Dynamical Predictability of Leading Interannual Variability Modes of the Asian-Australian Monsoon in Climate Models

The dynamical prediction of the Asian-Australian monsoon(AAM)has been an important and long-standing issue in climate science.In this study,the predictability of the first two leading modes of the AAM is studied using retrospective prediction datasets from the seasonal forecasting models in four operational centers worldwide.Results show that the model predictability of the leading AAM modes is sensitive to how they are defined in different seasonal sequences,especially for the second mode.The first AAM mode,from various seasonal sequences,coincides with the El Niño phase transition in the eastern-central Pacific.The second mode,initialized from boreal summer and autumn,leads El Niño by about one year but can exist during the decay phase of El Niño when initialized from boreal winter and spring.Our findings hint that ENSO,as an early signal,is conducive to better performance of model predictions in capturing the spatiotemporal variations of the leading AAM modes.Still,the persistence barrier of ENSO in spring leads to poor forecasting skills of spatial features.The multimodel ensemble(MME)mean shows some advantage in capturing the spatiotemporal variations of the AAM modes but does not provide a significant improvement in predicting its temporal features compared to the best individual models in predicting its temporal features.The BCC_CSM1.1M shows promising skill in predicting the two AAM indices associated with two leading AAM modes.The predictability demonstrated in this study is potentially useful for AAM prediction in operational and climate services.

Seasonal Transition Features of Large-Scale Moisture Transport in the Asian-Australian Monsoon Region

Using NCEP/NCAR reanalysis data for the period of 1957-2001, the climatological seasonal transition features of large-scale vertically integrated moisture transport （VIMT） in the Asian-Australian monsoon region are investigated in this paper. The basic features of the seasonal transition of VIMT from winter to summer are the establishment of the summertime ＂great moisture river＂ pattern （named the GMR pattern） and its eastward expansion, associated with a series of climatological events which occurred in some ＂key periods＂, which include the occurrence of the notable southerly VIMT over the Indochina Peninsula in mid March, the activity of the low VIMT vortex around Sri Lanka in late April, and the onset of the South China Sea summer monsoon in mid May, among others. However, during the transition from summer to winter, the characteristics are mainly exhibited by the establishment of the easterly VIMT belt located in the tropical area, accompanied by some events occurring in ＂key periods＂. Further analyses disclose a great difference between the Indian and East Asian monsoon regions when viewed from the meridional migration of the westerly VIMT during the seasonal change process, according to which the Asian monsoon region can be easily divided into two parts along the western side of the Indochina Peninsula and it may also denote different formation mechanisms between the two regions.

Characteristics of the Onset of the Asian Summer Monsoon and the Importance of Asian-Australian "Land Bridge"

Based on summarizing previous achievements and characteristics of Asian summer monsoon and the role using data as long and new as possible, the onset of Asian-Australian ＂land bridge＂ in the onset of summer monsoon are further discussed. In particular, the earliest onset area of Asian summer monsoon is comparatively analyzed, and the sudden and progressive characteristics of the onset of summer monsoon in different regions are discussed, Furthermore, the relationships among such critical events during the onset of Asian summer monsoon as the splitting of subtropical high belt over the Bay of Bengal （BOB）, the initiation of convection over Indo-China Peninsula, the westward advance, reestablishment of South Asian High, and the rapid northward progression of convection originated from Sumatra in early summer are studied. The important impact of the proper collocation of she latent heating over Indo-China Peninsula and the sensible heating over Indian Peninsula on the splitting of the subtropical high belt, the deepening of BOB trough, the activating of Sri Lanka vortex （twin vortexes in the Northern and Southern Hemispheres）, and the subsequent onset of South China Sea summer monsoon are emphasized.

Prediction of the Asian-Australian Monsoon Interannual Variations with the Grid-Point Atmospheric Model of IAP LASG(GAMIL)

Seasonal prediction of Asian-Australian monsoon （A-AM） precipitation is one of the most important and challenging tasks in climate prediction. In this paper, we evaluate the performance of Grid Atmospheric Model of IAP LASG （GAMIL） on retrospective prediction of the A-AM interannual variation （IAV）, and determine to what extent GAMIL can capture the two major observed modes of A-AM rainfall IAV for the period 1979-2003. The first mode is associated with the turnabout of warming （cooling） in the Nifio 3.4 region, whereas the second mode leads the warming/cooling by about one year, signaling precursory conditions for ENSO. We show that the GAMIL one-month lead prediction of the seasonal precipitation anomalies is primarily able to capture major features of the two observed leading modes of the IAV, with the first mode better predicted than the second. It also depicts the relationship between the first mode and ENSO rather well. On the other hand, the GAMIL has deficiencies in capturing the relationship between the second mode and ENSO. We conclude： （1） successful reproduction of the E1 Nifio-excited monsoon-ocean interaction and E1 Nifio forcing may be critical for the seasonal prediction of the A-AM rainfall IAV with the GAMIL; （2） more efforts are needed to improve the simulation not only in the Nifio 3.4 region but also in the joining area of Asia and the Indian-Pacific Ocean; （3） the selection of a one-tier system may improve the ultimate prediction of the A-AM rainfall IAV. These results offer some references for improvement of the GAMIL and associated seasonal prediction skill.

Atmospheric Circulation Cells Associated with Anomalous East Asian Winter Monsoon

Atmospheric circulation cells associated with anomalous East Asian Winter Monsoon （EAWM） were studied using the 1948/49 to 2002/03 NCEP/NCAR reanalysis and NCAR CAM3 AGCM simulations with monthly global sea surface temperatures from 1950 to 2000. Several atmospheric cells in the Pacific [i.e., the zonal Walker cell （ZWC） in the tropic, the Hadley cell in the western Pacific （WPHC）, the midlatitude zonal cell （MZC） over the central North Pacific, and the Hadley cell in the eastern Pacific （EPHC）] are associated with anomalous EAWM. When the EAWM is strong, ZWC, WPHC, and MZC are enhanced, as opposed to EPHC. The anomalous enhanced ZWC is characterized by air parcels rising in the western tropical Pacific, flowing eastward in the upper troposphere, and descending in the tropical central Pacific before returning to the tropical western Pacific. The enhanced MZC has characteristics opposite those of the enhanced ZWC in the central North Pacific. The anomalous WPHC shows air parcels rising in the western Pacific, as in the case of ZWC, followed by flowing northward in the upper troposphere and descending in the west North Pacific, as in the case of the enhanced MZC before returning to the western tropical Pacific. The anomalous EPHC is opposite in properties to the anomalous WPHC. Opposite characteristics are found during the weak EAWM period. The model simulations and the observations show similar characteristics and indicate the important role of sea surface temperature. A possible mechanism is proposed to link interannual variation of EAWM with the central-eastern tropical Pacific sea surface temperature anomaly （SSTA）.

T_(BB) DATA-REVEALED FEATURES OF ASIAN-AUSTRALIAN MONSOON SEASONAL TRANSITION AND ASIAN SUMMER MONSOOM ESTABLISHMENT

Based on TBB data from Meteorological Institute Research of Japan, study is carried out of the features of seasonal transition of Asian-Australian monsoons and Asian summer monsoon establishment,indicating that the transition begins as early as in April, followed by abrupt change in May-June; the Asian summer monsoon situation is fully established in June. The winter convective center in Sumatra moved steadily northwestward across the "land bridge" of the maritime continent and the Indo-China Peninsula as time goes from winter to summer, thus giving rise to the change in large scale circulations that is responsible for the summer monsoon establishment over SE Asia and India; the South China Sea to the western Pacific summer monsoon onset bears a close relation to the active convection in the Indo China Peninsula and steady eastward retreat of the subtropical TBB high-value band,corresponding to the western Pacific subtropical high.

东亚夏季风环流与ENSO循环的关系

采用NCEP/NCAR再分析资料和NCAR海温资料 ,对ENSO循环不同阶段东亚夏季风环流的变化进行了分析。计算了各年夏季风环流的强度系数及其与多年平均夏季风环流的相似系数和差异系数 ,分析它们与海温变化的关系。结果表明 :东亚夏季风环流强度有明显的年际变化和年代际变化 ,且与赤道东太平洋SST有较好的负相关关系 ,其中又以与三个月前的海温变化关系最好。在春季 (3～4月 )Nino 1+2区为冷、暖水时 ,当年夏季 (6～ 7月 )东亚季风区中 85 0hPa等压面上 >2m·s-1的经向风北伸纬度和东亚季风区的垂直经圈环流都有明显差异 ,在冷水期 >2m·s-1的经向风北伸纬度比暖水期高 ,季风环流圈的上升支北移 ,东亚夏季风环流较暖水期强。

青藏高原及周围地区的平均垂直环流特征

为得到青藏高原及周围地区平均垂直环流的图像，更好地理解该地区的降水气候，利用美国ＮＣＥＰ／ＮＣＡＲ１９６１～１９９０年３０年平均的ｗ、ｕ、ｖ、ｚ等再分析格点资料，计算分析了该地区的平均垂直运动场和经、纬圈环流图，分析了它们的年变化特征和可能的原因。结果表明，该地区冬、夏半年的平均垂直运动场和经、纬圈环流有很大的年变化；揭示了夏季在西北干旱区和华北半干旱区分别存在干旱和半干旱经圈环流，夏季在高原３２．５°Ｎ以南还存在巨大的向西纬圈环流等事实；也指出高原大气热源强度的年变化与随后高原及周边地区平均垂直环流的年变化联系密切。高原及周围地区的平均垂直环流特征也能较好地解释各地的降水气候。

热带环流演变与南海季风爆发

利用 1 95 8～ 1 997年的 NCEP/ NCAR再分析资料 ,分析了南、北半球中低纬环流的气候特征 ,并讨论了南海夏季风爆发与大尺度环流的关系。研究发现阿拉伯海经向环流管的上升气流和南半球纬向环流管的上升气流在 5月份同时到达南海 ,经向环流管低层的偏西风和纬向环流管低层的偏南风共同组成西南风 ,于是 5月份西南季风在南海地区首先爆发。此外 ,由于青藏高原地形及各经度海陆分布的影响 ,造成太阳辐射加热不均 ,是热带夏季风爆发的直接原因 。

中国东部夏季暴雨的年代际跃变及其大尺度环流背景

本文利用1960-2011年中国东部地面测站的逐日降水资料和JRA-55再分析资料探讨了夏季暴雨分布的年代际跃变及其相关联的大尺度环流异常特征。基于暴雨频数和占比（夏季暴雨占比是指5-8月暴雨降水量对总降水量的贡献百分比）的分析结果表明：中国东部夏季暴雨分布在20世纪70年代末和90年代初经历两次反相的经向＂三极子＂跃变。中国东部夏季暴雨的年代际演变过程可分为三个时段：1960-1979年为华南和华北暴雨偏多、江淮流域暴雨偏少的经向＂三极子＂分布;1980-1991年为南方和华北暴雨偏少、江淮流域暴雨偏多的经向＂三极子＂分布;1992-2011年为南方暴雨显著偏多、华北暴雨持续偏少,逐渐形成经向＂偶极子＂分布,并导致近十多年我国夏季＂南涝北旱＂的整体格局。1970年代末（1990年代初）跃变相关联的大尺度环流异常配置：东亚夏季风的减弱（增强）,西太平洋副高的增强西伸但南撤（北抬）,南亚高压的减弱南缩（增强东扩）,以及蒙古高原中低层的气旋式（反气旋式）环流异常。与此同时,低层局地环流也发生调整：华北和黄淮地区以及华南和江南地区均为反气旋式（气旋式）环流异常,而江淮流域和四川盆地受控于风场切变式辐合（辐散）异常;涡度场发生相应变化,南北方大部分地区的负（正）涡度异常不（有）利于低涡的发展,而江淮流域和四川盆地的正（负）涡度异常有（不）利于低涡的发展,进而引发江南和华南暴雨减少（增加）、江淮流域和四川盆地暴雨增加（减少）、黄淮和华北暴雨减少（增加）的经向＂三极子＂跃变。

北半球冬季风时期越赤道气流的初步分析

利用１９８０－１９８６年格点风资料，分析了各年北半球冬季风期间（１２－２月）东半球对流层低层及高空的越赤道气流通道。在该地区冬季低空具有气候意义的通道是１０５°Ｌ１２５°Ｅ、４５°Ｌ８０°Ｅ及１５０°Ｅ５条，其中以１００°一１３０“Ｅ为主要通道。高空则主要集中在８０°一１２０°Ｅ区间。在１９８３一１９８４、１９８４一１９８５年两个冬季，４５°Ｅ处出现较强的低空北风越赤道气流，这与高纬度大西洋东部上空持续的强阻塞形势有关。这支强越赤道气流与南印度洋及南太平洋多热带风暴也有联系。由平均经圈环流分析指出，冬季风期间在８０°一１２５°Ｅ存在着一个较完整的冬季风环流圈。

夏季东亚季风区Hadley环流上升支结构异常及其对长江流域降水的影响

利用ECMWF最新发布的Interim再分析资料,计算了东亚季风区Hadley环流质量流函数,并使用EOF分解、相关分析及合成分析等统计方法,分析了夏季东亚季风区Hadley环流上升支结构的异常特征及其对我国长江流域降水的影响。发现夏季东亚季风区Hadley环流上升支具有独特的双上升中心结构,两上升中心的位置分别对应东亚夏季风系统中的两条辐合带——热带季风槽及梅雨锋。上升支的主要异常模态表现为两个上升中心"跷跷板"型的反相异常。与梅雨锋对应的副热带上升中心强度与长江流域降水呈正相关关系,即当其偏强时,长江流域降水偏多,反之亦然。副热带支偏强时,低层西太平洋副热带高压偏南导致气流在长江流域异常辐合,其异常西南风水汽输送使得长江流域有异常水汽辐合,高层气流在长江流域异常辐散。同时鄂霍次克海附近阻塞活动偏强,东亚沿海地区500 h Pa高度场出现"+-+"的经向异常型。这些异常型均有利于长江流域的降水。

春季高原南侧低层前季风经圈环流特征分析

使用美国NCEP/NCAR再分析Ⅱ提供的风场资料,分析了春季青藏高原南侧大气低层前季风经圈环流的气候特征、年际变化和存在的原因及其与季风环流、夏季500 hPa环流形势之间的联系和物理机制。结果表明:前季风经圈环流是青藏高原与低纬地区之间从冬季Hadley环流型到夏季季风环流型转换的一种过渡形势;它的建立、结束时间及强度都有明显的年际变化,这种变化与其后季风环流出现的时间有密切的联系:当前季风经圈环流结束偏早时,往往预示着季风环流会提早建立;前季风经圈环流的异常与随后夏季环流形势也有一定的关系:当前季风经圈环流建立偏晚时,夏季500hPa西太平洋副热带高压面积偏大、强度偏强,且西伸更为明显;它们之间的这种联系实际上反映了前冬、春季青藏高原积雪对两者的共同影响。

亚澳越赤道季风环流的年际变化特征

利用1948~2004年美国月平均NCEP/NCAR(National Center for Environmental Prediction/National Center for At-mospheric Research)再分析资料,采用谐波分析和功率谱分析的方法,根据经向风的垂直切变定义了亚澳越赤道季风环流指数(IAAM),分析了亚澳季风(AAM)系统中越赤道季风环流的年际变化特征。结果表明:AAM的年际变化主要以2~3 a和准5 a周期振荡为主。利用合成分析,探讨了年际尺度上亚澳越赤道季风环流的空间分布特征。环流异常主要表现为整个东亚季风区冬季风环流的异常减弱和热带季风区夏季风跨赤道季风环流的异常加强,以及夏季赤道沃克环流和南亚高压的减弱。

The Analysis on the Features of the Atmospheric Circulation in Preceding Winters for the Summer Drought and Flooding in the Yangtze And Huaihe River Valley

Based on the annual variation of the rainfall departure percentage in summer in the lower-middle reaches of the Yangtze River and the Huaihe River valley, 7 cases for the abnormal drought and flooding summers (the drought years: 1981. 1984, 1985; the flooding yearst 1980, 1982, 1983, 1987) are selected. First we analyse the general circulation characteristics of the summer drought and flooding, and then the evolution processes of the general circulation patterns from preceding winters to summers are studied. It is found that during the two kinds of preceding winters for the drought and flooding summer, not only the general circulation patterns in the high-mid latitudes, the local Hadley cells in East Asia but also the activities of the cold surge in the lower latitude are different obviously. Spring, especially April, is the turning period of the general circulation in preceding winter for the drought or nooding summer evolution towards opposite direction. Hereafter, the drought or flooding circulation pattern is established and developed.

Possible mechanism of the effect of convection over Asian-Australian “land bridge” on the East Asian summer monsoon onset

The Asian-Australian “land bridge” is an area with the most vigorous convection in Asian monsoon region in boreal spring, where the onset and march of convection are well associated with the onset of East Asian summer monsoon. The convection occurs over Indo-China Peninsula as early as mid-April, which exerts critical impact on the evolution of monsoon circulation. Before mid-April there are primarily sensible heatings to the atmosphere over Indo-China Peninsula and Indian Peninsula, so the apparent heating ratios over them decrease with height. However, after mid-April it changes into latent heating over Indo-China Peninsula due to the onset of convection, and the apparent heating ratio increases with height in mid- and lower troposphere. The vertical distribution of heating ratio and its differences between Indo-China Peninsula and Indian Peninsula are the key factors leading to the splitting of boreal subtropical high belt over the Bay of Bengal. Such mechanism is strongly supported by the fact that the evolution of the vertical heating ratio gradient above Indo-China Peninsula leads that of 850 hPa vorticity over the Bay of Bengal. Convections over Indo-China Peninsula and its surrounding areas further increase after the splitting. Since then, there is a positive feedback lying among the convective heating, the eastward retreat of the subtropical high and the march of monsoon, which is a possible mechanism of the advance of summer monsoon and convection from Indo-China Peninsula to South China Sea.

WINTER QINGHAI-XIZANG T_(BB) FEATURES IN RELATION TO ATMOSPHERIC CIRCULATION PATTERN AND ASIAN-AUSTRALIAN MONSOONS

Predominantly in the context of Japan GMS-derived T_(BB) data,study is undertaken of the relationship between the winter thermal conditions of the Qinghai-Xizang Plateau(QXP)and anomaly in Asian-Australian monsoons during northern summer.Evidence suggests that anti- correlation of cold air activity of East Asia with that of Mid Asia is responsible for the counterpart of the ground thermal characteristics anomaly on an interannual basis between the SW and NE QXP;the winter thermal pattern bears a closer correlativity with the subsequent summertime Asian-Australian monsoons anomaly;as the thermal distribution is reversed,so are the convection features over North and South China,maritime continent,the NW and SW Pacific at tropical and equatorial latitudes,resulting in vast difference between East-Asian summer and Indonesian-North Australian winter monsoons;the subtropical monsoon-associated rainbelt over the mid-lower Changjiang basins exhibits the discrepancy in vigor and northerly shift timing.Besides,part of the results has been further borne out through analysis of temperature and precipitation records of the eastern portion of the country in monsoon climate.

THE MECHANISM OF THICKNESS OF GEOPOTENTIAL HEIGHT DIPOLE AND ITS RELATION WITH CLIMATE VARIABILITY IN ASIA-AUSTRALIA IN BOREAL WINTER

With the EOF of reanalysis data being analyzed, a northern-southern dipole is found in the upper troposphere geopotential height field of over the Asian-Australian monsoon region in the winter of the Northern Hemisphere(NH),which is defined as Asian-Australian dipole(AAD) in this study. Its intensity index is defined as AADI. Correlation and synthetic analysis illustrate that AADI is closely related to the weather and climate of Asian-Australian region in boreal winter. The index can reflect the simultaneous anomalies of temperature and precipitation on interannual and decadal scales in the boreal winter of Asian-Australian region. The superposition of the decadal and interannual signals is significant for the relationship between the AADI and climate change. The index can be used as an indicator of intensity of the Asian-Australian monsoon. In the years of strong AADI, the East Asia major deep trough is stronger,the Subtropical High is weaker and the Alaska ridge and the westerly jet are stronger than those in normal years.Enhanced meridional circulation between high and low latitudes exists in the years of strong AADI. These relationships reflect the intrinsic link between the anomalies in the upper troposphere geopotential height and climate in the Asian-Australian region.

2020年5月31日珠三角暴雨过程的中尺度特征

利用NCEP-FNL资料、国家级地面观测站和区域自动站资料,以及新一代多普勒天气雷达产品等资料,对2020年5月31日发生在珠三角地区且预报偏差较大的暴雨过程的中尺度系统生成环境条件、触发机制以及雷达回波演变特征进行分析,结果表明:该次暴雨过程是在高层辐散、中层弱波动过境、低层西南季风的有利天气背景下,由弱冷空气南下渗透,形成长时间维持的地面中尺度辐合系统,不断触发对流造成的。西南季风气流的风速脉动为对流提供水汽和不稳定能量,使对流不断发展增强。强降雨过程以低质心的暖云降水为主,强降雨后期东莞出现嵌有气旋式切变的强对流单体和较长生命史的弱HP型超级单体。