东亚夏季风环流和雨带的季节内变化

基于常规气象要素资料及变差度方法, 分析了东亚夏季风环流的演变特征, 发现东亚地区在夏季期间存在两次明显的次季节突变, 主要表现为西太平洋副热带高压 (副高) 的两次东退北跳, 第一次是在6月中旬, 第二次是在7月下旬。由于副高与雨带密切相关, 雨带在演进过程中也呈现出两次明显的突跳, 分别对应于江淮流域至日本一带梅雨期以及中国华北和东北雨季的开始。较第一次北跳而言, 副高的第二次北跳更为明显。副高的第一次北跳主要受南海地区对流活动加强的影响, 而第二次北跳则是暖池对流活动与高纬地区环流共同作用的结果。暖池地区向东北方向传播的Rossby波列以及高纬地区东传的Rossby波通过锁相作用使得副高强烈北跳。此外, 副高与其西部边缘凝结潜热的相互作用导致副高发生季节内的低频振荡。 风场变差度的分析表明, 高纬地区对流层中低层环流的调整随着夏季季节进程逐渐减弱, 这与中高纬地区温差的变化有关。而高纬地区高层环流的调整在夏季后半期随着高度的增加却逐渐增强, 这与高层环流从夏到冬的季节变化有关。从风场相似度的变化上还可以看到, 副高第二次北跳后东亚地区呈现出明显不同的环流状态。 南半球环流对于南海及暖池地区对流活动的增强有重要影响。6月中旬, 南海与暖池地区对流活动的增强是由于南海西边界西风加强并向东扩展造成的, 这与马斯克林高压 (马高) 的加强密切相关。而在7月中旬, 澳大利亚高压 (澳高) 的增强使其东北部的越赤道气流加强, 南半球大量冷空气侵入到暖池地区, 加强了暖池地区的不稳定性以及低层的辐合, 从而使暖池地区的对流活动增强。但在夏季前半期, 暖池对流活动也可调制澳高强度与其东北部越赤道气流强弱的关系, 使得二者呈现出相反的变化趋势。南半球冬季期间, 澳高在振荡中减弱, 这与澳洲大陆下垫面温度及上游马高的能量频散有关, 前者影响澳高的变化趋势 (减弱), 而后者影响澳高的低频振荡。     

热带大气季节内振荡与对西北太平洋台风生成数的影响研究

用典型年合成方法分析研究了1979—2006年西北太平洋多台风年和少台风年热带大气ISO的影响。结果表明,热带大气ISO对西北太平洋台风的生成具有明显的调节作用。多台风年,菲律宾以西地区大气季节内振荡较弱,东传不明显;菲律宾以东地区积云对流较强,热带大气30～60 d低频振荡也偏强,与台风生成相关的传播特征为源自赤道140～160°E附近季节内振荡的西北方向传播。这种异常的30～60 d低频振荡对周围大气环流有正反馈的作用,从而导致积云对流的进一步加强,这种强的积云对流会引起赤道西风异常,产生异常Walker环流,在菲律宾附近形成风场的低层辐合、高层辐散,增强那里的上升气流,有利于台风生成。少台风年,菲律宾以西地区低频活动较强,东传明显,菲律宾以东的低频活动较弱。这种西强东弱的低频活动形式,使得大气低层在菲律宾以西地区为低频东风异常,菲律宾以东地区为低频西风异常,导致在菲律宾以东的西北太平洋大气产生低层低频风辐散、高层辐合的形势,增强下沉运动,不利于台风的生成。     

OLR 低频振荡与西太平洋台风的发生

以影响我国西太平洋台风生成区域平均的OLR演变曲线为基础，对1975—1981年经过30—60天滤波处理的OLR场进行合成分析。结果表明，与西太平洋台风生成有关的系统，主要集中于东半球60°E以东的低纬地区。西太平洋和孟加拉湾南部的赤道地区构成一对低频偶极中心，它们之间有密切的联系。赤道印度洋中部地区是OLR低频振荡的一个重要源区，台风多发、间歇和转换阶段都有明显的特点，对台风的生成和间歇的中期预报有一定的参考意义。特别是夏季形势建立后，在30°N附近副高的东西摆动（有明显的低频变化特征）与ITCZ的变化     

赤道地区向西传播的40天周期低频波

本文用滤波和EOF位相合成技术对1981年7—12月份赤道地区出现的向西传播的40天周期低频波进行了分析。结果认为东太平洋地区从南半球到北半球的越赤道40天周期温度波是产生这种西传波的主要原因。这种波动主要产生于两个源地:一个是赤道150°E附近的对流层下层;另一个是110°W的赤道对流层上层。这两处产生的低频波性质不一样,前者与对流密切相关。通过计算整层积分的非绝热加热Q_1和水汽汇Q_2,结果表明Q_1加热中心在东太平洋也有越赤道传播。在150°E以西Q_2加热中心是向西北传播的,与低频波方向一致,Q_1的传播特征不明显,这说明西太平洋地区的热带对流可能有这种周期振荡。     

南半球环流与西太平洋副热带高压和台风群中期活动的关系

通过对南北半球环流6年资料的分析,发现在北半球夏季5～8月,南半球中纬西风指数、低纬西风指数、赤道气压指标与北半球西太平洋副热带高压和台风群的中期活动均有较好的关系。在台风群活跃的年份,台风群生成阶段前后,环流变化由南半球中纬先开始,随后南半球低纬和赤道地区环流也出现变化,赤道气压指标到达低值,此后,北半球西太平洋副热带高压的特征产生一系列的变化,上述南北半球环流系统变化的传播过程为准二周周期。     

OLR资料所揭示的热带地区低频振荡的变化特征

本文使用1979年1月至1984年12月射出长波辐射(OLR)资料,对热带地区低频振荡的一些特性进行了研究,认为正常年份30—60天振荡的合成功率谱最强,El Nino年最弱。低频波活动冬夏差异较大,其年际变化大值区冬季在赤道地区,夏季位置偏北,位于印度洋和西太平洋。就六年平均而言,低频波在西太平洋及印度洋地区有明显的经向传播,赤道地区低频波的纬向传播主要集中在北半球夏季。此外,30—60天OLR滤波场的强弱与印度季风的爆发和减弱有较好的对应关系。      

我国九大气候区降水特性及其物理成因的研究Ⅱ——我国各区降水与环流因子的关系

利用ECMWF的格点风和位势高度场资料，对1998年夏季热带内外地区大气低频振荡特征进行了分析研究。结果表明，亚洲季风区的季节内振荡非常显著。850hPa季节内振荡纬向风的经向传播揭示出南海地区的西风振荡受南北两支西风的共同影响。与长江流域特大洪涝密切相关的大气环流系统(乌拉尔阻高和西太平洋副高)对季节内振荡有较强的响应。在欧亚范围，持续性异常环流十分活跃，大气低频波的活动与环流持续异常的形成有密切的联系，季节内振荡是高低纬环流持续异常同时发生的纽带。其低频(30—60d)遥相关型的分布同江淮洪涝年的波列结构相类似。     

越赤道气流准双周振荡对西北太平洋台风路径的调制作用

利用美国联合台风预警中心（Joint Typhoon Warning Center,JTWC）热带气旋（tropicalcy—clone,TC）数据、NCEP／NCAR再分析风场资料,研究了越赤道气流准双周振荡对西北太平洋台风路径的调制作用。将西北太平洋台风路径划分为：西行路径、西北行路径、转向登陆中国路径、转向中日之间路径、转向登陆日本路径、转向日本以东路径和140°E以东路径。利用超前滞后回归方法,合成分析了6—10月不同路径台风对应的越赤道气流准双周振荡的低频环流演变过程。结果表明,925hPa越赤道气流及与其相联系的经向风存在明显10～20d准双周振荡现象,且对西北太平洋台风路径预报具有一定的指示作用。在西太平洋赤道地区,低频越赤道气流强度、演变特征影响着西北太平洋低频气旋的位置和移动方向,调节风场强辐合带与季风槽的位置与强度,继而对台风生成位置、移动路径产生重要的影响。初步认为,强向北低频越赤道气流分量有利于北侧低频气旋加强和向北传播,继而使得强辐合带、季风槽位置偏北,台风易于在此区域生成且沿着强辐合带位置移动。而弱向北低频分量或向南低频分量则不利于台风转向移动。     

秋季中低纬低频振荡传播与长江中下游的连晴、连阴雨

对1983年～1985年9月～11月份850hpa的经纬向风场进行了15d～25d的带通滤波分析。结果表明,110°E经向风低频振荡波存在阶段性地从南半球(北半球)越过赤道向北(南)传播,低频越赤道气流存在阶段性地明显向东、向西传播,它们在5天平均低频风场中表现为低频气旋、反气旋环流的周期性振荡变化,并都与长江中下游地区的连晴、连阴雨天气的时间分布相联系。     

1986年东亚夏季风的低频振荡及其与西风带环流的相互作用

本文用最大熵谱方法和后延相关分析了1986年夏季风时期60—150°E高空越赤道气流的低频振荡特征,指出30—50天的低频振荡非常显著。其传播特点在低频波位相的持续阶段和转换阶段有所不同。经过带通滤波的高空越赤道气流,其主要经验正交函数EOFI的时间序列能较好地描写东亚夏季风活动。 东业季风分量和东半球500hPa位势高度的后延相关揭示出东亚夏季风同中纬度环流有着明显的低频相互作用,其相互作用的“窗口”是在东亚季风活跃阶段打开的,它同亚洲区域低指数环流和副热带急流的南北振荡相联系。     

Interactions between the 30–60 day oscillation,the walker circulation and the convective activities in the tropical western pacific and their relations to the interannual oscillation

In this paper, interactions between the 30-60 day oscillation, the Walker circulation and the convective activities in the tropical western Pacific during the Northern Hemisphere summer are analyzed by using the observed data of wind fields and high-cloud amounts for the period from 1980 to 1989.The analyzed results show that the 30-60 day oscillation (hereafter called LFO) may be largely affected by the convective activities in the tropical western Pacific. The LFO in the tropical western Pacific during the strong convective activities around the Philippines stronger than those during the weak convective activities around the Philippines. Moreover, in the case of strong convective activities around the Philippines, the LFO in the tropical west-ern Pacific and tropical eastern Indian Ocean generally propagates westward, and it is intensified by the LFO with a westward propagating center of maximum oscillation from the east to 140oE. However, in the case of weak convective activities around the Philippines, the LFO gradually becomes stronger with a eastward propagating center of maximum oscillation from the eastern Indian Ocean to the tropical western Pacific.Corresponding to the 30-60 day oscillation, the Walker circulation is also in oscillation over the tropical Pacific and its circulation cell seems to shift gradually westward from the tropical western Pacific to the tropical eastern In-dian Ocean with strong convective activities around the Philippines. This may maintain the intensification of convective activities there. However, during the weak convective activities around the Philippines, the Walker circula-tion gradually moves eastward and an ascending flow may appear in the equatorial central Pacific. This may cause convective activities to be intensified over the equatorial central Pacific.The analyzed results also show that the LFO in the tropical western Pacific and East Asia may be associated with the interannual oscillation of the SST anomaly in the tropical western Pacific.     

INTERANNUAL CHANGE OF EXTRA-TROPICAL METHANE INDUCED BY THE QUASI-BIENNIAL OSCILLATION OF TROPICAL WIND AND ITS FORMATION MECHANISM

Researchers have paid much attention to the influence of the tropical zonal wind quasi-biennial oscillation (QBO) on tropical methane, while generally ignoring the change in extra-tropical methane. The present study analyzed the interannual changes in the methane mixing ratio in extra-tropics of both the Southern Hemisphere (SH) and Northern Hemisphere (NH) using Halogen Occultation Experiment (HALOE) satellite data. The results show that interannual changes in extra-tropical methane exhibit QBO features in both hemispheres that are obviously different from those in the tropics. The extra-tropical methane QBO perturbations usually occur in two layers and are longitudinally asymmetrical about the equator. The amplitude of the methane QBO disturbance in the extra-tropics is smaller than that in the tropics from 10 to 1 hPa but much larger in the layer from 30 to 10 hPa. The interannual relative changes in the methane mixing ratio are similar in both the NH extra-tropics and the tropics in the middle and upper stratosphere. Using the National Center for Atmospheric Research two-dimensional, interactive chemical dynamical radiative model (SOCRATES), simulation was conducted to investigate the mechanism of the extra-tropical methane QBO. The results indicate that the tropical stratospheric zonal wind QBO results in the QBO of the induced residual circulation. It is the transport of methane by the induced residual circulation that causes the methane QBO in the extra-tropics. The induced residual circulations in the middle and upper stratosphere are not always longitudinally symmetrical about the equator, resulting in different distribution of the methane QBO in the SH and NH extra-tropics.     

广西前汛期降水年代际变化与南半球印度洋海温的关系

利用NCEP/NCAR月平均再分析资料对广西前汛期降水年代际变化的环流差异及其与前期南半球印度洋海温的关系进行研究,结果表明:广西前汛期整体一致变化的降水分布型具有20年左右年代际振荡及3年左右的年际周期,桂南、桂北反相变化的降水空间型具有6年和准两年振荡.在前汛期降水偏多期,欧亚大陆地表温度偏高,热力作用增强,造成大陆热低压偏强,海陆差异加大,广西区域气柱不稳定性增强,上升气流显著增强,Hadley环流减弱,西太平洋副高及南亚高压减弱,南北半球越赤道气流增强,高原南侧南支槽气流加强,水汽输送增多,造成广西降水偏多;降水偏少期形势相反.相关分析表明前期2～3月南半球中纬度印度洋海温与广西前汛期降水年代际变化呈明显负相关,意味着南半球海温对广西前汛期降水年代际变化有调控作用,这种作用是通过海温异常影响越赤道气流从而影响亚洲季风的强弱而实现的.     

大气环流的变化与南海夏季风活动的关系

通过对近20年观测资料的计算分析表明，在南海夏季风活跃与中断期，南北半球副热带西风急流和热带东风急流以及青藏高压和墨西哥高压有明显的差异。从南半球向北传播的散度场低频振荡以及从北半球高纬向南传播的西风场低频振荡等的有利位相同时传入南海并共同作用可引起南海夏季风的活跃或中断。各种低频振荡的有利位相传播到南海并发生锁相的时间决定着南海夏季风明显的活跃或中断发生的时间。     

VARIATIONS OF 30—60 DAY OSCILLATION IN ATMOSPHERE BEFORE AND DURING 1982 EL NINO*

By using the upper-wind data from July 1980 to June 1983,the variations of the low-frequency oscillation(LFO) in the atmosphere before and during 1982 El Nino have been investigated.Before the El Nino,the LFO propagates from west to east over the equator of the Eastern Hemisphere and from east to west over 20°N.The eastward propagating LFO over the equator consists of zonal wavenumber 1 propagating eastward and zonal wavenumber 2 with a character of stationary wave.The oscillation of zonal wavenumber 2 can modulate the oscillation strength.After the onset of the El Nino,the propagating directions of the LFO over the equator and 20°N of the Eastern Hemisphere change to be westward and eastward,respectively.The LFO over the western Pacific weakens rapidly and one coming from middle and high latitudes propagates to the equator.From the phase compositions of streamline fields for the zonal wavenumber 1 of equatorial westward propagatirg LFO,it is found that the atmospheric heat source in the equator of the eastern Pacific(EEP)excites a series of the equatorial cyclones and anticyclones which move northward and westward and form the westward propagating LFO over the equator.With the wavelength of 20000km,this kind of equatorial wave is similar to the mixing Rossby-gravity wave.In its westward and northward movement,the circulation in East Asia is modified.This may be the mechanism of the influence of El Nino on the climate of China.     

RELATION BETWEEN SUMMER TYPHOON FREQUENCY ANOMALIES IN WEST PACIFIC AND ENSO EVENTS AND THE ANOMALOUS ATMOSPHERIC CIRCULATION CHARACTERISTICS

By using data of serially numbered typhoons in northwestern Pacific and NOAA OLR data and NCEP/NCAR reanalysis data of wind field, based on the statistics and study of the relationship between the calendar years with more (or fewer) summer typhoons and ENSO events, we compared the composites of OLR eigenvectors and tropical summer wind fields during El Nino and La Nina events with more or fewer than normal summer typhoons, respectively. The results show that, in summer, without remarkable systematic anomalies of Mascarene High and Australia High in South Hemisphere, the anomaly of Walker circulation will dominate and follow the rule of ENSO impacts to atmospheric circulation and typhoon frequency. Otherwise, when systematic anomalies of Australia High appear during the El Nino events, circulation anomalies in the South Hemisphere will dominate, and many more typhoons will occur. In 1999, which is a special year of La Nina events, northward and eastward monsoon was induced by the stronger Mascarene High, and fewer typhoons arose. The typhoon source are regions where weak vertical wind shear, warm pool in western Pacific and the area with monsoon troughs are overlapping with each other. Finally, this paper analyzes and compares the source locations and ranges of more (fewer) typhoons in the events of El Nino and La Nina, respectively.     

青藏高原大气低频振荡的源、汇特征分析

使用1998年美国NCAR再分析的逐日资料，研究了1998年夏季青藏高原大气低频振荡的源、汇特征。青藏高原对于纬向风的低频波来说，有时是低频振荡的源区，有时是汇区。通过计算低频涡度的通量散度，进一步研究了低频涡源、涡汇特征。青藏高原在不同时间和不同部位，低频涡源、涡汇具有不同的分布特征，传播特征也不尽相同，青藏高原上空100hPa低频涡源、涡汇受到了南亚高压的很大影响。     

THE SOUTHERN HEMISPHERE MID-LATITUDE QUASI-40-DAY PERIODIC OSCILLATION WITH ITS EFFECT ON THE NORTHERN HEMISPHERE SUMMER MONSOON CIRCULATION

By using the method of power spectrum combined with band-pass filtering with May—September1982 grid data from ECMWF,the spatial structure and propagation characteristics are tentatively examinedof the SH (Southern Hemisphere) mid-latitude quasi-40-day (ranging from 30—60 days) periodic oscilla-tion (QPO) together with the relation to NH (Northern Hemisphere) summer monsoon,with the result thatthere exists similar periodicity in the development of baroclinicity and activities of cold air at the same latitudes,and in response to this the zonal wind shows profound QPO with a nearly vertical axis of disturbance and thatthe air activities can act as periodic external forcing for the monsoon,which intensifies the west wind on thesouth side of the Mascarene or Australian high,and then the system itself,leading to the reinforcement of theSE trade wind on the north side,followed by the strengthening of cross-equatorial flow that,in turn,causesactive monsoon with its northward march over the eastern part of China.The process is responsible for thelow-frequency oscillation propagated in a meridional direction,which confirms the speculation of theauthor.     

黄河中下游旱涝年的低频波振荡特征

采用低通滤波和功率谱分析的方法，分析了黄河中下游地区旱、涝年500 hPa高度场的30—60天周期的低频振荡。从1986年和1988年的分析结果看出:在旱年的夏季，乌拉尔山地区的低频振荡周期短、谱值高；贝加尔湖地区的低频振荡周期长、谱值高；副热带地区的低频振荡周期短、谱值低。相反，在涝年的夏季，乌拉尔山地区、贝加尔湖地区和副热带地区的低频振荡具有与旱年相反的特征。