October in the period 1979-2008.The phases of the ITCZ ISO were determined based on 30-80-day filtered OLR anomalies averaged over the region(5-20 N,120-150 E).The number of TCs during the active phases was nearly three times more than during the inactive phases.The active(inactive) phases of ISO were characterized by low-level cyclonic(anticyclonic) circulation anomalies,higher(lower) midlevel relative humidity anomalies,and larger(smaller) vertical gradient anomalies of relative vorticity associated with enhanced(weakened) ITCZ convection anomalies.During the active phases,TCs tended to form in the center of the ITCZ region.Barotropic conversion from the low-level mean flow is suggested to be the major energy source for TC formation.The energy conversion mainly depended on the zonal and meridional gradients of the zonal flow during the active phases.However,barotropic conversion weakened greatly during the inactive phases.The relationship between the meridional gradient of absolute vorticity and low-level zonal flow indicates that the sign of the absolute vorticity gradient tends to be reversed during the two phases,whereas the same sign between zonal flow and the absolute vorticity gradient is more easily satisfied in the active phases.Thus,the barotropic instability of low-level zonal flow might be an important mechanism for TC formation over the WNP during the active phases of ISO.     

Modulation of low-latitude west wind on abnormal track and intensity of tropical cyclone Nargis (2008) in the Bay of Bengal

Tropical cyclone (TC) Nargis (2008) made landfall in Myanmar on 02 May 2008, bringing a storm surge, major flooding, and resulting in a significant death toll. TC Nargis (2008) displayed abnormal features, including rare eastward motion in its late stage, rapid intensification before landing. Using reanalysis data and a numerical model, we investigated how a low-latitude westerly wind modulated TC Nargis’ (2008) track and provided favorable atmospheric conditions for its rapid intensification. More importantly, we found a possible counterbalance effect of flows from the two hemispheres on the TC track in the Bay of Bengal. Our analysis indicates that a strong westerly wind burst across the Bay of Bengal, resulting in TC Nargis’ (2008) eastward movement after its recurvature. This sudden enhancement of westerly wind was mainly due to the rapidly intensified mid-level cross-equatorial flow. Our results show that a high-pressure system in the Southern Hemisphere induced this strong, mid-level, cross-equatorial flow. During the rapid intensification period of TC Nargis (2008), this strong and broad westerly wind also transported a large amount of water vapor to TC Nargis (2008). Sufficient water vapor gave rise to continuously high and increased mid-level relative humidity, which was favorable to TC Nargis’ (2008) intensification. Condensation of water vapor increased the energy supply, which eventuated the intensification of TC Nargis (2008) to a category 4 on the Saffir-Simpson scale.     

亚洲西风急流纬向非均匀性变化成因及其对东亚夏季气候的影响

利用美国NOAA（National Oceanic and Atmospheric Administration）的CMAP（Climate Prediction Center (CPC) Merged Analysis of Precipitation）月平均降水资料、NCEP/DOE（National Centers for Environmental Prediction/Design of Experiments）II的月平均再分析资料和中国气象局国家信息中心提供的中国160站逐月降水和平均气温资料,通过定义一个亚洲急流纬向非均匀性指数（I_Aja）,分析了1979～2019年夏季亚洲西风急流纬向非均匀性的年际变化特征,揭示了夏季亚洲急流纬向非均匀性变化异常的成因及其对东亚夏季降水和气温的影响。结果表明:夏季亚洲西风急流纬向非均匀性具有显著的年际变化特征,并存在6～8年和2年左右的振荡周期。当急流纬向非均匀性典型偏强（弱）年,东亚东部地区从低纬到高纬,降水异常主要呈现出偏多—偏少—偏多（偏少—偏多—偏少）的经向分布;气温则在中国西部地区和日本北部偏高（低）,贝加尔湖地区偏低（高）。引起夏季亚洲急流纬向非均匀性异常的可能原因如下:由大气非绝热加热异常而引起的热带和中纬度地区辐合/辐散运动造成的涡度源强迫,和来自西风带中波扰动能量的注入,两者共同作用形成并维持了与急流纬向非均匀性强弱变化相联系的异常环流,从而使亚洲急流东、西段强度差异增强（减弱）,进而有利于急流纬向非均匀性异常偏强（偏弱）。而上述西风带中波扰动能量的东传可能与北大西洋海表面温度异常有关。这对于深刻理解夏季亚洲急流纬向非均匀性异常的形成机理提供了有用的线索。     

Caribbean hurricanes: case study of interacting easterly and westerly waves

This observational study considers Caribbean cyclogenesis in the period 2003?C2009. Numerous events are identified from maximum of low-level relative vorticity and rain rate, and a case study is analyzed. Although fast moving tropical cyclones (TC) pose dangers to Caribbean Islands, it is the slower moving TC that inflict flood damage. The Atlantic warm pool enlarges through October as steering winds slacken. African easterly waves move over the warm pool and draw moist unstable air, while near-equatorial Kelvin waves from the Pacific surge into the Caribbean. The westerly flow accelerates around the northern Andes and is drawn into TC Omar 13?C15 October 2008. A combination of warm pool air and cyclonic vorticity provided by transient zonal waves sets off the process of cyclogenesis.     

一种对资源不稳定性敏感的EASY-backfill算法

利用合成技术对1995—2006年冬季（11月—次年2月）生成在西北太平洋上的34个热带气旋（tropicalcyclone,TC）个例进行分析,研究冬季西北太平洋TC生成的大尺度环流特征及其生成机制,结果表明：冬季TC生成的大尺度环流特征型为东风波西传型;北半球冬季对流层低层出现的跨赤道气旋对是冬季北半球TC形成的重要特征;太平洋中部赤道混合Rossby重力波西北传,与强对流中心重合,性质转为＂热带低压型扰动＂,为冬季热带气旋生成提供扰动源。对合成TC初始场的涡动扰动动能的收支分析表明,涡动有效位能和正压不稳定转换为TC形成提供了能量,这两种能量分别与积云对流加热和水平不均匀气流有关。正压不稳定能量转换为动能主要位于对流层中下层,而扰动有效位能的转换主要位于对流层中上层。低层热带东风波动从平均气流中获得正压不稳定能量,并与强积云对流耦合,热力和动力共同作用下形成TC。