EAST ASIA SUMMER MONSOON ONSET DATE CALCULATED FROM OBSERVED, REANALYZED AND COMBINED DAILY RAINFALL

In this paper, the East Asia summer monsoon onset date lines in East China are calculated by the definition similar to the traditional one, with the ECMWF reanalyzed 850 hPa daily wind and observed, reana-lyzed and combined daily rainfall during 1980 ~ 1993. To make the onset date line as close as possible to the previous work, the earliest onset date limits have to be applied for the regions with different latitude and the daily mean datasets have to be smoothed by space before calculation, therefore their space-resolution is reduced to about 3 longitude ×1 latitude. The results show that the multiyear mean summer monsoon onset date lines are quite similar to each other. Compared with the one from the reanalysis, the 14-year average onset date line form combination is obviously improved in the southern Sichuan Basin and the correlation between observed and combined onset date is also slightly higher over the Huaihe valley and Northeast China. Since daily rainfall combination also improved the long term daily mean and standard deviation through the pentad CMAP, if no better daily dataset is available, such a kind of daily rainfall combination can be used to get reasonable result in the Indian monsoon region without sufficient observatories or over the North Pacific without any ground obser-vation at all in future study.

THE ANALYSIS OF MECHANISM OF IMPACT OF AEROSOLS ON EAST ASIAN SUMMER MONSOON INDEX AND ONSET

RegCM4.3, a high-resolution regional climate model, which includes five kinds of aerosols(dust, sea salt,sulfate, black carbon and organic carbon), is employed to simulate the East Asian summer monsoon(EASM) from 1995 to 2010 and the simulation data are used to study the possible impact of natural and anthropogenic aerosols on EASM.The results show that the regional climate model can well simulate the EASM and the spatial and temporal distribution of aerosols. The EASM index is reduced by about 5% by the natural and anthropogenic aerosols and the monsoon onset time is also delayed by about a pentad except for Southeast China. The aerosols heat the middle atmosphere through absorbing solar radiation and the air column expands in Southeast China and its offshore areas. As a result, the geopotential height decreases and a cyclonic circulation anomaly is generated in the lower atmosphere. Northerly wind located in the west of cyclonic circulation weakens the low-level southerly wind in the EASM region. Negative surface radiative forcing due to aerosols causes downward motion and an indirect meridional circulation is formed with the low-level northerly wind and high-level southerly wind anomaly in the north of 25° N in the monsoon area, which weakens the vertical circulation of EASM. The summer precipitation of the monsoon region is significantly reduced,especially in North and Southwest China where the value of moisture flux divergence increases.

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.

THE EARLY SUMMER FLOOD PERIODS OF SOUTHERN CHINA AND THE SUMMER MONSOON CIRCULATION OF EAST ASIA

Based on the thermodynamic characteristics of the summer monsoon and foe change of the lower layer wind fields, the relation between the early summer flood periods of southern China, Including the first flood period of South China and the plum rains period of the middle-lower reaches of the Yangtze River and the activities of the summer monsoon is analysed. The establishment processes of the summer monsoon circulation of East Asia are investigated. It is shown that the beginning and ending of the flood periods are exactly in accordance with the arrival and departure of the fore boundary of the summer monsoon. The establishment process of the circulation from the very beginning of the arrival of the monsoon to the time of great prosperity of development are not the same for each year. They can be classified into four categories. Each category may have four or three stages. Besides, the structure of the summer monsoon regime of East Asia is not unitary. There exist four types of structure model of the monsoon regime of East Asia.

An East Asian Subtropical Summer Monsoon Index and Its Relationship to Summer Rainfall in China

Using the monthly mean NCEP/NCAR reanalysis data and the monthly rainfall observations at 160 rain gauge stations of China during 1961 1999, and based on major characteristics of the atmospheric circulation over East Asia and the western Pacific, a simple index for the East Asian subtropical summer monsoon （EASSM） is defined. The relationship between this index and summer rainfall in China and associated circulation features are examined. A comparison is made between this index and other monsoon indices. The results indicate that the index defined herein is reflective of variations of both the thermal low pressure centered in Siberia and the subtropical ridge over the western Pacific. It epitomizes the intensity of the EASSM and the variability of summer rainfall along the Yangtze River. Analysis shows that the Siberian low has a greater effect on the rainfall than the subtropical ridge, suggesting that the summer rainfall variability over the eastern parts of China is to a large extent affected by anomalies of the atmospheric circulation and cold air development in the midlatitudes. Taking into account of the effects of both the Siberian low and the subtropical ridge can better capture the summer rainfall anomalies of China. The index exhibits interannual and decadal variabilities, with high-index values occurring mainly in the 1960s and 1970s and low-index values in the 1980s and 1990s. When the EASSM index is low, the Siberian low and the subtropical ridge are weaker, and northerly wind anomalies appear at low levels over the midlatitudes and subtropics of East Asia, whereas southwesterly wind anomalies dominate in the upper troposphere over the tropics and subtropics of Asia and the western Pacific. The northerly wind anomalies bring about frequent cold air disturbances from the midlatitudes of East Asia, strengthening the convergence and ascending motions along the Meiyu front, and result in an increase of summer rainfall over the Yangtze River.

FIRST TRANSITION OF THE CIRCULATIONS IN ASIA AROUND MID－MAY FROM 7－YEAR MEAN ECMWF DATA

The time evolution of the general circulation over the South China Sea and surrounding areas during the period from April to June is studied using ECMWF data of 1980-1986. The first transition from the second (6-10 May ) to the third (11-15 May ) pentads is characterized by the distinct change of low-level (850 hpa ) winds from southeasterlies to southwesterlies along 15°N over the South China Sea, and by the sudden movement of the center of South Asian high in the upper troposphere (200 hpa ) from 10-15°N to 15-20°N over Southeast Asia. Corresponding to the abrupt change in circulations, the gradients of the temperature and humidity intensity along latitudes center on 30°N over East Asia at 850 hpa. The time sequence of the 850- 200 hpa layer thickness shows that the layer-mean temperature over the southeastern Tibetan Plateau-East China Plain region increases abruptly at the same time. The corresponding sudden increase of the vertically integrated heat source (Q1 )over the warming areas reveals that the heat source plays an evident role in the drastic changes. The time series of (Q1 ) over the northern part of the South China Sea shows that the drastic increase of the areamean (Q1 ) is also found but it is 5-10 days late than the change of corresponding wind fields. The time series of Xisha SST shows a continuous increase to about 29. 5°until May 10 when the abrupt changes in circulation occur.

“0506”华南持续性暴雨的季风环流背景

提出了确定东亚夏季风活动区域、划分热带季风和副热带季风活动区域的指标,利用大气对流层风速、位势高度、湿度、温度、OLR以及TBB等NCEP/NCAR资料,从月、候和过程平均多种时间尺度,诊断分析了2005年6月(简称“0506”)华南持续性暴雨的季风环流活动变化特征。结果表明:副热带高压强度偏强,西脊点位置偏西偏南,热带西太平洋(130°～140°E)区域越赤道气流偏强,华南处于气旋性低压异常区,无论是月时间尺度还是暴雨过程时间尺度都表现出这些明显特征;暴雨过程水汽除了来源于孟加拉湾和南海外,水汽通量异常部分主要来自南海和热带西太平洋,热带西太平洋水汽随着副高边缘气流经过南海向华南输送,从而为暴雨过程提供了丰富的水汽来源;2005年6月热带季风前沿在华南沿海地区停滞时间比气候平均偏长(2候),该特征是华南暴雨预报值得参考的信号;6月整个南海地区平均季风偏强,主要体现于经向风明显偏强,但华南持续性暴雨过程开始于南海地区夏季风非活跃期,这与热带季风季节内振荡向北传播到华南有关。以上季风活动变化特征为华南强降水提供了有利的动力条件和丰富的水汽来源。

东亚夏季风指数的年际变化与东亚大气环流

文中从夏季东亚热带、副热带环流系统特点出发 ,定义了能较好表征东亚夏季风环流年际变化的特征指数 ,并分析了东亚夏季风指数的年际变化与东亚大气环流及夏季中国东部降水的关系。文中定义的东亚夏季风指数既反映了夏季东亚大气环流风场的变化特征 ,也较好地反映了夏季中国东部降水的年际变化特征。此外 。

1982～1996年亚洲热带夏季风建立迟早的探讨(Ⅰ)热带季风环流的主要特征和季风建立指数

利用NCEP再分析的 1982～ 1996年的风场和高度场资料 ,分析讨论了亚洲热带夏季风环流的若干基本特征 ;从季风环流的主要特征出发 ,定义了一个季风建立指数 ;计算、确定和分析了东亚季风区和印度季风区 15年平均的季风建立时间 .结果表明 ,在 1982～ 1996年期间 ,平均而言 ,东亚热带季风 5月第 4候建立 ,维持到 10月第 2候 ;印度季风 6月第 2候建立 ,比南海热带季风晚约 4候 ,持续至 9月第

青藏高原加热对东亚地区夏季降水的影响

东亚地区降水主要集中在夏季,是亚洲夏季风系统的重要特征。本文利用NCEP再分析资料和CRU的降水资料,分析了青藏高原非绝热加热对东亚夏季降水的影响。结果表明,东亚地区夏季降水的分布形势与青藏高原非绝热加热变化有很好的相关关系。由于高原非绝热加热可在亚洲东部沿海地区强迫出类似Rossby波列的大气环流低频振荡结构,而此低频波可以影响到西太平洋副热带高压的形态和位置变化,从而使得东亚夏季降水的形势发生变化。而青藏高原非绝热加热的形态从春季到夏季有很好的持续性,春季高原加热与夏季东亚的降水形势分布也有很好的相关。本研究中采用的青藏高原非绝热加热指数可作为东亚夏季降水预测的一个指标,亚洲季风降水不仅受赤道太平洋海温的影响,青藏高原地区的非绝热加热对其也有显著的影响作用。

伴随南海夏季风爆发的大尺度大气环流演变

主要基于美国ＮＣＥＰ和ＮＣＡＲ的再分析资料（１９８０～１９９６ 年），针对南海夏季风爆发日期进行合成分析， 研究了伴随南海夏季风爆发的大尺度大气环流演变。其结果清楚地表明伴随南海夏季风爆发， 南亚和东南亚地区的对流层低层风场、对流层高层位势高度场以及大气湿度场和垂直运动场都有极显著的变化。南亚和东南亚８５０ ｈＰａ 上涡旋对的发展和活动以及５００ ｈＰａ 副高从南海地区的东撤对南海季风爆发起着重要作用。伴随南海夏季风的爆发， 在孟加拉湾到南中国海一带整层湿度和５００ ｈＰａ 垂直上升运动都出现了极明显的增加。对流层高层和对流层低层环流演变的特征也清楚表明， 南海夏季风爆发既是全球环流冬夏演变的一个部分， 又有显著的区域性特征。本文还指出南海夏季风在北部比中部和南部早建立的结论依据不足， 进而补充给出了亚洲季风爆发日期示意图。

夏季索马里急流的建立及其影响机制

定义了夏季索马里急流(SSMJ)的建立指数,从而确定了1948—2002年的SSMJ建立日期,并以此为基础研究了SSMJ建立过程以及前冬海陆热力异常和冬季风异常对其建立早晚的影响。结果表明:SSMJ的建立是亚洲季风爆发的最早信号;其建立过程受到马斯克林高压、中非低压以及阿拉伯高压等非洲—印度洋系统的影响,而印度低压的发展有利于SSMJ越过赤道后向东转向;夏季索马里急流建立时间的年际变化反映了北半球冬季海陆热力差异的年际异常;强(弱)东亚冬季风时,次年春季SSMJ建立偏迟(早)。

确定东亚─西北太平洋地区夏季副热带季风建立和活动范围的一种方法

文中利用1948—2002年NCEP/NCAR全球再分析资料,1958—2002年ECWMF再分析资料,中国国家气候中心的618个测站1961─2005年的逐日降水量资料,采用作者确定的东亚夏季风(EASM)建立标准,探讨东亚夏季风在南海爆发以后继续向北和向东推进活动范围、建立、持续和撤退的时空分布特征,结果表明:(1)给出东亚—西北太平洋地区的东亚夏季副热带季风(EASSM)的定义,计算其建立时间(候)和空间分布,发现EASSM除继续向北推进的方向外,在20°─25°N还有明显向东的分支,以及向西稍偏北的分支,107.5°E以西的季风建立等时线梯度十分密集,而以东的等时线梯度很小,季风建立的等时线的总体以110°─115°E为轴心、明显向中国华北和东北地区凸起的近于折线的分布,表明EASSM先在中国大陆建立,而同纬度的中国近海和西北太平洋地区的夏季副热带季风后建立;(2)撤退的等时线则近于与建立的等时线相反分布,沿120°E的方向凸向南,表明中国东北地区和近海地区先撤退;(3)给出东亚─西北太平洋地区EASSM建立的概率等值线的空间分布,我们尝试定义了<35%概率等值线的地区为其影响的边缘地带,北边界始于黑龙江省北部48°N附近向东南经由黑龙江省南部和俄罗斯滨海省,穿过日本海止于日本北海道南部,其西边界由东北向西南沿中蒙边境止于青藏高原。20世纪90年代北部边缘地带与多年平均比偏南3─5纬度,而西部边缘变动较小,只向东偏了1个经度。

华南6月降水异常及其与东亚—太平洋遥相关的关系

利用1959～2010年共52年的大气环流和降水资料,我们分析了华南前汛期季风降水(6月降水)的变化特征,发现6月华南降水与同期EAP(East Asia-Pacific,东亚—太平洋)遥相关型有显著的相关关系,两者之间的相关系数为0.35。EAP指数为正时,长江中下游以南的地区降水偏多,而长江以北和黄河之间的地区降水偏少。将华南6月降水分为与EAP相关的降水序列和与EAP独立的降水序列,比较了二者所对应环流异常的异同点。结果表明,与EAP相关的降水异常对应着EAP相关型的环流异常分布特征,降水为正异常时,850hPa风场从低纬度到高纬度呈现"反气旋、气旋、反气旋"的异常分布,湿的偏南风和干的偏北风在华南上空交汇,降水增多;而整个淮河流域上空为偏北风异常,导致南风带来的水汽输送减少,降水偏少,因此降水异常呈现偶极子分布。相比之下,与EAP独立的降水正异常对应的环流异常表现为热带西北太平洋上空的反气旋性环流异常,华南地区上空为显著的西南风异常,输送到华南地区的水汽增多,导致降水偏多。

我国南方初夏汛期和东亚夏季风环流

主要根据低层夏季风热力学性质与流场演变特点深入讨论我国南方初夏汛期包括华南前汛期和长江中下游梅雨期同夏季风活动的联系，分析了东亚夏季风环流建立过程的阶段性和类型，指出汛期的开始与结束同夏季风前沿位置变动一致。我国夏季风从开始到发展鼎盛时期低纬大气环流的演变过程各年不完全相同，主要可归纳为４类，每类包括４个或３个不同阶段，从而可认识到东亚夏季风体系结构的非单一性，它可有４种结构模型，即仅是东南季风Ｐ的单气流型、强东南季风Ｐ与弱西南季风Ａ的双气流型、强东南季风Ｐ和弱西南季风Ｍ的双气流型、强东南季风Ｐ和强（或弱）西南季风Ｍ以及弱西南季风Ａ的３气流型。

气溶胶对东亚夏季风指数和爆发的影响及其机理分析

利用高分辨率区域气候模式Reg CM4.3,通过引入沙尘、海盐、硫酸盐、黑碳和有机碳等5种气溶胶,对1995—2010年的东亚夏季风进行数值模拟,研究了自然和人为气溶胶对东亚夏季风的可能影响。结果表明:区域气候模式对东亚夏季风和气溶胶的时空分布都有较好的模拟效果,自然和人为气溶胶造成东亚夏季风指数减小约5%,且除我国东南部地区外,气溶胶使整个季风区的季风爆发时间推迟了1候左右。在我国东南部及近海地区,气溶胶通过吸收太阳辐射对中层大气起到加热作用,气柱受热会出现膨胀,从而造成了低层大气的位势高度下降并激发出气旋式环流距平,气旋式环流距平西侧偏北风能削弱东亚夏季风区低层的偏南气流。气溶胶的加入引起的地表负的辐射强迫造成了空气出现下沉运动并配合低层偏北风和高层偏南风距平,在25°N以北地区形成了间接经向环流距平,从而也削弱了东亚夏季风的垂直环流。气溶胶增加了我国季风区的水汽通量散度值,从而造成了夏季降水的明显减少,其中我国华北和西南地区为2个主要的降水减少区域。

影响东亚夏季风降水异常的前期海温信号

采用NCEP/NCAR大气再分析资料、Had ISST海温数据以及中国东亚季风区的实测降水资料,探讨前期冬季海温与东亚夏季风之间的关系。研究表明,影响东亚夏季风异常的海温变化存在2个关键区,分别位于南印度洋中部和北太平洋东部,关键区的海温与东亚季风区的降水在长江中游有显著正相关,与东亚夏季风指数有显著负相关。基于此,定义了印-太海温指数。强海温指数年的南亚高压、索马里越赤道气流、西太平洋副高偏强,有利于东亚夏季风北进,中国大部降水偏多;弱海温指数年则相反。

夏季索马里越赤道气流垂直结构的变化特征及其与东亚夏季风活动的关系

采用NCEP/NCAR和ERA40再分析月平均资料,对比分析夏季垂直尺度上五层(1000 hPa、925 hPa、850 hPa、700 hPa、600 hPa)索马里越赤道气流(SMJ)的时空演变特征。指出对流层中低层SMJ存在不同的垂直结构,且具有年际、年代际变化特征。夏季SMJ垂直结构主要呈现出全区一致、上下反相两种分布型("A"型和"B"型),细分为一致增强型("A+"型)、一致减弱型("A-"型)、上弱下强型("B+"型)和上强下弱型("B-"型)。进一步分析指出,SMJ垂直结构与东亚夏季风活动关系密切,SMJ"A+"("A-")型垂直结构对应东亚夏季风偏强(弱),我国内蒙古东部、华北地区降水偏多(少);夏季SMJ"B+"("B-")型垂直结构对应东亚夏季风偏弱(强),我国江南、日本地区降水偏少(多)。

南海夏季风爆发前后亚洲地区的大尺度环流突变

用１９８０—１９８６年的ＥＣＭＷＦ资料分析了南海季风爆发前后大气环流突变的平均特征。结果表明：南海季风的爆发一般发生在５月１０日前后，大气环流出现一次明显突变──高空南亚高压由１０—１５°Ｎ骤然北跳到１５—２０°Ｎ，南海北部西风转为东风；低空南海北部及附近地区西南风迅速加强并向东扩展，而中纬地区的偏北风也相应加强南压，青藏高原东南部到中国长江中下游一带为温度、湿度梯度大值区；中国西南地区出现低压环流。同时，青藏高原东南部及中国东部平原地区对流层大气发生急速增暖，大气热源和水汽汇明显增强。在南海季风爆发后南海北部大气热源亦显著增强，但比风场的突变落后５—１０天，而西沙海温的变化与季风爆发却比较一致。另外，地形对大气热源的分布有一定的影响，青藏高原东南坡的加热对南海季风的爆发可能比较重要。

东亚夏季风环流的动力统计诊断

本文对东亚夏季风环流作了动力统计诊断,其方法是将高、低层的偏差风场看作一个整体进行复EOF分析。诊断结果表明:偏差风场的第一模态直接与东亚夏季风环流有关,可称为季风模态;第二模态则与PJ波列关系密切,可称为PJ模态;季风模态存在两个平衡态,东亚夏季风也有两个平衡态;季风模态存在强弱的年际变化,其与东亚夏季风强弱的年际变化和菲律宾附近对流的年际变化均具有很好的对应关系;东亚夏季风和季风模态还存在明显的年代际变化。