ON THE PROCESS OF SUMMER MONSOON ONSET OVER EAST ASIA*

Using daily observational rainfall data covered 194 stations of China from 1961 to 1995 and NCEP model analyzed pentad precipitation data of global grid point from 1979 to 1997,the distribution of onset date of rainy season over Asian area from spring to summer is studied in this paper.The analyzed results show that there exist two stages of rainy season onset over East Asian region from spring to summer rainy season onset accompanying subtropical monsoon and tropical monsoon respectively.The former rain belt is mainly formed by the convergence of cold air and the recurred southwesterly flow from western part of subtropical high and westerly flow from the so-called western trough of subtropical region occurring during winter to spring over South Asia.The latter is formed in the process of subtropical monsoon rain belt over inshore regions of South China Sea originally coming from south of Changjiang (Yangtze) River Basin advancing with northward shift of subtropical high after the onset of tropical monsoon over South China Sea.The pre-flood rainy season over South China region then came into mature period and the second peak of rainfall appeared.Meiyu,the rainy season over Changjiang-Huaihe River Basin and North China then formed consequently.The process of summer tropical monsoon onset over South China Sea in 1998 is also discussed in this paper.It indicated that the monsoon during summer tropical monsoon onset over South China Sea is the result of the westerly flow over middle part of South China Sea,which is from the new generated cyclone formed in north subtropical high entering into South China Sea,converged with the tropical southwesterly flow recurred by the intensified cross-equatorial flow.     

ON THE PROCESS OF SUMMER MONSOON ONSET OVER EAST ASIA

Using daily observational rainfall data covered 194 stations of China from 1961 to 1995 andNCEP model analyzed pentad precipitation data of global grid point from 1979 to 1997,thedistribution of onset date of rainy season over Asian area from spring to summer is studied in thispaper.The analyzed results show that there exist two stages of rainy season onset over East Asianregion from spring to summer rainy season onset accompanying subtropical monsoon and tropicalmonsoon respectively.The former rain belt is mainly formed by the convergence of cold air and therecurred southwesterly flow from western part of subtropical high and westerly flow from the so-called western trough of subtropical region occurring during winter to spring over South Asia.Thelatter is formed in the process of subtropical monsoon rain belt over inshore regions of South ChinaSea originally coming from south of Changjiang (Yangtze) River Basin advancing with northwardshift of subtropical high after the onset of tropical monsoon over South China Sea.The pre-floodrainy season over South China region then came into mature period and the second peak of rainfallappeared.Meiyu,the rainy season over Changjiang-Huaihe River Basin and North China thenformed consequently.The process of summer tropical monsoon onset over South China Sea in 1998is also discussed in this paper.It indicated that the monsoon during summer tropical monsoononset over South China Sea is the result of the westerly flow over middle part of South China Sea,which is from the new generated cyclone formed in north subtropical high entering into SouthChina Sea,converged with the tropical southwesterly flow recurred by the intensified cross-equatorial flow.     

The Relationship Between Abnormal Meiyu and Medium-Term Scale Wave Perturbation Energy Propagation Along the East Asian Subtropical Westerly Jet

The East Asian subtropical westerly jet(EASWJ) is one of the most important factors modulating the Meiyu rainfall in the Yangtze-Huaihe River Basin, China. This article analyzed periods of the medium-term EASWJ variation,wave packet distribution and energy propagation of Rossby waves along the EASWJ during Meiyu season, and investigated their possible influence on abnormal Meiyu rain. The results showed that during the medium-term scale atmospheric dynamic process, the evolution of the EASWJ in Meiyu season was mainly characterized by the changes of3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves. The strong perturbation wave packet and energy propagation of the 3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves are mostly concentrated in the East Asian region of 90°-150°E, where the two wave trains of perturbation wave packets and wave-activity flux divergence coexist in zonal and meridional directions, and converge on the EASWJ. Besides, the wave trains of perturbation wave packet and wave-activity flux divergence in wet Meiyu years are more systematically westward than those in dry Meiyu years, and they are shown in the inverse phases between each other. In wet(dry) Meiyu year, the perturbation wave packet high-value area of the 10-15 d low-frequency variability is located between the Aral Sea and the Lake Balkhash(in the northeastern part of China), while over eastern China the wave-activity flux is convergent and strong(divergent and weak), and the high-level jets are strong and southward(weak and northward). Because of the coupling of high and low level atmosphere and high-level strong(weak) divergence on the south side of the jet over the Yangtze-Huaihe River Basin, the low-level southwest wind and vertically ascending motion are strengthened(weakened), which is(is not)conducive to precipitation increase in the Yangtze-Huaihe River Basin. These findings would help to better understand the impact mechanisms of the EASWJ activities on abnormal Meiyu from the perspective of medium-term scale Rossby wave energy propagation.     

2018年我国梅雨特征及梅雨期降水异常成因分析

2018年我国梅雨特征及梅雨期降水异常成因分析     

Impacts of the Zonal Position of the East Asian Westerly Jet Core on Precipitation Distribution During Meiyu of China

The east-west location change of the East Asian westerly jet （EAWJ） at 200 hPa during Meiyu and the associated spatial distribution variation of precipitation in the middle-lower reaches of the Yangtze River （MLYR） are investigated by using the 40-yr NCEP/NCAR （National Centers for Environmental Prediction/National Center for Atmospheric Research） pentad mean reanalysis data and daily precipitation observation data from 1958 to 1997. The results show that there are two areas over which the 200-hPa EAWJ center appears most frequently during the Meiyu period： one is the western Pacific （WP） and the other is the East Asian continent （EAC）. During the Meiyu period, the westerly jet over the EAC is weak, and the core of the westerly jet over the WP splits up with reduced intensity and disappears by the end of Meiyu. The changes in the location and intensity of the westerly jet are associated not only with the starting and ending dates of Meiyu, but also with the spatial distribution and intensity of precipitation in the MLYR. It is found that when the westerly jet core in the upper troposphere is located over the WP and is coupled with an 850-hPa southwesterly jet, heavy precipitation accompanied by strong convergence and plenty supply of water vapor, occurs in the lower reaches of the Yangtze River. If the 200-hPa westerly jet core is located over the EAC, and without an 850-hPa southwesterly jet, only weak precipitation occurs in the MLYR. Therefore, the longitudinal location of the EAWJ core plays an important role in determining the upper- to lower-level circulation structure and the spatial distribution of heavy precipitation in the MLYR during the Meiyu period.     

东亚地区夏季风爆发过程

利用中国194站1961～1995年日降水资料及NCEP1979～1997年候格点降水资料,探讨了亚洲地区自春到夏的雨季开始分布。结果表明,东亚地区自春到夏存在副热带季风雨季开始和热带季风雨季开始。前者于4月初开始于华南北部和江南地区,随后向南和向西南扩展,于4月末扩展到华南沿海和中南半岛,这个雨带主要是冷空气和副热带高压西侧转向的SW风以及南亚地区冬春副热带南支西风槽中西风汇合而形成的,是副热带季风雨季开始。后者是南海热带季风爆发后使原来由江南移到华南沿岸的副热带季风雨带随副热带高压北进而北进,前汛期雨季进入盛期,江南出现第二次雨峰,形成梅雨期和江淮及华北雨季。同时,热带季风雨带也自东向西传播到达南亚地区而形成热带季风雨季。还讨论了1998年东亚地区夏季风爆发过程,指出南海夏季风爆发期的季风由副高北侧形成的新生气旋进入南海造成南海中部西风和南海越赤道气流转向的SW季风加强汇合而形成,因而是东亚季风系统中环流系统季节变化造成的,和印度季风无关。在南海季风爆发期阿拉伯海仍由副热带反气旋控制,南亚仍是上述副热带反气旋北侧NW风南下后转向的偏西副热带气流所控制,索马里低空急流仍未爆发,赤道西风并未影响南海。     

江南南部初夏雨季的降水和环流气候特征

基于1961～2010年气象台站逐日降水资料、同期美国NCEP/NCAR的逐日再分析格点资料,通过气候平均、REOF分析、聚类分析等方法,分析了江南地区初夏降水的地域性和时段性特征,及西太平洋副高和高、低空急流等大气环流的相应演变过程。结果发现:（1）江南南部27.5°～29.5°N存在一个独立于华南前汛期和江淮梅雨的初夏雨季,该雨季平均发生时间为6月11～30日,比江淮梅雨早约8天左右。（2）西太平洋副高的西伸东退是江南南部初夏雨季发生发展的重要环流背景,6月第2候副高发生突变性加速西伸之后雨季开始,雨季期间850 hPa副高西伸脊点基本稳定在最西位置即133°E附近,6月第6候副高东退北抬后雨季结束。（3）低层急流大风带的形成和位置是江南南部初夏雨季阶段的重要动力条件,印度洋和孟加拉湾向东北延伸的低层急流与西太平洋副高西北侧的气流连通形成低层急流大风带,并与北侧上空的高空急流耦合,降水集中区位于低层急流大风带左侧、高空急流入口区右侧。     

ADVANCE AND RETREAT OF THE SUMMER MONSOON IN CHINA

By using the daily-14 year(1983—1996)NCEP/NCAR 2.5°×2.5° reanalysis data,wecarefully study in each pentad the advance and retreat of the summer monsoon in China and givento it a new definition.This definition considered the intensity of southwesterly winds at 850 hPatogether with its degree in temperature and moisture.The result revealed that:(1)The advance of the summer monsoon in China shows three abrupt northward shifts andfour relatively stationary stays.The four stable stages correspond to the peak of the pro-summerrainy period in South China,the“Meiyu”season in the Changjiang(Yangtze)-Huaihe RiverValleys.the rainy season in the downstream of the Huanghe(Yellow)Riyer Valleys and the rainyseason in northern China.The retreat of the summer monsoon is so fast that it totally retreatsfrom the mainland at about the mid-August.(2)The northward advance of summer monsoon in China is basically controlled by theseasonal variation latitudinally of the upper level planetary westerlies.It is in roughly accord withthe temporal variation in the position of 15 m s~(-1) isotaeh at 200 hPa.The fast retreat of thesummer monsoon is mainly due to the blocking effect of the Tibetan Plateau.(3)The advance of 500 hPa subtropical high of the western Pacific is also in aecordanee withthe advance of the summer monsoon in China.During the advancement of the summer monsoon,the eastward movement of the subtropical high shows great meaning that it creates the essentialcondition for the convergence of southward intrusion cold airs with the warm and humidsouthwesterly winds,which result in precipitation.There are three manifest eastward movementsof the subtropical high during its northward advancement.They coincide correspondingly to thebeginning of the peak of the pre-summer rainy period in South China,the“Meiyu”season in theChangjiang(Yangtze)-Huaihe River Valleys and finally the rainy season in northern China.Thewestern part of the subtropical high moves eastward to the region of Japan in late July and thebeginning of August.It then stays there for quite a long time which results in the straightmovement of cold airs intruding from the north to the east of Tibetan Plateau,i.e.the easternregion of China.This provides good condition for the fast southward retreat of the summermonsoon.(4)The intensifieation and development of the Tibetan high at 200 hPa are closely related to the eastward movement of the subtropical high,they often occur simultaneously.     

不同纬度天气尺度扰动影响2020年夏季梅汛期降水的数值模拟

利用观测资料和区域气候模式RegCM4.6,研究了高纬和低纬天气尺度扰动对2020年梅雨期降水的可能影响.观测分析表明:2020年6月、7月长江中下游降水在周期上表现为10 d以下的天气尺度扰动,在降水过程中存在多次中高纬度天气尺度扰动的南传与低纬扰动的北传.在此基础上,设计改变不同纬度天气尺度扰动(＜10 d)输入的...     

中国东部夏季极端降水事件及大气环流异常分析

主要利用1961~2014年中国东部地区438个台站的逐日降水资料和NCEP/NCAR的再分析资料,从大气内部动力角度对夏季不同极端降水情况下的环境场进行分析,结果表明:对长江中下游地区而言,在极端降水频次偏多年时,850 hPa风场及整层水汽输送距平场均表明东亚夏季风偏弱,有利于更多的水汽输送到长江中下游地区,500 hPa鄂霍次克海阻塞高压持续日数偏多,有利于冷空气南下,200 hPa东亚副热带急流偏南,且30°N以南偏西风异常有利于辐散,而在斜压波包从西北东南向传播为极端降水事件分发生集聚了能量;对华北地区极端降水频次偏多年而言,850 hPa风场及整层的水汽输送距平场均表明东亚夏季风偏强,有利于更多的水汽输送到华北地区,500 hPa高度距平场日本海正距平,贝加尔湖蒙古地区为负距平,华北地区东高西低,200 hPa东亚副热带急流偏北,从而导致我国华北地区极端降水频次偏多,能量传播也为西北东南向。这些结果表明极端降水的变化,与大气内部的动力作用和能量的传播有密切的关系。     

2011年南海夏季风爆发过程及其与长江中下游梅雨的联系

采用NCEP/NCAR再分析资料、FY2E-TBB及台站降水资料,对2011年南海夏季风爆发前后的环流特征进行分析。结果表明:2011年强对流活动由孟加拉湾扩展到南海地区,同时伴随着南亚高压移至中南半岛北部,西太平洋副热带高压向东撤出南海地区,南海夏季风于5月第4候(第28候)爆发;季风爆发后,印度-孟加拉湾季风槽形成,南海地区低空开始盛行西南气流,并伴有对流降水的发展和温、湿等要素的突变。随着季风活动的推进,我国雨带北抬,长江中下游一带进入梅雨期,出现降水大值区。通过分析发现长江中下游梅雨与南海夏季风均受副热带高压影响,且两者的强度为显著的负相关关系,梅雨开始时间与南海夏季风爆发时间呈显著的正相关关系。2011年南海夏季风偏弱,爆发时间偏早,长江中下游梅雨强度偏强,入梅时间异常偏早。     

水汽输送与江南南部初夏雨季及降水变化的联系

基于1961—2010年美国国家环境预报中心/大气研究中心(NCEP/NCAR)的逐日再分析格点资料,分析了初夏水汽输送的分布和演变过程及其与中国江南南部初夏雨季的关系。结果显示,初夏水汽输送总体上随夏季风前沿自南向北加强,有3次水汽通量突然增大的涌先后从中国南海北传到25°N及其以南、25°—30°N、30°N及其以北地区,水汽涌和相应峰的发生时间分别对应华南前汛期、江南南部初夏雨季、长江流域梅雨的开始和结束时间。江南南部在初夏雨季处在水汽通量高值区的北缘、水汽辐合区内。青藏高原南侧水汽辐散区是影响江南南部初夏雨季的直接水汽源,澳大利亚北部到印度洋和阿拉伯海南部地区的大面积水汽辐散区则是间接水汽源。经向水汽输送演变对雨季起(讫)具有标志性意义,纬向水汽输送也不容忽视。雨季开始(结束)时江南南部地区的南界(北界)中低层水汽流入(流出)显著增大,但北界(南界)水汽通量并未同步发生显著变化;雨季期间的纬向水汽输送明显增强,水汽通量大于经向水汽输送。雨季强、弱具有年代际变化,且与纬向水汽流入的相关比经向水汽流入的相关更显著。影响江南南部初夏雨季的水汽输送路径主要有两条,北支是从孟加拉湾北部经缅甸和云南、贵州的水汽输送,南支是经孟加拉湾、中南半岛、中国南海与西太平洋副热带高压西侧水汽汇合的水汽输送。强雨季年孟加拉湾北部的东北向水汽输送和中国南海的北向水汽输送都增强,弱雨季年则相反。孟加拉湾、中国南海南部和西太平洋暖池区是显著的水汽辐合区,是江南南部初夏雨季的水汽输送通道而不是水汽源,水汽辐合越弱(强)越有利于(不利于)江南南部初夏雨季的降水,其影响机制可能在于通道上的对流活动对江南南部初夏雨季水汽输送具有拦截作用。     

中国雨季的气候学特征

利用中国740站气候平均逐候降雨量对中国的主雨季进行定义,并对雨季（包括主雨季,春雨和秋雨）的气候学特征进行了讨论。结果表明:全国主雨季最早爆发于华南中部,最晚结束于华西地区。主雨季能持续4到14候不等,雨量占年总降水的30%～60%。主雨季在中国东部为季风雨季,自南向北推进;在西部受西风带影响,北方略早于南方, 且局地性强。中国雨季具有明显的区域性和阶段性特征。中国气候的夏季降水时间序列主要反映了季节循环特征, 但气候季节内振荡（CISO）对东部雨季的持续和推进具有明显的调制作用,其中长江中下游及其以南地区以30～60天周期为主。     

中国东部春季极端降水与同期欧亚大陆地表感热的可能联系

利用1960~2010年ERA-20C再分析资料和中国东部站点降水观测资料,探讨了我国东部春季极端降水与欧亚大陆地表感热通量的联系和可能影响途径。结果发现,当春季欧亚大陆中纬度巴尔喀什湖以西及贝加尔湖以南区域地表感热通量偏弱（强）,我国东部沿海地区地表感热通量偏强（弱）时,我国东部春季极端降水呈现南少（多）北多（少）的反相分布特征。当春季欧亚大陆中纬度关键区地表感热偏弱,低纬度关键区地表感热通量偏强时,春季副热带西风急流偏弱、位置偏北,我国东部北方地区大气斜压不稳定和对流不稳定偏强,北方地区极端降水偏强,而南方地区大气斜压不稳定和对流不稳定偏弱,南方地区极端降水偏弱。当春季欧亚大陆中纬度关键区地表感热偏强,低纬度关键区地表感热通量偏弱,我国东部极端降水的情况大致相反。     

2000年江淮梅雨的分析和中期预报着眼点

20 0 0年江淮梅雨期的环流特征 ,在其入梅时表现为 :西太平洋副热带高压 1 2 0°E脊线的季节性北移具有明显的两周振荡周期 ;南亚高压脊线北移至 2 5°N和印度季风的爆发——加尔各答稳定西风的结束均能很好地预示江淮梅雨的开始。在出梅时 :亚洲地区地转风急流轴入梅后持续北移到 47.5°N,5 0°N稳定的超长波脊先于西太平洋副高调整 ,梅雨期后期在菲律宾东部洋面生成的 3号台风的北上 ,均对出梅的环流调整起到了显著的作用。     

海温异常对东亚夏季风影响机理的研究进展

利用湖北省梅雨监测资料、国家气候中心新百项指数和NCEP/NCAR环流资料,分析了2020年湖北梅雨异常特征及其成因。结果表明：（1）2020年湖北梅雨持续时间异常长,为54 d,仅次于1996年;梅雨量异常多、强度强、雨日率大,均为1961年以来第1位。（2）高空西风急流、副热带高压、夏季风系统的冬夏调整季节性进程早,是造成湖北入梅早的主要原因。西风急流和副热带高压在7月下旬北跳,较常年时间偏晚,导致了出梅晚。入梅早、出梅晚,梅雨持续时间异常长。（3）在前冬El Niño事件、春夏热带印度洋海温偏暖和北大西洋三极子负位相的共同影响下,欧亚中高纬经向环流发展,冷空气活跃,副热带高压强且位置稳定,西侧水汽输送强,冷暖气流在长江中下游交汇,造成湖北省梅雨量异常偏多。

     

夏季北太平洋副热带高压系统的活动

文中根据 NCEP/NCAR再分析资料 ,分析了北太平洋副热带高压系统的变化。 5～ 9月由于亚洲夏季风的建立及活动 ,北半球副热带高压系统在 6 0～ 1 2 0°E出现断裂 ,夏季西太平洋副热带高压脊点平均伸展到 1 2 0°E,其年际变化反映了亚洲夏季风的强弱。强夏季风年 5 0 0h Pa西太平洋副热带高压脊线位于 3 0°N以北 ,并分裂成两个中心 ,印度低压强 ;弱夏季风年西太平洋副热带高压脊线位于 3 0°N以南 ,表现为北太平洋高压中心向西伸展的高压脊 ,印度低压弱。夏季西太平洋副热带高压的季内活动有两种模态 :第 1种表现为副热带高压系统以 2 0～ 3 0 d的周期从北太平洋中部的副热带高压中心一次次地向西扩张到 1 2 0°E以西 ,这类过程大多出现在亚洲夏季风强度偏弱年 ;第 2种模态表现为副热带高压系统以 2 0～ 3 0 d的周期一次次地由东向西扩充时 ,在 1 2 5～ 1 5 5°E停滞 ,这类过程大多出现在亚洲夏季风强度偏强年。江淮流域梅雨的中断和结束与北太平洋副热带高压系统 2 0～ 3 0 d季内振荡有关。西太平洋副热带高压 5～ 1 0 d的短期活动受 3 5～ 45°N西风带活动的影响 ,当西风槽在中国沿海和西太平洋地区向南伸展到 3 0°N以南后 ,西太平洋副热带高压有一次加强活动     

Characteristics of Subtropical Monsoon and Rainfall over Eastern China and Western North Pacific

Using the NCAR/NCEP (National Center for Atmospheric Research/National Centers for Environmental Prediction) reanalysis and the NOAA Climate Prediction Center's merged analysis of precipitation (CMAP)during 1981-2000, we investigated the seasonal evolution of the southwesterly wind and associated precipitation over the eastern China-subtropical western North Pacific area and its relationship with the tropical monsoon and rainfall, and analyzed the reasons responsible for the onset and development of the wind. It was found that the persistent southwesterly wind appears over southern China and the subtropical western Pacific the earliest in early spring, and then expands southwards to the tropics and advances northward to the midlatitudes. From winter to summer, the seasonal variation of surface heating over western China and the subtropical western Pacific may result in an earlier reversal of the westward tropospheric temperature gradient over the subtropics relative to the tropics, which may contribute to the earliest beginning of the subtropical southwesterly wind. Additionally, the strengthening and eastward expanding of the trough near the eastern Tibetan Plateau as well as the strengthening and westward moving of the western Pacific subtropical high also exert positive influences on the beginning and development of the subtropical southwesterly wind.In early summer,the northward expansion of the southwesterly wind over southern China is associated with a northward shift of the subtropical high, while the southward stretch of the southwesterly wind is associated with a southward stretch of the trough in the eastern side of the plateau. With the beginning and northward expansion of the subtropical southwesterly wind (namely southwest monsoon), convergences of the low-level air and water vapor and associated upward motion in front of the strongest southwesterly wind core also strengthen and move northward, leading to an increase in rainfall intensity and a northward shift of the rain belt. Accordingly, the subtropical rainy season occurs the earliest over southern China in spring, moves northward to the Yangtze-Huaihe River valley in early summer, and arrives in North China in mid summer.Compared with the subtropical rainy season, the tropical rainy season begins later and stays mainly over the tropics, not pronouncedly moving into the subtropics. Clearly, the Meiyu rainfall over the Yangtze-Huaihe River valley in early summer results from a northward shift of the spring rain belt over southern China,instead of a northward shift of the tropical monsoon rain belt. Before the onset of the tropical monsoon,water vapor over the subtropical monsoon region comes mainly from the coasts of the northern Indo-China Peninsula and southern China. After the onset, one branch of the water vapor flow comes from the Bay of Bengal, entering into eastern China and the subtropical western Pacific via southwestern China and the South China Sea, and another branch comes from the tropical western North Pacific, moving northwestward along the west edge of the western Pacific subtropical high and entering into the subtropics.     

2008年梅雨异常大尺度环流成因分析

利用NCEP再分析资料对2008年江淮梅雨异常特征及其大尺度环流成因进行了分析研究。结果表明： （1）2008年入（出）梅显著偏早、 梅雨期长度略偏短, 梅雨分布呈南涝北旱、 东多西少, 梅雨量偏少、 强度偏弱。（2）该年入梅显著偏早是东亚大气环流由冬季型向夏季型转换提前所致, 副热带高空西风急流北跳、 500 hPa西风带环流调整、 西太平洋副热带高压季节性北跳、 夏季风北涌至江淮流域的时间均早于常年。（3）该年出梅显著偏早的主导因素是冷空气活动。（4）南涝北旱梅雨型是受南亚高压东段脊线位置接近常年、 副热带高压脊线处于适宜梅雨发生纬度带的南段、 低空西南急流和水汽输送带北缘位置以及高空强辐散和中低空强辐合区位置偏南的影响。（5）东多西少梅雨型是冷空气路径偏西所致。（6）梅雨期夏季风北涌至江淮流域活动次数偏少是梅雨量偏少的重要因素。     

华南前汛期降水的年代际异常特征及其成因

利用1981—2013年中国160站逐月降水资料、NCEP/NCAR逐月再分析资料及NOAA海表温度资料,研究了华南前汛期降水年代际异常的时空特征及其可能成因。结果表明:1)华南前汛期降水在1992前后发生由异常偏少转为偏多的显著年代际转折,最显著异常中心位于广西东北部和广东北部。2)1990年代初发生的对流层高层南冷北暖(40°N附近为界)、对流层下暖上冷的年代际转折,使得高低层环流场均出现了有利于北方干、湿冷空气和孟加拉湾、西太平洋暖湿水汽在华南区域交汇并辐合上升的形势,造成华南前汛期降水发生偏少转偏多的显著年代际转折。年代际转折的前后两个时段中,位于热带的孟加拉湾槽、东亚沿岸EAP遥相关型波列中的西太平洋副高、阿拉斯加湾附近的脊,以及中纬度贝加尔湖以西以南脊的强度或位置均具有显著差异,故这些环流系统的年代际异常是华南前汛期降水年代际异常的重要原因。3)南太平洋关键区海温在1990年代初开始呈现增暖趋势,在偏暖(偏冷)时期,华南低空受异常气旋(异常反气旋)环流控制,对流层上层西风急流偏弱偏南(偏强偏北),造成华南地区降水异常偏多(偏少)。