Impacts of 30-60-Day Oscillations over the Subtropical Pacific on the East Asian Summer Rainfall

The relationships between the precipitation over East Asia (20°-45°N,110°-135°E) and the 30-60-day intraseasonal oscillation (ISO) over the Pacific during the boreal summer are studied in the paper.The daily wind and height fields of NCEP/NCAR reanalysis data,the 24-h precipitation data of 687 stations in China during 1958-2000,and the pentad precipitation of CMAP/NOAA from 1979 to 2002 are all analyzed by the space-time filter method.The analysis results,from every drought and flood summer in four different regions of East Asia respectively during 1958-2000,have shown that the flood (drought) in the East Asian summer monsoon region is absolutely companied with the strongly (weakly) westward propagations of ISO from the central-east Pacific,and depends little on the intensity changes of the East Asian summer monsoon. And the westward ISO is usually the low-frequency cyclones and anticyclones from the Bay of Alaska in northeastern Pacific and the Okhotsk in the northwestern Pacific of mid-high latitudes,and the ISO evolving in subtropical easterlies.In mid-high latitudes the phenomena are related to the westward propagating mid- ocean trough and the retreat of blocking high.Therefore the westward propagating ISO from the central-east Pacific to East Asia is indispensable for more rainfall occurring in East Asia in summer,which results from the long-wave adjustment process in the mid-high latitudes and ISO evolving in tropical easterlies.     

北半球副热带—中纬度太平洋大气季节内扰动的纬向传播与东亚夏季旱涝

用时空滤波和Morlet小波方法,分析了1958—2000年夏季东亚(20°—45°N,110°—135°E)不同纬带(由南到北分为4个区域)的降水分别与太平洋同一纬带上大气30—60 d振荡(ISO)沿纬圈传播的关系及其成因机制。发现太平洋上经向风ISO向西传播的强或弱,是东亚夏季风区降水偏多或偏少的必要条件。对逐年夏季的分析表明,无论当年东亚夏季风强与否,在所划分的几个东亚季风区所有涝的年份里,太平洋同一纬带上大气ISO向西传播都明显较强,而在这些区域绝大多数旱的年份里,相应的ISO向西传播明显较弱。进一步分析发现,经向风ISO的纬向传播对应着大气经向型环流系统的移动,向西传影响东亚夏季风区降水的ISO有来自低纬中东太平洋东风流中的低频气旋(如副热带东风带中ISO的演变);也有来自中高纬度阿拉斯加湾及鄂霍次克海一带低频低压(如洋中槽)和高压(如阻塞高压和东北太平洋高压)的向南向西频散。因此东亚夏季旱涝不但与热带季风有关,而且与中东太平洋副热带东风系统中ISO的向西传播、中高纬度长波调整时低频扰动向西南经北太平洋副热带的传播密切相关。     

亚洲-西太平洋夏半年季节内振荡：热带气旋活动强、弱年的对比

应用有限区域波数-功率谱和滞后线性回归统计方法，对比分析1979—2004年热带气旋（TC）活动强、弱年亚洲-西太平洋区域（AWP）夏半年大气季节内振荡（ISO）的传播特征及海气相互作用过程。分析结果表明：在整个AWP和西太平洋区域都存在TC强年ISO东传特征更显著的特征，ISO对流传播在纬向上伸展更偏东，强年沿赤道东传20～60 d周期波动更强，西太平洋区域是传播特征强弱年区别较显著的区域，与TC活动密切联系；TC强、弱年ISO过程中海气相互作用过程相同，海洋和大气间的能量交换对维持ISO西北传播时起作用；TC强年的海气相互作用比弱年更加强烈，尤其是对流凝结潜热的释放在强年比弱年强，这可能是导致TC强、弱年ISO特征区别的原因之一。     

南亚高压季节内变化与热带季节内振荡之间的关系

采用观测分析和数值试验等方法,分析夏季南亚高压与热带季节内振荡(ISO)之间的关系,并对两者之间的相互作用进行量化诊断,探讨其物理过程.主要结果表明:南亚高压ISO与热带ISO活动关系密切,当热带ISO处于印度洋位相(第1、2、3位相),则南亚高压东脊点位置偏西,当ISO处于太平洋位相(第5、6、7位相),则南亚高压东...     

强弱南海夏季风活动及大气季节内振荡

应用ＮＣＥＰ再分析资料和中国降水资料，分析研究了对应南海强、弱夏季风的环流形势及其与之相应的中国东部的降水异常。其结果表明，由强、弱夏季风所引起的中国气候异常是完全不同（甚至反相）的。分析大气季节内振荡（ＩＳＯ）的活动还表明，对应大气强（弱）南海夏季风，南海地区 ８５０ ｈＰａ也有强（弱）大气 ＩＳＯ；而强、弱南海夏季风环流（２００ ｈＰａ和 ８５０ ｈＰａ）主要由异常的大气ＩＳＯ所激发。本研究还揭示了南海地区大气ＩＳＯ的变化往往与江淮地区大气ＩＳＯ的变化反相，例如南海地区的强（弱）大气ＩＳＯ常与江淮流域的弱（强）大气ＩＳＯ相对应。对于大气ＩＳＯ的强度，一般多表现出局地激发特征，经向传播相对较弱。     

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。      

A STUDY ON RESPONSE OF PRECIPITATION IN CHINA TO MONSOON INTRASEASONAL OSCILLATION

Temporal and spatial evolution characteristics of the 30-60 day oscillation (intraseasonal oscillation, ISO) of summer rainfall in China and the effects of East Asian monsoon on the rainfall ISO are analyzed in this paper. Results show that the annual and decadal variations of the oscillation exist between 1960 and 2008, and the intensity is weakest in the late 1970s and early 1980s. In the typical strong years of the rainfall ISO obtained from empirical orthogonal functions (EOF mode 1), an anticyclone is in northwestern Pacific and a cyclone is in the east of China. In the typical weak years, the wind ISO is much weaker. The low-frequency zonal wind and water vapor transport from the low latitudes to mid-latitudes in the typical strong years, and the oscillation strength of diabatic heating is much stronger than that in the weak years of the rainfall ISO. The anomaly characteristics of the rainfall ISO show anti-phases between the Yangtze River basin and south of China. As for the typical strong years of the rainfall ISO in the Yangtze River basin (EOF mode 2), the main oscillation center of water vapor is in the east of China (20-30°N, 110-130°E). In the peak (break) phase of the rainfall oscillation, a low-frequency cyclone (anticyclone) is in the Yangtze River basin and an anticyclone (cyclone) is near Taiwan Island. In addition, the peak rainfall corresponds to the heat source in the Yangtze River basin and the heat sink in the Qinghai-Tibet Plateau. As for the typical strong years of the rainfall ISO in the south of China, the main oscillation center of water vapor is south of 20°N. In the peak (break) phase of the rainfall ISO, a low-frequency cyclone (anticyclone) is in the south of China and an anticyclone (cyclone) is in the Philippines. The peak rainfall corresponds to the heat source in the south of China and the South China Sea, and the heat sink in the west of Indochina.     

The impact of ocean–atmosphere coupling on the predictability of boreal summer intraseasonal oscillation

The impact of ocean–atmosphere coupling on the simulation and prediction of the boreal summer intraseasonal oscillation (ISO) has been investigated by diagnosing 22-year retrospective forecasts using the Seoul National University coupled general circulation model (CGCM) and its atmospheric GCM (AGCM) forced with SSTs derived from the CGCM. Numerous studies have shown that the ocean–atmosphere coupling has a significant effect on the improvement of ISO simulation and prediction. Contrary to previous studies, this study shows similar results between CGCM and AGCM, not only in regard to the ISO simulation characteristics but also the predictability. The similarities between CGCM and AGCM include (1) the ISO intensity over the entire Asian-monsoon region; (2) the spatiotemporal evolution of the northward propagating ISO (NPISO); and (3) the potential and practical predictability. A notable difference between CGCM and AGCM is the phase relationship between precipitation and SST anomalies. The CGCM and observation exhibits a near-quadrature relationship between precipitation and SST, with the former lagging about two pentads. The AGCM shows a less realistic phase relationship. The similar structure and propagation characteristics of ISO between the CGCM and AGCM suggest that the internal atmospheric dynamics could be more essential to the ISO than the ocean–atmosphere interaction over the Indian monsoon region.     

热带大气季节内振荡的异常与1997年El Nio 事件的发生

利用美国NCEP资料分析热带大气季节内振荡（ISO）与ENSO的关系，揭示了1997年El Ni o事件的发生，与赤道中西太平洋地区大气 ISO在 1996年冬到 1997年春期间的异常增强有重要关系。而且该地区 ISO的异常增强并不主要由赤道印度洋地区移来，主要表现为ISO在印度尼西亚上空被激发增强和东传。印度尼西亚地区ISO的被激发，则是由于东亚冬季风强异常在该地区引起的强对流活动的结果。     

中国东南部与川渝陕甘地区夏季降水 30～60天CISO的反相现象

利用1979～2010年NCEP/NCAR逐日再分析资料和我国753站逐日降水资料,研究了中国东南部和川渝陕甘地区夏季降水30～60天气候季节内振荡（Climatological IntraSeasonal Oscillation,CISO）的反相现象。结果显示两个区域夏季降水CISO在7月中旬之前存在显著的反相关系,且在传播特征上有明显差异。川渝陕甘地区降水局地低频振荡显著,中国东南部降水CISO北传特征清晰。低频环流分析表明,对流层低层低频槽、脊的更替形成了川渝陕甘地区降水CISO的“驻相”振荡,低频气旋式、反气旋式环流位置的移动则产生了中国东南部降水CISO的北传。CISO垂直环流显示上升支与下沉支正好处于川渝陕甘地区与中国东南部,较好地反映了低频降水在两区域的相反变化。中国东南部、长江中下游与川渝陕甘地区三者之间存在动力学上的关联,这可部分地解释川渝陕甘地区和中国东南部之间降水CISO的反相关系。这种反相关系的形成还与气候态下的西太平洋副热带高压和贝加尔湖附近大陆高压脊的季节内变化有关。揭示中国东南部与川渝陕甘地区之间降水CISO的反相现象对于丰富人们对我国夏季风区低频振荡局地特征的认识和理解降水季节内振荡产生机制有重要意义。     

大气季节内振荡的研究进展及其在延伸期预报中的应用

回顾和总结了大气季节内振荡(ISO)的主要研究成果,尤其是新近的研究成果,主要包括大气ISO 的基本特征和活动规律、与ENSO 的相互作用、动力学机制及其对我国天气和气候的影响;重点讨论了季节内振荡在延伸期预报中的可能应用,并从季节内振荡的角度提出了制作延伸期预报的思路和流程.     

INTRASEASONAL OSCILLATIONS IN ASIA TO WESTERN PACIFIC REGION IN BOREAL SUMMER: CONTRASTIVE ANALYSIS FOR ACTIVE AND INACTIVE YEARS OF TROPICAL CYCLONES

Comparative analysis is carried out by using finite-domain power spectrum and lagged regression methods for the propagating characteristics and air-sea interaction processes of intraseasonal oscillations (ISOs) in the Asia to western Pacific (AWP) region during the boreal summer between the active and inactive tropical cyclone (TC) years from 1979 to 2004.The results show as follows.(1) There exist more significant eastward propagating characteristics of the ISO in the active TC years over the whole AWP region.The ISOs of convection propagate zonally with more eastward extension in the years with active tropical cyclone activities,during which the 20-60-day period is strengthened,western Pacific becomes an area with evident characteristics of the propagation that is closely related to TC activities.(2) The air-sea interaction processes are the same in both active and inactive TC years,and the energy exchanges between the air and the sea play a role in maintaining the northwestward propagation of ISOs.(3) The air-sea interaction is more intensive in the active TC years than in the inactive ones.It is particularly true for the latent heat release by condensation as the result of convection,which may be one of the reasons resulting in significant differences in characteristics of ISOs between the active and inactive TC years.     

2007 年南海夏季风季节内振荡的北传及影响因子

利用2007年全球降水气候计划GPCP(the Global Precipitation Climatology Project)卫星红外窗口导出的全球降水指数GPI(the Global Precipitation Index)的日降水资料及频率-波数分析方法,分析2007年南海夏季风季节内振荡(Intraseasonal Oscillation,ISO)的传播特征,并使用美国国家环境预报中心(NCEP)/美国大气研究中心(NCAR)再分析的逐日资料,探讨影响其传播的主要因子.结果表明,南海夏季风ISO有明显的北传趋势,并且明显比南传分量占优.影响南海夏季风ISO北传的主要因子是平均纬向风垂直切变和平均经向风对异常水汽的输送.之所以异常经向风对平均水汽的输送及海-气相可作用的影响在南海地区不重要,而在印度季风区有一定的贡献,是因为平均水汽和纬向风分布在两个地区的差异.     

THE EXCITING MECHANISM OF TROPICAL INTRASEASONAL OSCILLATION TO EL NINO EVENT

The data analyses indicated that the occurrence of EL Nino event is closely related to intraseasonal oscillation (ISO) in the tropical atmosphere: The intraseasonal oscillation is very strong in tile tropics (particularly over the equatorial western Pacific) prior to the occurrence of El Nino; But the ISO is evidently reduced and the quasistationary system is enhanced after the outbreak of El Nino. A simple air-sea coupled model study shows that the periodical self-excited oscillation can be produced in the air-sea-coupled system, but the pattern is different from the observed ENSO mode. When there is external (atmospheric) forcing with interannual time scale, a coupled mode, which looks like the ENSO mode, will be excited in the air-sea system. Synthesizing the results in data analyses and the theoretical investigation. the mechanism of ISO in the tropical atmosphere exciting the El Nino event can be suggested : The interannual anomalies (variations) of the tropical ISO play an important role in the exciting El Nino event through the air-sea interaction.     

Relationship between summer rainfall anomalies and sub-seasonal oscillation intensity in the ChangJiang Valley in China

Sub-seasonal variability of summer (May–October) rainfall over the ChangJiang Valley exhibits two dominant timescales, one with a quasi-biweekly (QBW) period (10–20 days) and the other with an intraseasonal oscillation (ISO) period (20–60 days). A significant positive correlation (at a 99% confidence level) was found between the summer precipitation anomaly and the intensity of the QBW and ISO modes in the region. By examining the composite structure and evolution characteristics, we note that the QBW mode is characterized by a northwest–southeast oriented wave train pattern, moving southeastward. The perturbations associated with the ISO mode propagate northwestward in strong ISO years but southeastward in weak ISO years. A marked feature is the phase leading of low-level moisture to convection in both the QBW and ISO mode. When the summer rainfall is strong in the ChangJiang Valley, large-scale atmospheric conditions in the strong QBW/ISO activity region are characterized by deeper moist layer, convectively more unstable stratification and greater ascending motion. Such mean conditions favor the growth of the QBW and ISO perturbations. Thus, a significant positive correlation between the summer precipitation and the strength of sub-seasonal variability arises from the large-scale control of the summer mean flow to perturbations.     

亚洲夏季风30～60天季节内振荡对中国东部地区持续性极端降水的影响

本文基于1979～2015年中国台站观测的格点化高分辨率降水和NCEP II大气再分析逐日资料,探讨了亚洲季风区夏季30～60天大气季节内振荡（ISO）与长江中下游地区持续性降水异常的关系,重点揭示了南亚和东亚子季风区ISO的相互作用及二者协同引起长江中下游持续性极端降水的物理机制。合成分析表明,南亚和东亚ISO是通过高层辐散环流发生相互作用。在ISO位相1～3（5～7）,异常活跃（抑制）对流从赤道印度洋北传至孟加拉湾—印度次大陆区域,其伴随的高层异常辐散（辐合）环流通过补偿效应,引起南海—热带西北太平洋的异常高层辐合（辐散）,加强了局地的异常下沉（上升）运动,有利于南海—西北太平洋的异常抑制（活跃）对流发展并维持。南海—西北太平洋的异常抑制（活跃）对流伴随着显著的斜压散度,并进一步激发出一个连接南海和长江中下游的经向垂直环流圈,引起长江中下游强烈的异常上升（下沉）运动和低层水汽辐合（辐散）,使得降水持续性偏多（少）,极端降水的发生概率持续地偏高（低）,有利于（不利于）形成持续性极端降水事件。研究还表明,亚洲季风区ISO的强度存在显著的年际变化,并对长江中下游持续性极端降水的发生频次和持续时间具有调制作用。在ISO偏强（弱）年,长江中下游持续性极端降水的发生频次较高（低）,且持续时间较长（短）。     

对流凝结加热的垂直分布与低纬大气的30～60天低频振荡

运用一个包含Wave-CISK机制的斜压半地转8层模式和本征函数展开方法，研究了三种不同的对流凝结加热廓线对低纬大气的30—60天低频振荡的影响。研究表明，不同的加热廓线分布时30～60天低频振荡具有不同的相速和周期，并且低频振荡特征相速的量级都是o(10m／s)，由Wave-CISK机制激发的低频CISK—Kelvin波和CISK—Rossby波都是稳定的。同时，还进一步揭示了不同加热廓线对低纬大气30～60天低频振荡垂直结构的影响。     

谱模式SAMIL对南亚季风区大气季节内振荡向北传播的模拟

基于与NCEP资料结果的比较,研究了中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室发展的大气环流模式SAMIL对夏季南亚季风区季节内振荡（ISO）向北传播特征的模拟,并结合目前对ISO北传机制的理解对模拟结果进行了分析讨论。SAMIL在夏季南亚地区模拟出相当强度的季节内振荡的活动,并且模拟的ISO也表现出与NCEP资料相似的从赤道向北传播的特征,但传播的速度要慢于NCEP资料的结果。模拟的北传ISO具有与NCEP资料相似的结构特征,涡度和水汽场明显的呈经向不对称,涡度和水汽的正异常位于ISO对流的北面,最大的上升运动和最强的行星边界层辐合也位于ISO对流的北面。ISO 结构的经向不对称性正是模式模拟的ISO具有向北传播特征的原因;而模式对夏季南亚季风区高低层风场和行星边界层水汽的合理模拟起了关键的作用。同时,根据关于ISO北传机理的已有研究,模式的结果也表明南亚地区夏季风场的垂直结构是那里ISO向北传播的重要机制。     

有关全球变暖对热带大气季节内振荡特征的影响及其数值模拟的研究

首先用资料诊断分析了全球变暖前后,热带大气低频振荡特征变化。研究发现,全球变暖前后,热带低频振荡的强度、分布、传播及周期等发生了一系列变化:ISO活跃区及强度范围均有不同程度的增大,气候变暖有缩小ISO强度季节差异趋势;气候变暖后,纬向东传波的大气季节内振荡能量增加,而西传波的能量相对减少;偏暖阶段,ISO振荡主周期相对分散,特别是较长周期的小波能量明显增大。然后利用动力学分析及数值模拟进一步探讨了热带低频振荡与海温的联系,对热带ISO的数值模拟进一步发现,大气模式与海洋模式耦合时,所模拟的热带季节内振荡较强;热带低频振荡的变化可能与全球变暖后海温场发生的变化有关。     

Investigation of dominant modes of monsoon ISO in the northwest and eastern Himalayan region

This study investigates the altitudinal variation of dominant modes of summer monsoon intra-seasonal oscillation (ISO) over the Northwest (NWH) and Eastern Himalayan (EH) region using (i) spatially scattered 133 number of station rainfall observations and (ii) latitudinal transect-wise (LT) rainfall variation, obtained from an observed interpolated gridded rainfall data for the period 1995–2004. The altitudinal variation of dominant modes of monsoon ISO were investigated by exploring the strong and weak phases of the principal components of 10–90 days bandpass rainfall data of June to September with respect to location specific station height. Investigation of frequency of days for light and moderate rainfall along with the occurrence of total seasonal rainy days has revealed existence of a rainfall maximum around 2100 m height for the NWH region. Similarly, the total seasonal rainy days of EH region was found to have maxima between 1100 and 1400 m height. Analyses of the spatially scattered station rainfall observation for the NWH region showed that the strong periods of ISO modes exist around 747.9 (±131.7) m and 2227.2 (±100.2) m heights. Over the EH region, the dominant modes of the monsoon ISO were found to be centred around 1200 m. Significant alterations of strong and weak phases of monsoon ISO as a response to altitudinal variation in the mountain surface were observed when latitudinal transect-wise variation of monsoon ISO modes were investigated.