夏季新疆降水异常与印度降水的关系

利用1960—2003年6~8月新疆75个气象站降水量、印度地区降水量和NCEP/NCAR再分析资料,分析了新疆夏季降水量与印度降水之间的关系,以及印度降水变化与新疆降水异常联系的可能的物理机制。研究表明,印度降水与新疆夏季降水量之间存在显著的反相关关系,它们之间的线性相关系数为-0.39。夏季印度降水变化与其西北侧的西亚—中亚地区对流层平均温度呈显著正相关,从而与对流层高层南亚高压中心东西振荡和高压西部强弱,西亚地区副热带西风急流（西亚急流）强度和南北位置振荡,500 hPa伊朗高压南北和东西振荡密切联系,夏季印度降水变化通过与这些影响新疆降水的系统联系而与新疆夏季降水呈显著的反相关关系。Eliassen-Palm通量（EP通量）的动力学诊断分析进一步表明,印度季风偏强（偏弱）导致南亚高压西侧由南向北进入亚洲西风急流入口处的波作用通量偏强（偏弱）,使得西亚急流偏强（偏弱）、偏北（偏南）。

夏季中国华北降水、印度降水与太平洋海表面温度的耦合关系

本文基于1951～2014年的站点观测资料以及再分析资料,应用多变量经验正交分解法(MEOF)研究了年际尺度上华北夏季降水、印度夏季降水与海表面温度之间的耦合关系(主要模态)。结果表明:当印度夏季降水偏强时,若同期夏季赤道中东太平洋海温表现为LaNi?a位相,则西太平洋暖池对流加强,副热带高压偏西偏北,有利于华北夏季降水与印度夏季降水一致增强。反之,当印度大部降水偏弱时,若同期夏季赤道中东太平洋海温表现为ElNi?o位相,则华北夏季降水和印度夏季降水一致减弱。然而,两地夏季降水的协同变化关系并不总是成立。当赤道中东太平洋海温异常随时间演变表现为冬春El Ni?o衰减型时,伴随着印度洋偶极子(IOD)正位相的衰减过程,这会减弱东亚夏季风,使得华北夏季降水偏少。此时印度半岛夏季降水增强区集中在其西部,无法形成连接印度和华北夏季降水异常的环半球遥相关(CGT)波列,可能使得华北夏季降水异常与全印度夏季降水异常成相反形势。这些结论揭示了中国华北夏季降水、印度夏季降水和海表面温度之间的耦合关系,有助于进一步理解海温外强迫对两地夏季降水之间相关关系的作用,从而对华北夏季降水的预测具有参考意义。

北大西洋多年代际振荡正、负位相期间欧亚夏季副热带波列季节内活动特征及与印度降水的联系

利用美国国家环境预测中心与国家大气研究中心(NCEP/NCAR)逐日再分析资料,针对北大西洋多年代际振荡(AMO)两个不同位相,对逐候200 h Pa经向风异常进行EOF分析,发现在AMO正、负位相期间,欧亚副热带波列的季节内活动存在明显差异。利用超前—滞后回归,对比了不同AMO位相下副热带波列及其相联系的印度夏季降水的季节内活动演变特征,分析有关的大气环流,探究波列影响降水的机制。结果表明:在AMO负位相期间,由格陵兰岛以南北大西洋经大不列颠岛、地中海、黑海—里海向南亚北部传播的副热带波列的季节内演变,在印度中部引起下沉,导致中部及西北部季节内降水减少,波列负位相相反;在AMO正位相期间,副热带波列西起冰岛以南北大西洋经丹麦南部、俄罗斯西部、中亚向南亚东北部传播,对应该波列的季节内演变,辐合上升区在印度中部和东西两侧,使得该区域季节内降水增加,波列负位相相反。于是,AMO通过调制夏季欧亚副热带波列的季节内活动,可以对印度夏季降水的季节内变化空间型及演变发挥显著影响。

东绒布冰芯净积累量与印度夏季风降水的关系

珠穆朗玛峰东侧东绒布冰川积累区的降水主要是由印度夏季风环流带来的.根据东绒布冰芯记录恢复的净积累量与印度中北部和印度半岛东部地区的夏季风降水量具有基本一致的周期,三者有着较好的相关性,因此可以通过冰芯净积累量来反映更长时间序列印度某些地区夏季风降水量的变化趋势.东绒布冰芯净积累量的相对变化幅度大于印度夏季风降水量的变化,表明高海拔地区的降水比低海拔地区具有更高的敏感性.

印度洋海表温度的变化及其对印度夏季季风降水影响的诊断研究

对印度洋海表温度(SST)的主要特征及变化趋势进行分析,并研究了其与印度夏季季风降水(ISMR)和季风环流的关系,揭示出:从北印度洋到南半球中高纬度印度洋,SST最显著的变化模态是全海盆一致的变化,近50 a来总体趋势是上升的,在1976,1986年以及1996年间分别有一次跳跃性增温,与太平洋SST变化趋势基本一致.除了长期变化趋势外,南印度洋中高纬度比热带地区有更显著的模态分布.在印度洋SST升温的背景下,ISMR具有逐渐减少的趋势,但两者相关较弱.印度洋SST发生跳跃后的不同阶段,许多海区SST与ISMR相关均发生变化,但在春季,热带外南印度洋具有一对相对稳定区,其分布与EOF分析的第2模态相似.根据它们的分布,文中定义了春季南半球偶极子(SIOD),在正SIOD(PSIOD)情况下印度降水偏多,而负SIOD(NSIOD)则反之.环流分析表明,PSIOD(NSIOD)通过与大气的相互作用,对夏季马斯克林高压具有增强(减弱)作用,进而使得索马里越赤道气流增强(减弱),在印度地区低空产生异常的辐合(辐散),高层辐散(辐合),从而影响印度季风环流,使得印度季风降水偏多(少).

热带印度洋降水的年际变化特征分析

对热带印度洋海区逐月降水资料的分析表明,热带印度洋海区降水年际变化的主要特征表现为东、西方向反位相的偶极子模态,该模态与热带印度洋海区低空纬向风场异常有较强的相关,并且与太平洋ENSO事件存在显著相关。另外对偶极子型降水主要模态的周期分析表明,偶极子型降水距平还存在1.5 a和4 a左右的变化周期。

印度低压异常特征及与印度、中国同期降水相关的分析

用夏季的月、季平均1 000 hPa位势高度场定义了一组描述印度低压的环流指数,包括:印度低压指数P、面积指数S、中心位置指数(λc,c)。用NCEP/NCAR再分析资料计算了1948—2008年逐年5—8月(用5,8月代表)、夏季(6—8月,用6,8月代表)印度低压的上述环流指数,用来分析夏季各月印度低压的气候和异常特征,并研究了P和λc指数与印度、中国同期降水的相关关系。分析结果表明:(1)印度低压5月形成后逐月西移、发展,7月达到最强(大)。(2)印度低压指数P和中心位置指数λc、c均存在显著年代际变化。P和c均在1960年代末发生年代际转变,它们的标准化距平P'(c')由负转正(由正转负),印度低压由强转弱(中心位置由偏北转偏南);λc'在1980年代以前为负,以偏西为主,之后转为偏东。(3)印度低压环流指数P、λc与印度同期降水显著相关,与中国同期降水也有一定相关。P与两国同期降水以负相关为主,即印度低压强年,两国某些地区降水异常增多;而λc与两国同期降水以正相关为主,即印度低压中心偏东,两国某些地区同期降水偏多;反之亦然。

南方涛动与印度夏季降水相关联系的历史演变

用 1 8 71～ 1 994年南方涛动指数 ( ISO)与印度夏季降水资料 ,研究了它们的相关关系及其历史演变。结果表明 :不同季节两者相关程度不同 ;同一季节两者的相关关系是随时间而变的不稳定关系。

索马里急流与印度夏季降水关系的年代际变化

利用1958—2014年印度热带气象研究所的全印度降水量、欧洲中期天气预报中心(ECMWF)、日本气象厅(JRA-55)及美国环境预报中心与国家大气预报中心(NCEP/NCAR)的再分析资料,研究了索马里急流(SoMali Jet,SMJ)与印度夏季降水的相关关系。结果表明:1983年之前,SMJ与印度夏季降水有显著的正相关关系,这跟以往的认识一致,但在1983年之后两者的正相关关系显著减弱。研究进一步发现,在1983年之前,南印度洋及索马里沿岸有越赤道气流正异常,阿拉伯海、印度西北地区存在西南风正异常,异常强的SMJ对应着异常强的阿拉伯海西南季风,从而增加印度夏季的水汽输送,进而加强季风降水,且SMJ与南亚高压联系密切;但在1983年后,阿拉伯海北部及印度西北地区的西南风正异常减弱了,特别是在印度西海岸的西南风正异常显著减弱了,而且其与南亚高压的关系也显著减弱了,从而导致SMJ与印度夏季降水的正相关关系显著减弱。另外,在印度夏季降水与ENSO、SMJ相关关系减弱的同时,初步显示它与东亚/太平洋遥相关型(EAP)的相关关系却有所加强。

El Ni?o衰退年印度洋海盆模态对印度夏季季风降水的影响

采用全印度陆地降水资料、NCEP/NCAR大气资料以及Had ISST资料集的SST资料,使用经验正交函数分解(Empirical Orthogonal Function,EOF)和相关分析回归分析等统计方法,在前人的基础上,系统研究了厄尔尼诺-南方涛动(El Nino-Southern Oscillation,ENSO)及印度洋海盆模态(Indian Ocean Basin mode,IOB)对印度夏季季风降水的影响,指出厄尔尼诺衰退年印度夏季降水在反对称模态和北印度洋二次增暖共同的作用下表现出初夏减少晚夏增加的特点。通过观测分析发现,厄尔尼诺在其发展年和衰退年对印度夏季季风降水的影响截然不同。在发展年,中东太平洋的异常增暖引起Walker环流的改变,印度洋区域的下沉气流抑制印度大陆降水。在衰退年,厄尔尼诺对印度洋进行"充电",产生印度洋海盆增暖模态。在这个过程中,局地海气相互作用引起衰退年的印度降水有初夏减少晚夏增加的特点。其中春季印度洋的反对称风场(赤道以北为东北风异常,赤道以南为西北风异常)对印度夏季风有一定的减弱作用,这种异常环流减弱了初夏印度降水。同时反对称风场会造成在夏季北印度洋的二次增暖,又会促进了晚夏降水的异常增加。通过水汽输运通量的诊断分析进一步验证了上述海盆模态对印度夏季降水的作用。

THE VARIATION OF EVAPORATION OVER SOUTH CHINA AND ITS RELATIONSHIPS TO PRECIPITATION

The evaporation rate over South China is estimated based on the Climate Prediction Center Merged Analysis of Precipitation(CMAP)data and the NCEP/DOE reanalysis II data from 1979 to 2007. The temporal variation of evaporation over South China and its relationship to precipitation are discussed. Climatologically,the evaporation rate over South China is the largest in July and smallest in March.In spring and summer,the evaporation rate is approximately one half of the precipitation rate.However,the evaporation rate is approximately equal to the precipitation rate in fall and winter.The year-to-year variation of the evaporation rate over South China is quite in phase with that of the precipitation rate in the period from February to May but out of phase with that of the precipitation rate in early winter.Over South China there is a pronounced decreasing trend in the evaporation in colder seasons and a positive correlation between the evaporation variation and the rainfall variation in spring.In summer,the abnormality of rainfall over South China is closely related to the anomalous evaporation over the northeastern part of the South China Sea and its eastern vicinity.In winter,the rainfall variation in South China has a close linkage with the evaporation variation in a belt area covering the eastern Arabian Sea,the Bay of Bengal,the southeastern periphery of the Plateau,the southern part of South China Sea and the central part of Indonesia.

Extreme Rainfall Events and Associated Natural Hazards in Alaknanda Valley, Indian Himalayan Region

Entire Himalayan region is vulnerable to rain-induced （torrential rainfall） hazards in the form of flash flood, cloudburst or glacial lake outburst flood Flash floods and cloudburst are generally caused by high intensity rainfall followed by debris flow or landslide often resulting into blockade of river channels. The examples of some major disasters caused by torrential rainfall events in last fifty years are the flash floods of 1968 in Teesta valley, in 1993 and 2000 in Sutlej valley, in 1978 in Bhagirathi and in 1970 in Alaknanda river valleys. The formation of landslide dams and subsequent breaching is also associated with such rainfall events. These dams may persist for years or may burst within a short span of its formation. Due to sudden surge of water level in the river valleys, havoc and panic are created in the down stream. In Maknanda valley, frequencies of such extreme rainfall events are found to be increasing in last two decades. However, the monthly trend of extreme rainfall events has partly indicated this increase. In most of the years extreme rainfall events and cloudburst disaster were reported in August during the later part of the monsoon season.

Recent Changes of Northern Indian Ocean Summer Rainfall Based on CMIP5 Multi-Model

This study evaluates the simulation of summer rainfall changes in the Northern Indian Ocean (NIO) based on the fifth phase of Coupled Model Intercomparison Project (CMIP5). The historical runs of 20 CMIP5 coupled General Circulation Models (GCMs) are analyzed. The Multi-Model ensemble (MME) of the CMIP5 models well reproduces the general feature of NIO summer rainfall. For a short period 1979?2005, 14 out of 20 models show an increased trend in the mean rainfall and a similar spatial distri-bution to the Global Precipitation Climatology Project (GPCP) observations in MME. The increasing of the convergence in the equatorial IO results in the increase of rainfall significantly. The equatorial rainfall trend patterns seem modulated by the SST warm-ing in the tropical Indian Ocean, which confirm the mechanism of 'warmer-get-wetter' theory. For a long period 1950?2005, the trend of monsoon rainfall over India shows a decrease over the most parts of the India except an increase over the south corn er of the Indian Peninsula, due to a weakened summer monsoon circulation. The pattern is well simulated in half of the CMIP5 models. The rainfall over the north India is different for a short period, in which rainfall increases in 1979?2005, implying possible decadal varia-tion in the NIO summer climate.

Prediction and mechanistic analysis of May precipitation in North China based on April Indian Ocean SST and the Northwest Pacific Dipole

North China May precipitation(NCMP)accounts for a relatively small percentage of annual total precipitation in North China,but its climate variability is large and it has an important impact on the regional climate and agricultural production in North China.Based on observed and reanalysis data from 1979 to 2021,a significant relationship between NCMP and both the April Indian Ocean sea surface temperature(IOSST)and Northwest Pacific Dipole(NWPD)was found,indicating that there may be a link between them.This link,and the possible physical mechanisms by which the IOSST and NWPD in April affect NCMP anomalies,are discussed.Results show that positive(negative)IOSST and NWPD anomalies in April can enhance(weaken)the water vapor transport from the Indian Ocean and Northwest Pacific to North China by influencing the related atmospheric circulation,and thus enhance(weaken)the May precipitation in North China.Accordingly,an NCMP prediction model based on April IOSST and NWPD is established.The model can predict the annual NCMP anomalies effectively,indicating it has the potential to be applied in operational climate prediction.

The Interannual Variations of Summer Precipitation in the Northern Indian Ocean Associated with ENSO

Using rainfall data from the Global Precipita- tion Climatology Project （GPCP）, NOAA extended reconstruction sea surface temperature （ERSST）, and NCEP/NCAR reanalysis, this study investigates the interannual variation of summer rainfall southwest of the Indian Peninsula and the northeastern Bay of Bengal associated with ENSO. The composite study indicates a decreased summer rainfall southwest of the Indian Penin- sula and an increase in the northeastern Bay of Bengal during the developing phase, but vice versa during the decay phase of E1 Nifio. Further regression analysis dem- onstrates that abnormal rainfall in the above two regions is controlled by different mechanisms. Southwest of the Indian Peninsula, the precipitation anomaly is related to local convection and water vapor flux in the decay phase of E1 Nifio. The anomalous cyclone circulation at the lower troposphere helps strengthen rainfall. In the northeastern Bay of Bengal, the anomalous rainfall depends on the strength of the Indian southwest summer monsoon （ISSM）. A strong/weak ISSM in the developing/decay phase of E1 Nifio can bring more/less water vapor to strengthen/weaken the local summer precipitation.

Perspectives on the non-stationarity of the relationship between Indian and East Asian summer rainfall variations

The relationship between Indian and East Asian summer rainfall variations is non-stationary in observations as well as in historical simulations of climate models.Is this non-stationarity due to changes in effects of external forcing or internal atmospheric processes？ Whilst ENSO is an important oceanic forcing of Indian and East Asian summer rainfall variations,its impacts cannot explain the observed long-term changes in the Indian-East Asian summer rainfall relationship.Monte Carlo test indicates that the role of random processes cannot be totally excluded in the observed longterm changes of the relationship.Analysis of climate model outputs shows that the Indian-North China summer rainfall relationship displays obvious temporal variations in both individual and ensemble mean model simulations and large differences among model simulations.This suggests an important role played by atmospheric internal variability in changes of the Indian-East Asian summer rainfall relationship.This point of view is supported by results from a 100-years AGCM simulation with climatological SST specified in the global ocean.The correlation between Indian and North China or southern Japan summer rainfall variations displays large fluctuations in the AGCM simulation

RELATIONSHIP BETWEEN DIPOLE OSCILLATION OF SSTA OF INDIAN OCEAN REGION AND PRECIPITATION AND TEMPERATURE IN CHINA

The work is a general survey using SSTA data of the Indian Ocean and of precipitation at 160Chinese weather stations over 1951～1997 (47 years). It reveals that the dipole oscillation of SST, especially the dipole index of March～May, in the eastern and western parts of the ocean correlates well with the precipitation during the June～August raining season in China. As shown in analysis of 500-hPa Northern Hemisphere geopotential height height by NCEP for 1958～1995, the Indian Ocean dipole index (IODI) is closely related with geopotential height anomalies in the middle- and higher- latitudes in the Eurasian region. As a negative phase year of IODI corresponds to significant Pacific-Japan (P J) wavetrain, it is highly likely that the SST for the dipole may affect the precipitation in China through the wavetrain. Additionally, correlation analysis of links between SST dipole index of the Indian Ocean region and air temperature in China also shows good correlation between the former and wintertime temperature in southern China.

Weaker connection between the Atlantic Multidecadal Oscillation and Indian summer rainfall since the mid-1990s

Previous studies have shown that the Atlantic Multidecadal Oscillation （AMO） can play an important role in modulating the variabilityoflndian summer monsoon rainfall （ISMR） over a 50-60-yr timescale. A significant positive correlation between the AMO and ISMR is found both in observations and models. However, instrumental records show that the relationship becomes non-significant or even of opposite sign after the mid-1990s, suggesting a weakening of the AMO-ISMR connection. The mechanism for the breakdown of the AMO-ISMR connection is investigated in the present work, and the results suggest that a substantial warming in the Indian-tropical western Pacific Ocean plays a role. The warming weakens the meridional gradient of tropospheric temperature between Eurasia and the indian Ocean, and reduces the meridional sea level pressure gradient between the Indian Subcontinent and Indian Ocean, weakening the Indian summer monsoon. Thus, warming in the Indian-tropical western Pacific Ocean seems responsible for the weakened connection between the AMO and ISM.

Relationship Between Upper-Ocean Heat Content in the Tropical Indian Ocean and Summer Precipitation in China

An analysis of the Ishii ocean heat content(OHC) in the tropical Indian Ocean from the surface to 700-m depth shows that the OHC changes dramatically on the interannual timescale in the Indian Ocean.The first mode of empirical orthogonal function(EOF1) of the OHC shows that there is a strong air-sea interaction pattern in the Indian Ocean with a positive(negative) loading in the east and a negative(positive) loading in the west.This seesaw oscillation pattern influences the summer precipitation in China with a North-South reversed distribution.Composite analysis shows that during a positive(negative) OHC episode,an anomalous cyclonic(anticyclonic) circulation over the western Pacific and South China weakens(enhances) the monsoonal northward flow in the lower troposphere;meanwhile,anomalous meridional circulation connects the descending(ascending) branch over the Southeast Indian Ocean and the ascending(descending) branch in South China as well as a descending(ascending) branch over North China.Analysis of the mechanism behind these features suggests that(1) the accumulation of OHC-induced vorticity is related to the wave activity over the mid-latitudes and that(2) the meridional teleconnection induced by the Indo-Pacific air-OHC interaction appears over East Asia and the western Pacific.Both of these patterns can cause summer precipitation anomalies in China.

Variability of Summer Monsoon Rainfall in India on Inter-Annual and Decadal Time Scales

Indian Summer Monsoon Rainfall （ISMR） exhibits a prominent inter-annual variability known as troposphere biennial oscillation.A season of deficient June to September monsoon rainfall in India is followed by warm sea surface temperature （SST） anomalies over the tropical Indian Ocean and cold SST anomalies over the westem Pacific Ocean.These anomalies persist until the following monsoon,which yields normal or excessive rainfall.Monsoon rainfall in India has shown decadal variability in the form of 30 year epochs of alternately occurring frequent and infrequent drought monsoons since 1841,when rainfall measurements began in India.Decadal oscillations of monsoon rainfall and the well known decadal oscillations in SSTs of the Atlantic and Pacific oceans have the same period of approximately 60 years and nearly the same temporal phase.In both of these variabilities,anomalies in monsoon heat source,such as deep convection,and middle latitude westerlies of the upper troposphere over south Asia have prominent roles.